At the last race in 2023, the RZ had an intake leak, and I tried to fix it using high temp RTV. Not one of my better ideas.
I hadn’t touched the bike since the race, so it was still sitting in the garage with 1/8” of orange silicone smeared all over the intake rubbers, reed blocks, and the intake manifolds on the engine.
Last week, I decided that it was time to clean up my mess. It took a while, but all the parts are now silicone free.
So, there I was, sitting in the garage, removing bright orange silicone off the rough surface of the motor.
It was a mind numbing job, and since I couldn’t damage the cylinders by scraping them with my fingernail, I decided to numb my mind before getting started.
As I was working away, I started thinking about the upcoming race season, and what I could do to make the bike faster.
Well, to make the RZ faster (mph) would require more HP, which means engine mods, and that costs big $$$, so it ain’t going to happen.
However, my 1/4 mile times would be a quicker if I could launch harder, without pulling a wheelie.
And I could launch harder if the bike was longer, lighter or lower.
Well, to make the bike longer I’d have to lengthen the swing-arm, (again), and that costs big $$$, so it ain’t going to happen.
And I don’t know if I can make the RZ much lighter. I already put it on a diet.
- Replaced the oem pipes with Toomey pipes, Y-boot.
- Removed the stock air-box, oil-tank, oil pump, center-stand, turn signals, rear fender, rear signal/plate bracket, mirrors, ignition switch, horn, speedometer, speedometer drive, rad cover, passenger pegs, and belly-pan.
- I cut off the back half (passenger section) of the foot-peg brackets.
(Note: I had to cut them off because they hit the FZR400 swing-arm, but I didn’t cut up any good parts. They were already broken.)
- I’m using a smaller, lighter battery.
- I have a lighter, 16” Ninja 250 front wheel and tire.
- I’m only running 1 brake caliper and 1 rotor on the front.
- I’ve got an aluminum swing-arm on the rear.
- I installed Mikuni TM28 carbs, which are a few ounces lighter than the oem carbs.
- I’m using V-Force 4 reeds which are a few grams lighter than the oem reed blocks.
- I cut 2” of foam out of the seat.
- And I never have more than 3/4 gallon (3 liters) of fuel in tank.
Not really sure what else I can get rid of. I kinda need the rest of the parts.
Drag Bike - Plan B
Moderator: rztom
Re: Drag Bike - Plan B
So, can I make it lower? Well, maybe.
A lot of racers use tie-down straps, (connected between the triple tree and lower fork leg), to compress the forks.
It’s a simple way to lower the front end, and it’s not terribly expensive.
And I like modifications that are easy and cheap, because I’m lazy and poor.
With that thought, I stopped scraping silicone and picked up a tape measure.
I took a look at my forks to see what I could do and as it turns out, not much.
I’ve already shortened the forks and only have 3” of fork travel. When I’m sitting on the bike, I only have 2.5” of fork travel left.
The minimum fork travel allowed is 2”, so I could only shorten the forks by 1/2”, and that would only lower the steering stem by 1/4” to 3/8”.
And, I’d have to source parts, and build brackets, and it would add weight to the bike.
And after thinking about it for a bit, I decided that it’s not worth the effort.
So, lowering the front end is not an option.
And there’s not much more that I can do on the rear end of the bike either.
I’ve already installed longer dog-bones to lower the rear ride height and I can’t lower it any further, because I only have 2-3 mm (1/8”) of clearance between the rear tire and fender, when the shock is fully compressed.
Ding, ding, ding. The light came on and I had an epiphany. Install a smaller tire.
I’m currently running a 150/70-18 rear tire on a 4.00” wide, FZR400 rim.
If I installed a 140/70-18 tire, it would shorten the tires’ sidewall by 7 mm (0.275”).
This would lower the rear axle 7 mm, and it would also give me more clearance between the tire and fender.
Then I could get longer dog-bones and lower the rear ride height by another 7 mm (0.275”).
This combination would lower the rear ride height by 14 mm, (0.55”).
Seems like a lot of expense for a pretty small gain.
However, if I installed a 130/70-18 tire, it would lower the rear axle by 14 mm, and longer dog-bones would lower the ride height by another 14 mm.
That would, be a total of 28 mm (1- 1/8”). Hmmm.
I fired up the computer and took a look at the Bridgestone website. It shows tire diameter specs for the 150/70-18 tire mounted on a 4.25” rim.
I’m running a narrower 4.00” rim, so my tire is pinched in a bit, which will make it taller.
The Bridgestone tire diameter spec for the 130/70-18 tire, is when it’s mounted on a 3.50” rim.
I’d be putting it on a wider rim, so it would flatten out, and its diameter would be a bit smaller than the Bridgestone spec.
(The 130/70-18 tire specs say it can be used on 3.00” to 4.00” wide rims)
Maybe, I can get another 2-4 mm of clearance due to the variance in rim width.
It’s possible that I could lower the rear ride height, and the seat, which supports the majority of my weight, by 30-32 mm (1-1/4”).
Hmmm. Lower bike. Lower center of gravity. Harder launch. Faster 1/4 mile times.
I was really liking the idea, but before I started ordering parts off the Internet, I decided to take a break, have one for the road, and think about it for a few minutes.
A lot of racers use tie-down straps, (connected between the triple tree and lower fork leg), to compress the forks.
It’s a simple way to lower the front end, and it’s not terribly expensive.
And I like modifications that are easy and cheap, because I’m lazy and poor.
With that thought, I stopped scraping silicone and picked up a tape measure.
I took a look at my forks to see what I could do and as it turns out, not much.
I’ve already shortened the forks and only have 3” of fork travel. When I’m sitting on the bike, I only have 2.5” of fork travel left.
The minimum fork travel allowed is 2”, so I could only shorten the forks by 1/2”, and that would only lower the steering stem by 1/4” to 3/8”.
And, I’d have to source parts, and build brackets, and it would add weight to the bike.
And after thinking about it for a bit, I decided that it’s not worth the effort.
So, lowering the front end is not an option.
And there’s not much more that I can do on the rear end of the bike either.
I’ve already installed longer dog-bones to lower the rear ride height and I can’t lower it any further, because I only have 2-3 mm (1/8”) of clearance between the rear tire and fender, when the shock is fully compressed.
Ding, ding, ding. The light came on and I had an epiphany. Install a smaller tire.
I’m currently running a 150/70-18 rear tire on a 4.00” wide, FZR400 rim.
If I installed a 140/70-18 tire, it would shorten the tires’ sidewall by 7 mm (0.275”).
This would lower the rear axle 7 mm, and it would also give me more clearance between the tire and fender.
Then I could get longer dog-bones and lower the rear ride height by another 7 mm (0.275”).
This combination would lower the rear ride height by 14 mm, (0.55”).
Seems like a lot of expense for a pretty small gain.
However, if I installed a 130/70-18 tire, it would lower the rear axle by 14 mm, and longer dog-bones would lower the ride height by another 14 mm.
That would, be a total of 28 mm (1- 1/8”). Hmmm.
I fired up the computer and took a look at the Bridgestone website. It shows tire diameter specs for the 150/70-18 tire mounted on a 4.25” rim.
I’m running a narrower 4.00” rim, so my tire is pinched in a bit, which will make it taller.
The Bridgestone tire diameter spec for the 130/70-18 tire, is when it’s mounted on a 3.50” rim.
I’d be putting it on a wider rim, so it would flatten out, and its diameter would be a bit smaller than the Bridgestone spec.
(The 130/70-18 tire specs say it can be used on 3.00” to 4.00” wide rims)
Maybe, I can get another 2-4 mm of clearance due to the variance in rim width.
It’s possible that I could lower the rear ride height, and the seat, which supports the majority of my weight, by 30-32 mm (1-1/4”).
Hmmm. Lower bike. Lower center of gravity. Harder launch. Faster 1/4 mile times.
I was really liking the idea, but before I started ordering parts off the Internet, I decided to take a break, have one for the road, and think about it for a few minutes.
Re: Drag Bike - Plan B
I returned to the shop with a new point of view and some new questions.
What are the downsides to a smaller tire?
And as always, there was the big question; How Much ?
A new tire will cost $180, and another $75 for some longer dog-bones.
Better make that $150 for dog-bones, because I’d get 2 sets with different lengths.
I’d get a long set that should work, if all my calculations and measurements are correct.
And I’d get a 1 mm shorter set, to increase the ride height by 4 mm, in case I miscalculate and the tire hits the fender.
Add in some sales tax and the parts would cost about $350 Can ($260 US).
Next question. Will the narrower tire provide enough traction off the line?
I raced the RZ for a few years using the 120/80 tire, and didn’t have any traction issues off the starting line.
But, that was the stock wheelbase RZ, and I could only launch it at 3500-4000 rpm.
Now I have a longer swing-arm and I’m launching at 6700 rpm.
Just for fun, I measured the width of the contact patch on 2 of my race tires.
It’s pretty easy to see how much of the tire is on the pavement, because it wears fast and flattens out.
On the 150/70-18 tire, the contact patch is 80 mm (3.15”) wide.
On the 120/80-18 tire, the contact patch is 55 mm (2.17) wide.
I’m thinking that a 130/70-18 tire, on the 4.00” rim, should give me a contact patch that’s at least 64 mm (2.52”) wide, and that should provide enough traction.
Another consideration is that the smaller tire will have a smaller circumference, and that will affect the gearing.
I currently have 15T and 16T front sprockets, and 40T and 41T rear sprockets.
According to the Gearing Commander website, the 15T front sprocket will be too small, unless I get a 37T or 38T rear sprocket.
If I use the 16T front sprocket, then the 41T rear sprocket would be perfect for the 1/8 mile, where I cross the finish line at 88 – 90 mph, in 4th gear.
And, the 16T front will also work well with the 41T rear, for the 1/4 mile races, if I’m using all 6th gears.
At some of the slower 1/4 mile tracks, where I only have a top speed of 105-107 mph, a 42T rear sprocket would work better.
If I want to run the 1/4 mile in 5th gear, then I would need to get a 38T rear sprocket.
While I was mulling this over and looking at the cost vs. benefits, I had another eureka moment.
What are the downsides to a smaller tire?
And as always, there was the big question; How Much ?
A new tire will cost $180, and another $75 for some longer dog-bones.
Better make that $150 for dog-bones, because I’d get 2 sets with different lengths.
I’d get a long set that should work, if all my calculations and measurements are correct.
And I’d get a 1 mm shorter set, to increase the ride height by 4 mm, in case I miscalculate and the tire hits the fender.
Add in some sales tax and the parts would cost about $350 Can ($260 US).
Next question. Will the narrower tire provide enough traction off the line?
I raced the RZ for a few years using the 120/80 tire, and didn’t have any traction issues off the starting line.
But, that was the stock wheelbase RZ, and I could only launch it at 3500-4000 rpm.
Now I have a longer swing-arm and I’m launching at 6700 rpm.
Just for fun, I measured the width of the contact patch on 2 of my race tires.
It’s pretty easy to see how much of the tire is on the pavement, because it wears fast and flattens out.
On the 150/70-18 tire, the contact patch is 80 mm (3.15”) wide.
On the 120/80-18 tire, the contact patch is 55 mm (2.17) wide.
I’m thinking that a 130/70-18 tire, on the 4.00” rim, should give me a contact patch that’s at least 64 mm (2.52”) wide, and that should provide enough traction.
Another consideration is that the smaller tire will have a smaller circumference, and that will affect the gearing.
I currently have 15T and 16T front sprockets, and 40T and 41T rear sprockets.
According to the Gearing Commander website, the 15T front sprocket will be too small, unless I get a 37T or 38T rear sprocket.
If I use the 16T front sprocket, then the 41T rear sprocket would be perfect for the 1/8 mile, where I cross the finish line at 88 – 90 mph, in 4th gear.
And, the 16T front will also work well with the 41T rear, for the 1/4 mile races, if I’m using all 6th gears.
At some of the slower 1/4 mile tracks, where I only have a top speed of 105-107 mph, a 42T rear sprocket would work better.
If I want to run the 1/4 mile in 5th gear, then I would need to get a 38T rear sprocket.
While I was mulling this over and looking at the cost vs. benefits, I had another eureka moment.
Re: Drag Bike - Plan B
By this time, I had pretty much abandoned any thoughts of working on the bike, and I still had some unanswered questions, so the search continued.
Hey Google, how does Tire Diameter affect Torque? The short answer, it doesn’t.
Torque is produced by the engine and transmitted through the front sprocket, chain, rear sprocket, and sprocket bolts, to the wheel.
And that’s not going to change, no matter what size tire is on the bike.
However, the Force that’s being applied to the pavement, at the contact patch of the tire, and accelerating the bike forward, is affected by the tire diameter.
And the Internet experts all seemed to agree that smaller diameter tires will accelerate faster than large diameter tires.
OK, fine, but why?
And that question opened a can of worms that started with a dozen Google searches and ended with me reading a couple of chapters of Tony Foale’s book on Motorcycle Handling and Chassis Design.
It took a while, but I think I found what I was looking for.
Torque is the rotational version of Force.
It is the measure of a twisting tendency created by an offset application of a Force.
Its value is determined by multiplying the Force by the degree of offset.
Torque (T) = Force (F) x Length (L).
On the rear wheel, the Length (L) of the offset is the Radius (r) of the rear tire.
So, the driving Force (F), at the tires’ contact patch, is equal to the engine Torque (T) divided by the Radius (r) of the tire.
F = T/r
Engine Torque doesn’t change, so as the tire gets smaller, the Force at the contact patch gets bigger.
I knew the diameters of the tires, so I looked at my dyno runs and picked a nice round Torque number of 30 ft/lb and did some math.
(I was looking for a % increase, so the actual Torque value is not important, because it’s a constant in the equation.)
According to my calculations, the driving Force will increase 4.4%, if I replace the 150/70-18 tire, with a 130/70-18 tire.
That sounded pretty good, but there was more.
According to Newton’s 2nd Law: Force = Mass x Acceleration.
Therefore, Acceleration = Force / Mass.
The smaller rear tire will increase the Force, and it will also decrease the Mass, both of which will result in faster Acceleration.
If I go with the 4.4% increase in Force, and use a bike/rider weight of 480 pounds, then decrease the weight by 4 pounds, for the lighter tire, the RZ’s Acceleration should increase by a total of 5.3%.
If I assume that 1 pound of rotating weight is equal to 3.5 pounds of sprung weight, then the lighter tire would increase the RZ’s acceleration by 7.5%.
By the time I’d satisfied my curiosity, it was way past being late in the evening, and well into early morning, and I’d spent the 1st part of the night trying to lose touch with reality.
So, when it comes to the math and physics, it’s entirely possible that I’ve made incorrect assumptions, calculation errors, or stupid mistakes in applying the formulas.
If I’ve said anything in the above post that is totally stupid, please let me know.
I was trying to keep it all straight in my head, but it was like trying to cram an entire high school Physics course into a couple of hours.
Force, Weight, Mass, Velocity, Acceleration, Torque, Moments of Inertia, Angular Acceleration.
And to really understand Velocity and Acceleration, you need to have a pretty good knowledge of differential calculus, which I don’t.
So, the Force and Acceleration numbers that I calculated don’t mean very much, if anything at all.
It’s all just theory and speculation at the moment.
Will the RZ’s Acceleration increase by 4%, 5%, 7%? I haven’t got a clue.
I do know that a smaller tire will weigh less than a bigger tire.
And I trust Sir Isaac Newton when he says that less mass will equal faster acceleration.
And the Internet experts say smaller tires will accelerate faster than bigger tires.
So, in theory, installing a smaller, lighter tire on the RZ should make it faster.
But, in the real world, will it make a noticeable improvement in my 1/4 mile times?
It’s an interesting question and I must admit, I am curious to see what the smaller tire would do at the track.
But, am I $350.00 worth of curious?
Hey Google, how does Tire Diameter affect Torque? The short answer, it doesn’t.
Torque is produced by the engine and transmitted through the front sprocket, chain, rear sprocket, and sprocket bolts, to the wheel.
And that’s not going to change, no matter what size tire is on the bike.
However, the Force that’s being applied to the pavement, at the contact patch of the tire, and accelerating the bike forward, is affected by the tire diameter.
And the Internet experts all seemed to agree that smaller diameter tires will accelerate faster than large diameter tires.
OK, fine, but why?
And that question opened a can of worms that started with a dozen Google searches and ended with me reading a couple of chapters of Tony Foale’s book on Motorcycle Handling and Chassis Design.
It took a while, but I think I found what I was looking for.
Torque is the rotational version of Force.
It is the measure of a twisting tendency created by an offset application of a Force.
Its value is determined by multiplying the Force by the degree of offset.
Torque (T) = Force (F) x Length (L).
On the rear wheel, the Length (L) of the offset is the Radius (r) of the rear tire.
So, the driving Force (F), at the tires’ contact patch, is equal to the engine Torque (T) divided by the Radius (r) of the tire.
F = T/r
Engine Torque doesn’t change, so as the tire gets smaller, the Force at the contact patch gets bigger.
I knew the diameters of the tires, so I looked at my dyno runs and picked a nice round Torque number of 30 ft/lb and did some math.
(I was looking for a % increase, so the actual Torque value is not important, because it’s a constant in the equation.)
According to my calculations, the driving Force will increase 4.4%, if I replace the 150/70-18 tire, with a 130/70-18 tire.
That sounded pretty good, but there was more.
According to Newton’s 2nd Law: Force = Mass x Acceleration.
Therefore, Acceleration = Force / Mass.
The smaller rear tire will increase the Force, and it will also decrease the Mass, both of which will result in faster Acceleration.
If I go with the 4.4% increase in Force, and use a bike/rider weight of 480 pounds, then decrease the weight by 4 pounds, for the lighter tire, the RZ’s Acceleration should increase by a total of 5.3%.
If I assume that 1 pound of rotating weight is equal to 3.5 pounds of sprung weight, then the lighter tire would increase the RZ’s acceleration by 7.5%.
By the time I’d satisfied my curiosity, it was way past being late in the evening, and well into early morning, and I’d spent the 1st part of the night trying to lose touch with reality.
So, when it comes to the math and physics, it’s entirely possible that I’ve made incorrect assumptions, calculation errors, or stupid mistakes in applying the formulas.
If I’ve said anything in the above post that is totally stupid, please let me know.
I was trying to keep it all straight in my head, but it was like trying to cram an entire high school Physics course into a couple of hours.
Force, Weight, Mass, Velocity, Acceleration, Torque, Moments of Inertia, Angular Acceleration.
And to really understand Velocity and Acceleration, you need to have a pretty good knowledge of differential calculus, which I don’t.
So, the Force and Acceleration numbers that I calculated don’t mean very much, if anything at all.
It’s all just theory and speculation at the moment.
Will the RZ’s Acceleration increase by 4%, 5%, 7%? I haven’t got a clue.
I do know that a smaller tire will weigh less than a bigger tire.
And I trust Sir Isaac Newton when he says that less mass will equal faster acceleration.
And the Internet experts say smaller tires will accelerate faster than bigger tires.
So, in theory, installing a smaller, lighter tire on the RZ should make it faster.
But, in the real world, will it make a noticeable improvement in my 1/4 mile times?
It’s an interesting question and I must admit, I am curious to see what the smaller tire would do at the track.
But, am I $350.00 worth of curious?
Re: Drag Bike - Plan B
The Pro’s and Con’s of installing a smaller, lighter rear tire,
Pro’s
If I pick an average 1/4 mile time is 12.5 seconds and an acceleration increase of 5%, and then apply simple Drag Race math, (which is based on hunches, assumptions, and wishful thinking, and has nothing to do with physics or calculus), then the RZ would run the 1/4 mile 5% quicker, which would give me a time of 11.88 seconds. Whoop. Whoop.
Personally, I think it’s impossible that my 1/4 mile time would decrease by 0.6 seconds, and I’d probably fall off my bike if they handed me a time slip that read 11.anything seconds.
Con’s
- I don’t have $350 to spend on a physics experiment, just to satisfy my curiosity.
- If the RZ doesn’t run faster 1/4 mile times, then it’s a waste of time and money.
- If the RZ does run faster 1/4 mile times, then it’s still a waste of time and money,
because winning a bracket race has nothing to do with how fast your bike is.
Pro’s
If I pick an average 1/4 mile time is 12.5 seconds and an acceleration increase of 5%, and then apply simple Drag Race math, (which is based on hunches, assumptions, and wishful thinking, and has nothing to do with physics or calculus), then the RZ would run the 1/4 mile 5% quicker, which would give me a time of 11.88 seconds. Whoop. Whoop.
Personally, I think it’s impossible that my 1/4 mile time would decrease by 0.6 seconds, and I’d probably fall off my bike if they handed me a time slip that read 11.anything seconds.
Con’s
- I don’t have $350 to spend on a physics experiment, just to satisfy my curiosity.
- If the RZ doesn’t run faster 1/4 mile times, then it’s a waste of time and money.
- If the RZ does run faster 1/4 mile times, then it’s still a waste of time and money,
because winning a bracket race has nothing to do with how fast your bike is.
Re: Drag Bike - Plan B
This is how many if us work on bikes i reckon... you think, you research and then you realise the reality is something like the last line you wrote Steve.
A thought... could you not make an adjustable threaded dogbone from an old set?
A thought... could you not make an adjustable threaded dogbone from an old set?
80 XT500 Supermoto!
81 RD350LC Resto
82 RD421LC Hybrid
82 RD350LC decapitation project
82 RD250LC JDM '251LC' YPVS
83 RZ350 Resto
84 RZ500 Resto
85 RZ350 F1 Resto
86 RZ350 F2 Resto
86 RZ350 F2 Hybrid
Like Watches? www.PloProf.com & www.DeskDivers.com
81 RD350LC Resto
82 RD421LC Hybrid
82 RD350LC decapitation project
82 RD250LC JDM '251LC' YPVS
83 RZ350 Resto
84 RZ500 Resto
85 RZ350 F1 Resto
86 RZ350 F2 Resto
86 RZ350 F2 Hybrid
Like Watches? www.PloProf.com & www.DeskDivers.com
-
evan_calgary
- Posts: 1381
- Joined: Tue Nov 12, 2013 2:34 am
Re: Drag Bike - Plan B
Attempting to change your effective gear ratio. Calculators like below can help!
https://tiresize.com/gear-ratio-calculator/
If you are running out of top end then you may gain to lose. If not, may be overall positive.
You are lowering the back end with overall smaller tire diameter, isn't this going to negatively impact your other issue of wheelies?
https://tiresize.com/gear-ratio-calculator/
If you are running out of top end then you may gain to lose. If not, may be overall positive.
You are lowering the back end with overall smaller tire diameter, isn't this going to negatively impact your other issue of wheelies?
Re: Drag Bike - Plan B
Thanks for the link, Evan.
I’m not sure how the smaller tire would affect the top end speed.
With the big tire, the engine is still gaining rpm at the finish line, but the acceleration slows down quite a bit when you shift from 5th to 6th.
Last year I geared the bike high (16/40 up from 15/41), and crossed the finish in 5th gear.
Lowering the rear ride height (rear part of the frame), and leaving the front end alone, would increase the rake, change the weight bias and change the bike’s center of gravity.
But it’s late in the evening, and I’m having a difficult time trying to visualize how the frame is moving and whether the C of G is moving forward or rearward, as the frame pivots around the front axle.
A smaller tire and longer dog-bones would lower the rear ride height and lower the bike’s center of gravity, which should help prevent wheelies.
And the lighter tire on the rear would move the C of G forward, but I’m not sure how that would affect the way the bike performs, because the tire is un-sprung weight,
So, I have absolutely no idea how a smaller tire would affect the wheelie issue.
Only 1 way to find out.
Hey Jon. I did look into adjustable dog-bones, but most of what I found are made for specific models of bikes, and I couldn’t find any with the bolt diameters that I needed, (12 mm on one end and 14 mm).
And if I did manage to find something, I’d have to convert my puny $Can to $US, at a 30% loss, and pay Shipping, Customs and Duty, so the Ebay parts end up getting pretty expensive.
I opted to support my current machinist. He designed the dog-bones in a CAD system and sent that info to another company that cut them out of 3/8” Al. using a high pressure water jet.
I’m not sure how the smaller tire would affect the top end speed.
With the big tire, the engine is still gaining rpm at the finish line, but the acceleration slows down quite a bit when you shift from 5th to 6th.
Last year I geared the bike high (16/40 up from 15/41), and crossed the finish in 5th gear.
Lowering the rear ride height (rear part of the frame), and leaving the front end alone, would increase the rake, change the weight bias and change the bike’s center of gravity.
But it’s late in the evening, and I’m having a difficult time trying to visualize how the frame is moving and whether the C of G is moving forward or rearward, as the frame pivots around the front axle.
A smaller tire and longer dog-bones would lower the rear ride height and lower the bike’s center of gravity, which should help prevent wheelies.
And the lighter tire on the rear would move the C of G forward, but I’m not sure how that would affect the way the bike performs, because the tire is un-sprung weight,
So, I have absolutely no idea how a smaller tire would affect the wheelie issue.
Only 1 way to find out.
Hey Jon. I did look into adjustable dog-bones, but most of what I found are made for specific models of bikes, and I couldn’t find any with the bolt diameters that I needed, (12 mm on one end and 14 mm).
And if I did manage to find something, I’d have to convert my puny $Can to $US, at a 30% loss, and pay Shipping, Customs and Duty, so the Ebay parts end up getting pretty expensive.
I opted to support my current machinist. He designed the dog-bones in a CAD system and sent that info to another company that cut them out of 3/8” Al. using a high pressure water jet.
Re: Drag Bike - Plan B
Ya. That was pretty much a rhetorical question.
The 130/70-18 Bridgestone BT46 rear tire arrived in early March.
It was promptly weighed, mounted on a rim, and then measured.
The new tire weighs 13.3 pounds, which is 3.2 pounds lighter than the old 150/70-18 tire.
Not as light as I’d hoped, but every pound helps.
The measured radius of the new 130/70 tire is 319 mm, which is 0.6 mm smaller than the Bridgestone spec.
The measured radius of my used 150/70 tire is 329 mm, which is 5.0 mm smaller than the Bridgestone spec. Hmmm.
I assume that the 150/70 tire is smaller than the Bridgestone spec because I’ve done over 100 burnouts, and 1/4 mile runs, on this tire.
So, at the moment, the new tire only gives me an extra 10 mm of clearance between the tire and rear fender, and not the 14 mm that I was hoping for.
A new set of 97 mm long dog-bones would lower the rear fender 8 mm, and the tire shouldn’t hit the fender.
A new set of 98 mm long dog-bones would lower the rear fender 12 mm, and the tire might hit.
I took a gamble and ordered one set of 98 mm dog-bones.
The new tire might brush the fender, when the shock is fully compressed, but I’m pretty sure that I can fix the problem with a few long burn outs.
The new dog-bones arrived at the end of March. I took them home, set them on the workbench, beside the tire, and didn’t touch them again until May 19th.
Re: Drag Bike - Plan B
Short tangent in case you were wondering what was so incredibly important that it prevented me from touching my RZ for over 6 weeks.
I’m on the Board of Directors for the Canadian Motorcycle Drag Racing Association (CMDRA), and this year we had a company put up some serious money to sponsor a Heads-Up Class for really quick Harley street bikes.
The CMDRA also wanted to open the Super Gas class to include metric bikes, instead of being restricted to just Harleys.
(Super Gas is an Index Class, and the winner is the person who runs closest too, but not faster than, 9.70 seconds).
This meant that we had to make some changes to the Rulebook for the 2024 season.
And the current Rulebook is something that has pissed me off since I joined the CMDRA, 6 years ago.
Some of the rules are vague, inconsistent, contradictory, and poorly worded.
The Table of Contents was disorganized and incomplete, the formatting was terrible, the overall layout was confusing, and it had a number of spelling, punctuation and grammar mistakes.
IMO, it sucked, and I told the rest of the board members exactly what I thought. I’m pretty sure that at one point in the meeting, I used the phrase, ‘piece of sh*t’.
After a lively discussion, one of the directors got tired of listening to me bitch and complain, and said, ‘If you think the Rulebook is that bad, then you why don’t you volunteer to fix it’.
I said, ‘Fine, I will’.
And with one little sentence, my big mouth took on a job that used up all my free time for a month and a half.
PS: IMO, the Rulebook is better, but it’s still not good. I’d only give it 6 out of 10.
It definitely needs some more revisions and we’ll continue to work on it, but we had to get it published before the 2024 season started.
I’m on the Board of Directors for the Canadian Motorcycle Drag Racing Association (CMDRA), and this year we had a company put up some serious money to sponsor a Heads-Up Class for really quick Harley street bikes.
The CMDRA also wanted to open the Super Gas class to include metric bikes, instead of being restricted to just Harleys.
(Super Gas is an Index Class, and the winner is the person who runs closest too, but not faster than, 9.70 seconds).
This meant that we had to make some changes to the Rulebook for the 2024 season.
And the current Rulebook is something that has pissed me off since I joined the CMDRA, 6 years ago.
Some of the rules are vague, inconsistent, contradictory, and poorly worded.
The Table of Contents was disorganized and incomplete, the formatting was terrible, the overall layout was confusing, and it had a number of spelling, punctuation and grammar mistakes.
IMO, it sucked, and I told the rest of the board members exactly what I thought. I’m pretty sure that at one point in the meeting, I used the phrase, ‘piece of sh*t’.
After a lively discussion, one of the directors got tired of listening to me bitch and complain, and said, ‘If you think the Rulebook is that bad, then you why don’t you volunteer to fix it’.
I said, ‘Fine, I will’.
And with one little sentence, my big mouth took on a job that used up all my free time for a month and a half.
PS: IMO, the Rulebook is better, but it’s still not good. I’d only give it 6 out of 10.
It definitely needs some more revisions and we’ll continue to work on it, but we had to get it published before the 2024 season started.
Re: Drag Bike - Plan B
The RZ is back together with the smaller rear tire and longer dog-bones. Yippee.
In theory, the 130/70-18 was going to lower the rear ride height of the RZ by 14 mm, and the 3 mm longer dog-bones would lower it another 12 mm, for a total of 26 mm (1.02”).
I estimated that the new tire would be 4 pounds lighter than my current tire.
In reality, the smaller tire only lowered the ride height by 10 mm and the longer dog-bones only lowered it another 11 mm, for a total of 21 mm (0.83”).
And the new tire is only 3.2 pound lighter than the old tire.
A bit less than I’d hoped for, but I’m pretty sure that the rear tire is not going to hit the fender, and it will get smaller, and lighter, after a day at the track.
In theory, the 130/70-18 was going to lower the rear ride height of the RZ by 14 mm, and the 3 mm longer dog-bones would lower it another 12 mm, for a total of 26 mm (1.02”).
I estimated that the new tire would be 4 pounds lighter than my current tire.
In reality, the smaller tire only lowered the ride height by 10 mm and the longer dog-bones only lowered it another 11 mm, for a total of 21 mm (0.83”).
And the new tire is only 3.2 pound lighter than the old tire.
A bit less than I’d hoped for, but I’m pretty sure that the rear tire is not going to hit the fender, and it will get smaller, and lighter, after a day at the track.
Re: Drag Bike - Plan B
I had the drag bike parked in the garage beside my 83 RZ, and as I walked up behind them the other day, I thought, “damn, the 130/70 tire on the drag bike looks huge compared to the 120/80 tire on the 83 RZ.
So, I dug out my calipers and measured the overall width of the Bridgestone tires on all 3 of my bikes.
120/80-18 tire, on a 2.75” wide rim, has a maximum width of 119.6 mm.
150/70-18 tire, on a 4.00” wide rim, has a maximum width of 150.5 mm.
130/70-18 tire, on a 4.00” wide rim, has a maximum width of 139.7 mm.
Wow. The 130/70 tire is 10 mm (3/8”) wider that I expected, after measuring the other 2 tires.
I assume that the extra width is because the tire is on a 4.00” wide rim, instead of the 3.50” rim that Bridgestone specifies as the optimum width.
With that good news, I don’t think I’ll have any traction issues at the starting line.
Speaking of good news.
One set of 98 mm dog-bones cost $80 Can ($60 US).
One Bridgestone BT46 rear tire cost $0.
The bike shop that I work for, donated the tire. So a big shout out to Universal Cycle.
So, I dug out my calipers and measured the overall width of the Bridgestone tires on all 3 of my bikes.
120/80-18 tire, on a 2.75” wide rim, has a maximum width of 119.6 mm.
150/70-18 tire, on a 4.00” wide rim, has a maximum width of 150.5 mm.
130/70-18 tire, on a 4.00” wide rim, has a maximum width of 139.7 mm.
Wow. The 130/70 tire is 10 mm (3/8”) wider that I expected, after measuring the other 2 tires.
I assume that the extra width is because the tire is on a 4.00” wide rim, instead of the 3.50” rim that Bridgestone specifies as the optimum width.
With that good news, I don’t think I’ll have any traction issues at the starting line.
Speaking of good news.
One set of 98 mm dog-bones cost $80 Can ($60 US).
One Bridgestone BT46 rear tire cost $0.
The bike shop that I work for, donated the tire. So a big shout out to Universal Cycle.
Re: Drag Bike - Plan B
After I finished working on the Rulebook, and before I installed the new tire and dog-bones, I did one other minor modification to the RZ.
I moved the battery. Since I couldn’t get rid of any weight, I decided to relocate some of it instead.
With the lengthened FZR400 swing-arm, I have 6” of clearance between the rear tire and the cross-brace on the swing-arm.
I made a new battery box that rests on the swing-arm, and then I bolted it to the cross-brace.
The new mounting location will lower the 5 lb. battery, and battery box, by 11” and it will move it back 6”.
The good news - This will put more weight on the rear tire, which will improve traction.
And it will lower the bike’s Center of Gravity, so it will be less likely to pull a wheelie.
The bad news – It will move the bike’s Center of Gravity back, take weight off the front end, and make the RZ more prone to pulling a wheelie.
I just hope the good news outweighs the bad.
I moved the battery. Since I couldn’t get rid of any weight, I decided to relocate some of it instead.
With the lengthened FZR400 swing-arm, I have 6” of clearance between the rear tire and the cross-brace on the swing-arm.
I made a new battery box that rests on the swing-arm, and then I bolted it to the cross-brace.
The new mounting location will lower the 5 lb. battery, and battery box, by 11” and it will move it back 6”.
The good news - This will put more weight on the rear tire, which will improve traction.
And it will lower the bike’s Center of Gravity, so it will be less likely to pull a wheelie.
The bad news – It will move the bike’s Center of Gravity back, take weight off the front end, and make the RZ more prone to pulling a wheelie.
I just hope the good news outweighs the bad.