Swapping Forks, wheels, and swing-arm.

[Off Road]

If you want some accurate suspension information, check out the Links at the end of this post
If you want an hour of entertainment, read on, and on, and on. Sorry it's a long one.

Upgrading the suspension on my RZ sounds like it might have some benefits.
Maybe install a set of R1 forks, so that I have some compression & rebound damping.
Some wide 17" wheels with huge brake discs, so I can run some sticky tires, and be able to stop on a dime.
And it will look really cool.

And it's going to make some changes to the steering geometry, and handling characteristics, of the bike.
I've been on the net, and found a bit of info on the subject.
Here's a summary of the info that I thought was interesting / important.
Most of it is really obvious stuff, however, some of it did surprise me.


RAKE:
- When you are hard on the brakes and the front end dives, the rake decreases and the trail also decreases.
- A steeper rake decreases the amount of friction between the fork tubes and the sliders, which gives better resonse to small bumps.
- When the front fork compresses, the front wheel moves up, and back, in relation to the steering head.
A steeper rake angle, decreases the amount of backward movement, which decreases the 'change in trail', as the forks compress.

RZ350 rake is 26*. Yamaha R1 rake is 24*.


TRAIL:
The magical property, that keeps the front wheel self centered, and provides high speed stability.
But keeping the wheel pointing straight, isn't all that good, when you're trying to flick the bike through the S's, or power out of corner.

A steeper rake decreases trail.
A smaller front tire, decreases trail.
A smaller fork offset, increases trail.

RZ350 trail is 3.78". Yamaha R1 trail changed from 3.6", to 4.1", to 4.0", to 3.8".


WHEELBASE:
No magic here, Shorter bikes turn faster.

RZ350 wheelbase 54.5". R1 wheelbase changed from 54.9" to 55.7".


OFFSET:
Less fork offset results in more trail.

RZ350 offset is 1.94", 2002 R1 offset is 1.0". Some R1 models had 1.4" of offset.


SQUAT:
The force that sucks the rear end down, and extends the forks, when you're accelerating.
If I remember correctly, it's trying to rotate the top of the motor around the bike's center of gravity.


ANTI-SQUAT:
The force that pushes the rear of the bike up, when it's accelerating.
From what I understand, this dyno video is a really good example of anti-squat.
http://www.youtube.com/watch?v=9sSrQQ4Kx4M" onclick="window.open(this.href);return false;

Some of the things that affect anti-squat are the swing-arm angle, swing-arm length, and sprocket size.


SWING-ARM:
Under acceleration, the rear tire is applying a force to the swing-arm, which transfers it to the frame, at the swing-arm pivot.
Since the swing-arm is at an angle, the force is trying to push the bike forward and up, at the same time.
A steeper swing-arm angle increases anti-squat.
And a longer swing-arm decreases anti-squat.

Under acceleration, the top run of the chain, is tight between the front & rear sprockets,
The distance between the chain, and the swing-arm pivot, helps determine how much anti-squat you have.
Anti-squat increases when the chain gets closer to the pivot.

The swing-arm angle also plays an important part in how well the bike hooks up and accelerates, in a straight line, and when exiting corners.


SPROCKETS:
Just a smaller front sprocket, will bring the top of the chain, closer to the swing-arm pivot, and increase the anti-squat.
Just a larger rear sprocket, will change the angle between chain & swing-arm pivot, and increase anti-squat.
A combination of the two, screws things up, and requires complicated math to figure out.


REAR SHOCK:
A longer rear shock will increase the swing-arm angle, raise the rear ride height, steepen rake, and reduce trail.
It will also shorten the wheelbase, and increase the anti-squat, due to the steeper swing-arm angle.

The shock is connected to the frame through a linkage that pivots.
The linkage uses a lever so that large wheel travel results in a small shock travel.
Changes to the shock mount/pivot setup, could change the leverage, which may require changes to the spring rate,
which could lead to changes in the front springs, so that they match the rear.
(well, that's what someone said on one of the sites I read)


FRONT TIRE:
When the diameter of the front tire is reduced, the trail decreases.


REAR TIRE:
A larger rear tire will raise the rear ride height, steepen the rake, and reduce the trail.
It will also flatten the swing-arm angle, which will reduce the anti-squat.


Seems like a lot of things affect the anti-squat.
If you don't have enough anti-squat, then when you accelerate out of a corner,
the rear end will compress, rake will increase, the trail will increase, and the bike might want to drift wide.

In addition, a guy named John Robinson, wrote a book on motorcycle chassis tuning, said the following

"During acceleration, the full weight of the machine can be transferred to the rear wheel.
The suspension needs to be able to carry this load AND maintain a centre of gravity at a height which will give maximum traction.
If the centre of gravity is too low, the wheel will spin too easily; if it’s too high, the bike will overturn too easily"

So, if there's not enough anti-squat, it's a burnout.
And if there's too much anti-squat, it's an out of control wheelie.
Got it.

Now that I've read all this info, I'm ready to install the R1 front end, wheels, and swing-arm.

- Longer forks - will increase rake, increase trail, increase wheelbase, increase ride height and center of gravity
- Smaller offset - will increase trail, shorten wheelbase, increase ride height and center of gravity
- Smaller front tire - will decrease trail, ride height and center of gravity
- Longer swing-arm- increase wheelbase, reduce anti-squat
- Longer shock - increase swing-arm angle, increase anti-squat, raise rear ride height, shorten wheelbase, increase rake, decrease trail, raise the center of gravity
- Larger Rear tire - decrease anti-squat, increase rear ride height, increase rake, decrease trail, raise the center of gravity.
After I sort out and optimize the above items, I can adjust the compression & rebound damping.
And it's done!!!

I installed FZR400 suspension on my RZ.
After a bunch of measuring, and calculating, I determined that my new rake was 0.5* steeper, and trail was 1mm longer.
When riding the bike, I really don't notice this difference, because,
a - I'm not a very talented rider,
b - It's a Street bike, not a race bike.
What I do notice, is the awesome brakes, and incredible radial tires.
And it looks really cool.
Definitely worth the price of admission.

So, a couple of important things,

1 - I'm not trashing the idea of installing R1 suspension components on an RZ.
I'm just posting this to pass on information to people who might find it interesting or maybe even useful.
I have spent a number of hours with a rake & trail calculator, evaluating the RZ with both R1 components, and FZR400 suspension components.
And it's not that bad. With the R1 parts, the rake angle gets larger & trail gets quite a bit bigger, (mostly due to the small offset on the tree's).
However, by adjusting fork and shock length, you can make some changes to the rake, trail, wheelbase, and steering height.
And it's done!!!

2 - I've read a lot of things recently, and I'm still trying to understand most of it.
And, it's more than possible that I made some glaring errors in the above Summary.
So, don't trust anything that I said, If you're interested in the subject, look into it on your own.

The following links will give you much better information, than I did.

Basic info on rake,trail, wheelbase, and swing-arm angle.
http://sportbike.natkd.com/motorcycle_geometry_101.htm" onclick="window.open(this.href);return false;

Link on Squat, Anti-squat, Squat Adjustment, Sprocket size & Squat,
Sag, Fine tuning sag & ride height.
http://www.peterverdone.com/archive/rideheight.htm" onclick="window.open(this.href);return false;

Swing-arm angle, Chain Pull, and Anti-squat
http://www.sportrider.com/tech/146_0404 ... _geometry/" onclick="window.open(this.href);return false;

Sprocket size, and how it affects Anti-squat.
http://www.insidemotorcycles.com/featur ... squat.html" onclick="window.open(this.href);return false;

Link on shock linkages and Leverage.
Interesting section on matching front and rear spring rates
http://www.promecha.com.au/leverage_linkages.htm" onclick="window.open(this.href);return false;

Swing arm, Anti-squat, and using the rear brake.
http://www.msgroup.org/forums/mtt/topic ... PIC_ID=164" onclick="window.open(this.href);return false;

Excerpts from a book on Google. Good info on linkages, levers, and Anti-squat.
But it would be helpful to have a mech. engineering degree, to understand it all.
http://books.google.ca/books?id=l2Zizg8 ... le&f=false" onclick="window.open(this.href);return false;

Yamaha R1 engineering info.
Interesting info on page 2 about the swing-arm angle and fork offset.
http://www.yamaha-motor.eu/designcafe/e ... lse&page=2" onclick="window.open(this.href);return false;

Yamaha R1 info on Wikipedia.
http://en.wikipedia.org/wiki/Yamaha_YZF-R1" onclick="window.open(this.href);return false;

[silverstrom]

Steve....I copied this to Tech Tips. Awesome info! Thanks

John

[2smoke]

Excellent post.
Thanks for taking all the time to compile all this information!

[JonW]

superb info, Im still digesting it LOL

[Ricino]

I'm also grateful, it is beautiful to visualize all those forces when looking at different bikes.
Also very useful for my project.

[Off Road]

Another long post and It will be a while before you get to the end.
Still reading?? OK, but you were warned.

By now, everyone has read the above links, and has a comprehensive knowledge of the changes that are going to happen, when suspension components are swapped out.
We know the steering geometry is going to change, and we know why it's going to change, but we don't know what the end result is going to be.
I really wanted to know.
Right from the first moment I saw the new tires on my FZR400 wheels.




On a stock RZ, the front tire is 0.5" smaller than the rear tire.
On the FZR wheels, the front tire is 3' smaller than the rear.
The front end is going to drop, the rear end is going to go up.
That will steepen the rake & decrease trail.

More internet searches.
Found sites that said smaller front wheels, also decrease trail.
Found sites that said 3" was about the minimum amount of trail that you want to have.
Saw video's of guys doing stoppies, when all of a sudden, the front wheel starts violently shaking,
which results in huge tank-slapper, and ends with him going over the bars and crashing.
Caused by not having enough trail.

Well, I don't bounce well, and I heal slow, so before I took my chassis experiment out on the pavement,
I wanted to see if I could calculate my new rake & trail, and convince myself that it was going to be safe to ride.


I started by using the following Rake & Trail calculator.
http://www.rbracing-rsr.com/advchoppercalc.html" onclick="window.open(this.href);return false;

Before I go further, here's some Important Info for you to consider as you read on:
1 - The above link gives definitions for rake, trail, offset, etc., so I won't repeat them here.
2 - The rake & trail calculator needed the dimensions in inches, so we're all working in inches today.
3 - My measurements were made in mm and converted.
4 - I measured using a metric tape measure, so the dimensions may not be 100% accurate
5 - The rake and trail calculator was designed to calculate what happens when you change fork length.
It was not designed to calculate for changes in wheel diameter, or fork offset.
I made some assumptions on how the calculator worked, and manipulated it to provide the info I wanted.

On a more positive note, I have spent a lot of hours using the calculator, learning how it works.
And even more time time trying to understand how a bike responds when offset, wheels, and fork length are changed.
Does it tilt, or move up, or rotate?
If nothing else, it was a fun exercise.
- It would be great if someone could evaluate what I've done, and confirm that it's valid.
- Or point out my mistakes, and provide a correct procedure.


To help explain the procedure, I have included a lot of numbers, calculations, and math equations.
Unfortunately, it makes for pretty boring reading.
But, if you're up for it, check out my next post. It makes this one look short.

[Off Road]

Calculating Rake and Trail for an RZ350 with FZR400 Forks, Trees and Wheels.
using the RB Racing Rake and Trail calculator.

http://www.rbracing-rsr.com/advchoppercalc.html" onclick="window.open(this.href);return false;

I started out by putting stock RZ info into the calculator:

Rake, 26*, from the manual
Wheelbase, 54.5", from the manual
Offset, 1.94', measured
Fork Length, 28.25", measured
Front Tire, 24.37" OD, from Bridgestone site, BT45 info
Rear Tire, 24.92" OD, from Bridgestone site, BT45 info

With this info, the calculator gave me a trail measurement of 3.78", (the same trail value as in the Manual)
It also gave me 2 more dimensions.
The distance between the rear axle and the top of the steering head (47.25")
The distance from the ground to the top of the steering head (36.73')

Both of these dimensions were useful in manipulating the program.
For me it helped to think of the bike as a large triangle, with the 3 corners being the rear axle, the front axle, and the top of the steering head.


1 - CHANGING FORK OFFSET
Imagine the stock RZ with both wheels on the ground, firmly fixed in space so that the frame can't move.
Remove the forks and front wheel.
Then remove the RZ tree (1.94" offset), and replace it with the FZR tree (1.54" offset).

When the forks are put back on, they are closer to the steering head.
This reduces the wheelbase, and lowers the center-line of the front axle.
In order for the front wheel to slide back on, the forks need to be raised in the tree.

Experimenting with the Rake & Trail calculator, I found that there is only 1 combination of Fork Length & Wheel-base,
that retains the original RZ Height at the steering head, and the original RZ length from rear axle to the top of the steering head.
Basically, it's a trial & error approach, until you hit the correct combination of the two.

When I put in my new values for fork length (28.06"), and wheel-base (54.06"), the steering head hasn't moved, so the rake hasn't changed.

Changing the offset to 1.54", in the calculator
- increased the trail to 4.23"
- shortened the wheelbase to 54.06".
- rake stays at 26*



2 - CHANGING FORK LENGTH
At this point, I can use the Rake & Trail calculator as it was intended.
I can install the longer FZR front forks (28.8").
The calculator tells me that
- rake is increased to 26.7*,
- trail is increased to 4.4",
- wheelbase has increased to 54.39
- the height at the top of the steering neck has increased to 37.22".

Enter the New Wheelbase (54.39"), New Fork length (28.8"), and New Rake (26.7*),
into the calculator Before Changing Front Wheel Diameter.



3 - CHANGING FRONT WHEEL DIAMETER
Again, imagine my bike, firmly held in place with the tires just touching the floor.
I remove the RZ front wheel (24.37" Dia.), and install the FRZ wheel (23.31" Dia.)
The front tire is now 1.06" smaller.
In order to maintain the same rake, when the bike is dropped back on the ground,
the rear tire also needs to have 1.06" smaller diameter, and is now 23.86".
At this time both tires are 0.53" off the ground.
If I drop the bike straight down, until the wheels touch the ground, the rake, and the wheelbase, remain unchanged.

Changing the front tire size to 23.32", in the calculator,
- decreased the height at the top of the steering neck, by 0.53".
- Decreased the Trail to 4.13"



4 - CHANGING REAR TIRE DIAMETER
When the front tire diameter was decreased by 1.06", the rear tire diameter needed to decrease, by the same amount, to 23.86", in order to keep the rake constant.
The new FZR rear tire diameter is 26.22", which is 2.36" larger, than the "calculated" rear tire diameter.
This will raise the rear axle by 1.18" (half the diameter).

Again, think of the bike as a triangle, rear axle, front axle, and top of the steering head.
Assume that the front wheel is fixed, so it can't move.
When I raise the rear axle by 1.18", and the whole bike will rotate around the front axle, and tilt forward.
The distance between the front and rear axles does not change, but the rake will get steeper, and the wheelbase will get shorter.

Time to review some high school math, (with a drawing that looks like it was done by a 5 year old).

Imagine a Right angle Triangle.
The Hypotenuse is the distance between the front axle and the rear axle (original wheelbase).
The Opposite side is the Axle height increase due to the larger rear wheel.
The Adjacent side is the new wheelbase.




From this information, I can calculate the angle that the bike tilted, and the new wheelbase.

The Sine of angle (x), = the Opposite side / Hypotenuse, = Height increase / Original wheelbase.
Sine (x) = 1.18" / 54.39" = 0,021695165
Angle (x) = 1.24*

Tilting the bike 1.24*, steepens the rake from 26.7*, to 25.56*.

Now that the angle is known, the new wheelbase can be determined.
The Tangent of angle (x), is the Opposite side / Adjacent side = Height increase / New wheelbase.
Tan 1.24* = 1.18" / New wheelbase.

Therefore:
New wheelbase = 1.18" / Tan 1.24* = 1.18" / 0.02170029 = 54.37".

The last step was to put the new rake (25.56*) & wheelbase (54.37") into the calculator, to find the new trail value.

The final Rake Value is 25.56*
And the final Trail value is 3.87"



5 - CHANGING FORK LENGTH AGAIN
When I had the final rake, fork length, and wheelbase, in the calculator,
It was simple to change fork length, (raised or lowered in the trees), and get new rake & trail numbers.



Now for the funny part, I made an error, and all my calculations are Wrong.
I forgot to account for swing-arm angle, and made the calculations, "assuming" the angle was "stock".

I'm using an FZR400 swing-arm with FZ600 dogbones.
My swing-arm angle is flatter, which will lower the rear ride height, which will increase rake, and increase trail.
So all of my rake values are too small (off by about 0.5*)
And all my trail values are too short (off by around 0.02")
Some day when I'm bored, I'll re-calculate, and post the new info.

Here are some charts I made, showing my results, at this time.

The first chart shows the changes to rake and trail, as each new item was installed.
The second chart shows what happens to rake, trail, and wheelbase, when I shorten the forks, (raise the tubes up above the top tree).
If I raised the forks 1.5", I could get real close to the FZR400 geometry of 24* rake, and 3.5" trail
However, I'd end up losing 3/4" ground clearance.




So, what's it all mean in the real world, where the rubber meets the road. Damned if I know.
I was just playing with numbers, trying to convince myself that my bike was safe to ride.

According to my numbers, the modified bike has more rake, and more trail, than both the RZ and the FZR400.
The extra rake and trail would suggest that the bike should have good high speed stability.
And it does.
Is the stability, due to the rake and trail, or is it due to the awesome radial tires that I installed.
I don't know.

And even if I got real lucky, and my calculations are correct. Now what?
Well, nothing really.
You still need to ride the bike and test it.
Then make adjustments to the preload, and sag, and ride height, and fork length, and shock, and fork oil, and tire pressure, and...
Then take it for another ride, and test it again.
Increase front sag, or raise the rear ride height, or shorten the forks, to reduce rake & trail, and make it turn quicker.
Or just the opposite, to give it more high speed stability. Or some combination both.
All the exact same things you'd do, with or without Rake & Trail numbers.


And that's how I spent my summer vacation.
I tried to write the procedure so that if someone was interested, they could use it to evaluate the rake & trail, for their modifications.
Not really sure if I succeeded.
Anyway, it's up to you to decide if it's useful information, or just the results of another idiot with a keyboard.

FWIW, I also evaluated the rake & trail, when installing R1 forks and wheels.
However, I made an incorrect assumption, which led to some math errors.
I'll post the R1 info after I fix my mistakes.

(I have edited this post to correct mistakes in the chart.)

[L.B]

My head hurts....
Steve that is really good info once understood, thank you for taking the time out & sharing with all of us.
Now, back to re-reading it all again.

[silverstrom]

Added those posts to Tech Tips too. If you add more to the topic, maybe you can update the info there also. Excellent work my friend.

[JonW]

So question is, who will work out the RGV forks/swinger on the YPVS then?

[Off Road]

If I make 1 more math calculation, I'm afraid that my head will explode.
Before that happens, I thought I'd make a short post.
I thought wrong, so smoke em if ya got em.

... I have spent a lot of hours using the calculator, learning how it works.

Bwahaha.

I spent more than, a lot of hours, just learning the terms involved.
On the net searching for info, found the RB Racing Rake & Trail calculator.
It was designed to calculate a new Rake & Trail, when you put longer forks on your Harley.
It took a few minutes to figure out what the program was doing, and how to use it to compensate for offset changes.

Changes to fork length were simple, that's what the calculator was designed for.
Changes to the front wheel diameter seemed to be pretty straight forward.
Smaller front wheel, lowers the bike, and decreases trail.

Changes to the rear wheel diameter, made no difference in the calculator.
The rear wheel could be 1' in diameter or 6', the dimensions didn't change.
So, what the hell is the calculator doing with the rear wheel info?

Oh well, move on, calculate the new rake & trail, using my old math lessons.
The rear tire is going to get bigger, which will raise the rear axle, and tilt the bike.
But, bigger than what?

Assume that the rear tire is the same diameter as the new front tire, and calculate from there.
Wrong, but didn't know it at the time.
Assume that the rear tire is the same diameter as the OEM rear tire, and calculate from there.
Wrong, but didn't know it at the time.

Eventually, I realized that on a RZ, with 26* rake, the rear tire diameter is 0.55" bigger, than the front.
In order to keep the same rake, when the front tire is made x" smaller, the rear tire also needs to be x" smaller.
Then the rear tire diameter increase, is based on this as a starting point.

While I was working on this, I assumed that the rear wheel goes straight up. Wheelbase doesn't change.
Wrong, again.
Sometime during the exercise, I realized that as the rear tire increased, the bike rotated around the front axle, and the wheelbase got shorter.
This discovery was followed by a whole new set of calculations.

(It all seems so obvious now, but at the time, my assumptions seemed like good ideas.)

By this time, I've spent a few minutes on the rake & trail calculator.
I've got it to calculate for changes in offset, fork length, and front wheel diameter.
And I'm pretty happy with the results that I got from my calculations, for rear tire diameter.

However, there is still 1 tiny inconsistency.
When I enter my final rake and wheelbase numbers, into the calculator,
the calculator comes up with a new, (longer), dimension, between the rear axle, and the top of the steering head.
Why? And, What the hell is the calculator doing with the rear wheel diameter info?

Hmmm? Well, it is what it is. Accept it and move on.
Summer arrived, and I rode the bike for 4 moths.




A couple of months ago, I was checking out RZResurections project (R1 forks, trees, front wheel, longer swing-arm, huge 180 rear tire),
and got to wondering about how this was going to change his steering geometry.
Sent him a PM and asked for his swing-arm length, fork length, and fork offset.
He replied with swing-arm is 21.25", forks are 28.35", and offset is 1.0".

1.0", WTF. That's half as much offset as the RZ. That can't be right. Get a new ruler, buy some glasses, just say no.
So, I reply with, "Hi Jeff. Would you kindly remeasure & confirm the offset".
He confirms 1", and send some links with R1 specs.

I got sidetracked for a while, evaluating R1 steering geometry, to see how Yamaha compensated for the small offset.
Eventually found some time to look at the R1 information from Jeff.

Fired up the rake and trail calculator, and started the process by trying to recreate my FZR calculations.
Spent the next 2 nights trying to remember what I did last time, and more importantly, why I did it.
Eventually, I was able to recreate my results from the FZR400 parts swap.

Then, I entered all the R1 info into the calculator, made up a chart, and sent the results to Jeff.
There was a discrepancy between my calculated wheelbase, and his measured wheelbase.
It's possible that this is because I didn't account for a change in swing-arm angle.
I told Jeff I'd look into it some more and let him know.

This started a search of the net, looking for info on swing-arm angle.
Got sidetracked again for a few days, reading info on how it affects ride height , rake, wheelbase, anti-squat.
Last week, I started this thread to post up the links with some interesting info, before I lost them.


I still didn't have a good way of accounting for swing-arm angle, but I wanted to redo the calculations, for the R1 swap, anyway.
When I originally did the calculations, I changed swing-arm length first, and then made the R1 front end changes.
Don't know why I did it that way, and it's probably not the most useful info,
This time, I will calculate for changes to the front end first.
After that, I can look at how swing-arm length, and angle, affect the setup.


So, I figure I've got my shit together, and I have a new plan, so I fired up the rake & trail calculator.
Spent the first night trying to remember what I did last time, and more importantly, why I did it.
Eventually, I was able to recreate my results from the FZR400 parts swap.

Then got sidetracked again.
Started looking at what happened to the steering geometry on my bike, when I shortened the FZR400 forks.
Well, that took 5 minutes, or a couple of nights, I don't recall.
Time flies when you inputting numbers in a calculator, and observing what changes happen.
So, while everything was still fresh in my mind, I decided to make a post on the procedure I used.

But, now I was ready to start looking at the R1 swap. So, I fired up the rake & trail calculator again.
Entered all the RZ and FZR info (rake, wheelbase, offset, fork length, tire size), from memory, without looking it up.
Went through the procedure for the FZR setup, and it all went smooth.

However, I noticed that in my above chart, I made a typo. Damn.
When the forks,trees, and front wheel were changed, the distance from rear axle to top of the neck, stays at 47.25", I accidentally typed in 47.26".
I'll correct it.



The only time the axle to neck length changed in the calculator, was when I made manual changes to rake, due to larger rear tire.
Well, why does the calculator change that dimension? And what the hell is it doing with the rear tire size?
This sidetracked me for a long 5 minutes.

As far as I can tell, the rake and trail calculator treats the rear axle as a fixed point,
In the calculator, when you enter in a wheelbase, it positions the front axle relative to the rear, as far as distance goes, but does not consider any height difference between the 2 axles.
If the calculator does assume a height difference, it is fixed, and is not changed by changing rear tire diameter

When you enter the rake, fork length, and front tire diameter, it fixes the top of the steering head, relative to the front wheel,
and then calculates the distance between the steering head and rear wheel.

So, there's no way that the calculator will let me raise the rear end, and steepen the rake by 1.24*
But it will allow me to shorten the forks, and steepen the rake by 1.24*
I'm excited, so I start reducing fork length until I get a rake of 25.56*.

Right about this time, the light came on, and I realized the 26.7* - 1.24* is NOT 25.56*.
It's 25.46*.
Son of a bitch. A subtraction error. The final calculations are wrong, the charts are wrong, it needs to be redone.
I turned off the computer and went home.



Today, I fired up the calculator, and input the FZR400 trees, forks, and front wheel info.
Then, instead of increasing the rear tire to get 25.46* rake, I shortened the forks to get 25.46* rake.
From this starting point, I shortened the forks another 1/2", then 1",
and compared it to what happened with the large rear wheel and shorter forks.

In both cases, shortening the forks by 1/2"
Steepened rake by 0.48*
Decreased trail by 0.11"
Decreased wheelbase by 0.21"
Decreased neck height by 0.34"
Shortening the forks by 1", doubled all of the above numbers.

I started the day thinking I might be able to figure out what the calculator was doing with the rear wheel position.
Ended up confirming that changing the fork length will affect rake, trail, wheelbase, and steering stem height.
And the amount of change, is constant, and is not influenced by the bikes original wheelbase, and steering stem height.

And that brings us up to the here and now.
The next thing to do is enter the R1 info into the calculator, and tabulate the results.
Should only take about 5 minutes. Really.

After that's done, I'll try and answer the 3 questions that most of you probably have.
1- Why the hell would I do this?
2- Was it useful?
3- Did I learn anything?

[RZResurection]

My god Steve, amazing work. I've been looking over the posts and see if there was anything intelligent that I could add. Sometimes I have to scrape hard to find anything intelligent. ;)

My biggest thing that I can make note (which you've touched on) - be careful of over simplifying the relationships of the working suspension system. Sometimes it's easy (and needed) to simplify some things or make some assumptions, just to make the overall calcs easier. Problem is that some assumptions are wrong and do affect the overall outcome.

My advice would be to get some (not always easy to get - at least accurately) empirical measurements, just to verify rake/trail measurements. I always say the empirical measurements are to give it the 'sniff test'. Just to make sure everything smells ok.

It funny, when we were communicating about some of my measurements/data, I could tell that you were tying nicely to tell me that you thought I was on crack WRT the offset measurements. I guess it helps when I actually know the proper year of my R1 forks. Lol. I've got so many different parts on my bike. I just started to write them all down the other day. That all goes back to that 'intelligent' thing.

After the exercise(s), I would imagine that you could answer question 3 quite easily. The cool part is that you included me and you shared with the forum.

Again, excellent work Steve.

[Off Road]

My advice would be to get some (not always easy to get - at least accurately) empirical measurements, just to verify rake/trail measurements.

I've been thinking of getting a digital angle finder, so I could measure the rake and also the swing-arm angle.
But that's as far as it got. I haven't put any effort into actually buying one.

My biggest thing that I can make note (which you've touched on) - be careful of over simplifying the relationships of the working suspension system.

It's probably the most complicated thing I ever looked into.
Earlier this year, a friend gave me a copy of Tony Foale's book called, "Motorcycle Handling and Chassis Design, the art and the science".
Basically, it's 498 pages of complex math.
I was lost and confused before I finished reading the introduction.

I'm trying to look at the bike as a simple 2 dimensional object, being tilted, or rotated in space.
And at the moment, I'm confused about how to account for the swing-arm angle, and more importantly, the new pivot point, that gets added to the equation.
In his book he talks about all this, and then considers all the forces involved, when the bike's leaned over and braking for the corner,
and a second later when all the forces change, while the bike's still leaned over, but accelerating hard out of the corner.

I think Tony starts calculating these forces on like page 10, and after that, the book starts to get complicated.
I really should have paid more attention in school.

[Off Road]

The next thing to do is enter the R1 info into the calculator, and tabulate the results.
Should only take about 5 minutes. Really.

Bwahaha.


Fired up the rake & trail calculator last night, and started putting in the R1 information.
Put in the R1 offset, and calculated new fork length, trail and wheelbase.
Put in the R1 forks, calculated new rake, trail and wheelbase.
Put on the R1 front wheel, calculated new trail.
Went really smooth, and probably only took me 15 or 20 minutes.


Now that I had the steering geometry for the Complete R1 front end swap,
I calculated new rake, trail, and wheelbase values, when the forks were shortened by 1/2" or 1".
Just in case someone was interested.

Next on the list was accounting for the steeper rake, due to installing a larger rear time.
Simple, I've already done the math.
Sine of the angle = Axle height gain / wheelbase.

Ummm? Which wheelbase? I just calculated 3 of them.
As the forks got shorter, the wheelbase got shorter.
And a shorter wheelbase, results in a bigger angle.

If you put a large rear tire on a long wheelbase bike, it will steepen the rake by x*
If you put a large rear tire on a short wheelbase bike, it will steepen the rake by x* + y*.
Well, that can't be right. How can 1 tire steepen the rake, by 2 different amounts?

Spent a few minutes looking into this simple little puzzle. Still working on it.
The rake angle is not a linear system, and the math is still a mystery, but I can see it in my mind.
If you put a really big rear tire, on a short bike, it will make a bigger change in rake angle,
than if you put the same tire on a really long bike.

Back to my original question, "Which wheelbase should I use?"
Seemed like a good time to do some math.

The longest wheelbase was with the forks in the oem position.
The shortest wheelbase was with the forks shortened by 1".
A 1" change in fork length, resulted in a 0.44" change in wheelbase.
Installing the large rear tire on the Short wheelbase bike, steepens the steering angle by 0.463*
Installing the large rear tire on the Long wheelbase bike, steepens the steering angle by 0.459*

So, a 0.44" change in wheelbase, results in a 0.004* change in angle. Let's call that insignificant.
And since the rake & trail calculator on uses 2 decimal places, I'll be entering 0.46* anyway.

When I did the FZR400 angle calculations, I used the long wheelbase, (when the forks were in the oem position).
I'll do the same when calculating the R1 geometry, which is the next item on my To-Do list.


Oh. And while I was playing with these numbers, I discovered that I wrote down the wrong wheelbase number, on my sketch.
Note to self: Double check your work, before posting it on a worldwide forum, where it will be viewed for all eternity. Dumb ass.

[aaronmvrider]

wow... lay off the acid .... good times.... wish i was with you when you decided to delve deep into the unknown