Swapping Forks, wheels, and swing-arm.

[Off Road]

Hey Aaron. Nobody likes a quitter.
When I'm alone, in the dark, imagining shorter forks, or larger tire, it easy to lose track of how the bike is reacting.
Is it rotating around the front axle or the rear?
And then you start questioning how you arrived at a certain calculation.
And then it all falls apart, and I have to start over again.

For me, it's best if I keep a clear head.
But some days, I do miss the 70's.


Sat down the other day, to do the R1 calculations, but first, I wanted to look up some "really important" info from Tony Foale's book.
Started scrolling through the pages, stopped and read a few things that caught my eye.
Scrolled some more, saw a heading marked Trail. Stopped to read again.
Did you know the are 2 types of trail?
Ground trail,(measured along the ground),
and real trail, ( a line from from the contact patch of the front tire, that intersects the steering axis line, at a right angle)

Did you know that Trail decreases when the front wheel is turned?
Did you know that the rate of trail decrease, and amount of decrease, depends on the rake angle?
(FYI. Larger rake gives a larger rate of decrease, and larger amount.)

Interesting, I didn't know that. Started scrolling through the pages, resuming my search for.............Oh crap.
Forgot what it was. Still don't remember.

Moved on to the R1 calculations.
Calculated for offset, fork length, and front wheel diameter.
Calculated for Jeff's larger rear tire.
Put all the numbers into a chart.

Hmmmm? Jeff is using a ridiculously small 180/70- 17 rear tire.
So, I recalculated the changes to rake & wheelbase, using a larger rear tire diameter.

The 'height increase', due to the new wheel, went from .43", to .63". Roughly a change of 50%.
The 'steering angle change', went from .46* to .67*. Again, it's close to a 50% change.
The wheelbase got shorter by .002".
That just doesn't sound right. I suspect a math error, but can't find it.
Oh well. Guess it's time to get a pen, and a fresh sheet of paper, and get this R1 info wrapped up.

But, what can you really expect, when a guy who graduated 38th, out of a class of 39,
tells you that he going to do his best.

[Off Road]

The 'height increase', due to the new wheel, went from .43", to .63".
The 'steering angle change', went from .46* to .67*.
The wheelbase got shorter by .002".
That just doesn't sound right. I suspect a math error, but can't find it.

Redid the math, calculating wheelbase & rake angle, down to 7 and 8 decimal places.
When the rear axle height is increased by .43", the wheelbase will get .002" shorter.
And if you raise the rear axle another .20", the wheelbase will get another .002" shorter.

OK. That make sense. BUT.....

The rear axle height did not increase from .43" to .63". The rear tire diameter increased by that amount.
The axle height increase was half of that.
Which means my last set of calculations are all wrong.

Somebody better cut down another tree, cause I'm going to need more paper.

[Off Road]

Calculating Rake and Trail for an RZ350 with 2002/03 R1 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;

The rake and trail calculator was not designed to compensate for changes to offset, or wheel diameter,
and it required some manipulation, to have it account for these changes.
In this write-up, I will try to explain the reasons for the assumptions that I made,
and I've also included all my measurements and calculations.

It make for pretty boring reading, so feel free to skip down to the charts, where I list the final results.


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 'Really Important' 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.



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 R1 tree (1" offset)
Re-install the forks, (so that they are sticking way up above the top tree),
put on the front wheel, and then lower the forks until the wheel touches the ground,
The rear wheel, frame, and top of the steering head, have not moved,
so there is only 1 fork length possible, and this is going to result in 1 wheelbase.

Experimenting with the Rake & Trail calculator, I found that, yes, there is only 1 combination of Fork Length & Wheel-base,
that retains the "original RZ Height at the steering head"(36.73"),
and the "original RZ length from rear axle to the top of the steering head"(47.25").
Basically, it's a trial & error approach, until you hit the correct combination.

Changing the offset to 0.09843" (25mm), in the calculator, gave the following results:
- Rake stayed at 26*
- Fork Length decreased to 27.79"
- Wheelbase decreased to 53.44"
- Trail increased to 4.85"



CHANGING FORK LENGTH

At this point, I can use the Rake & Trail calculator as it was intended.
I can install the longer R1 front forks (28.35").

Changing the fork length to 28.35", in the calculator, gave the following results:
- Rake Increased to 26.54*
- Wheelbase increased to 53.69"
- Trail increased to 4.98"
- Height of the steering neck increased to 37.11".

Before calculating changes in Front Wheel Diameter, Enter the New Wheelbase (53.69") and New Rake (26.54*), in the calculator,


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 R1 wheel (23.74" Dia.)
The front tire is now 0.63" smaller.
In order to maintain the same rake, when the bike is dropped back on the ground, the rear tire also needs to have a 0.63" smaller diameter,

Changing the front wheel diameter to 23.74", in the calculator, gave the following results:
- Rake stayed at 26.54*
- Wheelbase stayed at 53.69"
- Trail decreased to 4.83"
- Rear tire decreased to 24.29"
- Height of the steering neck decreased to 36.79".



CHANGING REAR TIRE DIAMETER

When the front tire diameter was decreased by 0.63", the rear tire diameter needed to decrease, by the same amount, to 24.29", in order to keep the rake constant.
The new 180/55-17 rear tire diameter is 24.72", which is 0.43" larger, than the "calculated" rear tire diameter.
This will raise the rear axle by 0.215" (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 .215", 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.

Using a Right angle Triangle.
The Hypotenuse is the distance between the front axle and the rear axle (original wheelbase).
The Opposite side is the 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) = 0.215" / 53.69" = 0,00400447
Angle (x) = 0.23*

Tilting the bike .23*, steepens the rake from 26.54*, to 26.31*.

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 0.23* = 0.215" / New wheelbase.
Therefore:
New wheelbase = 0.215" / Tan 0.23* = 0.215" / 0.0040045 = 53.6896".

Changing the Rear wheel diameter to 24.72", in the calculator, gave the following results:
- Rake decreased to 26.31*
- Wheelbase basically stayed at 53.69"
- Trail decreased to 4.77"
- Height of the steering neck increased to 36.85".



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.



Here are the charts, showing results using 2002-03 R1 trees.

The first chart shows the changes to rake and trail, as each new item was installed.
Please note that I made 2 calculations for changes to rear wheel size.
One for the 180/70-17 rear tire, and another for the larger, oem, RZ rear tire

The second chart shows what happens to rake, trail, and wheelbase, when I shorten the forks, (raise the tubes up above the top tree).
In this chart, I started the calculations, assuming that the oem, RZ rear tire was being used.





On the 2002/03 R1 trees, the fork offset is 25mm (1")
On the 1998-01 R1 tree, the fork offset is 35mm (1.38")
More offset will result in less trail.

I went through the procedure again, starting with the new offset of 1.38".
Calculated the new fork length, and wheelbase, needed to keep the rake at 26*
Then lengthened the forks. Which Increased rake, trail. wheelbase.
Then a smaller front tire diameter. Which Decreased the trail
Then the larger rear tire diameter, Which Decreased trail and steepened rake.


Here are the charts, showing results using 1998-01 R1 trees.
In both of these charts I assumed that the oem RZ rear tire diameter was being used.

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).







From these calculations, it seems that installing either R1 front end,
results in very minor changes to the rake angle, and the height of the steering head.
The R1 front end swap does make some bigger changes to the Trail and Wheelbase.

Installing the 98-01 R1 front end, on an RZ, Increases the Trail by 0.5", and shortens the wheelbase by 0.5".

Installing the 02-03 R1 front end, on an RZ, Increases the Trail by 1.0", and shortens the wheelbase by 0.8".

On the R1,Yamaha uses a steeper (24*) rake, to make up for the small offset, and still maintain a fairly small amount of trail.
Using the small offset trees on the RZ, with it's 26* neck rake, results in larger trail values.

Here's some of my thoughts on different ways to steepen the rake and decrease the trail.

- Shorten the forks, But this will lower the bike, and further decrease the already short wheel base.
- Longer shock or shorter dogbones, will raise the rear ride height, steepen rake, decrease trail.
Again, this will shorten the wheelbase by some amount.
- Longer swing-arm will also raise the ride height and change the rake and trail.
- Larger rear tire, will raise the rear end and steepen the rake.
- A Smaller front tire. (and FWIW, I like this idea best)
It will drop the front of the bike down, which will steepen rake and decrease trail.
The smaller diameter front tire will cause a further decrease in trail.
The smaller tire will also provide quicker steering.

My final thoughts.
I make no claim that any of the above information is correct.
It's up to you to evaluate it, and see if it's valid, and/or useful.
Oh, and the 2002/03 R1 trees, wouldn't be my first choice.

[Off Road]

I did up a couple of quick charts to summarize my results.


The first chart compares Rake, Trail, and Wheelbase when,
FZR400, 98-01R1, and 02-03 R1 forks, trees, and wheels, are installed on an RZ350.

The second chart shows what happens to Rake, Trail, and Wheelbase when, the forks are shortened by 1".






In all 3 cases, when the forks are shortened by 1":
- Rake gets steeper by 1*
- Trail decreases by 0.2"
- Wheelbase decreases by 0.4"

I calculated the changes to Rake, Trail and Wheelbase when the Rear ride height is Increased by 1".

In all 3 cases, when the rear ride height is increased by 1":
- Rake gets steeper by 1*
- Trail decreases by 0.2"
- Wheelbase decreases by 0.02"


Please Note: When raising the rear ride height, I assumed a 2" larger rear wheel, which would raise the rear axle by 1".
This change has a very minor affect on the wheelbase.
In most cases, the rear ride height increase ,will be done by lengthening the shock, or shortening the dog-bones.
Both of these methods will steepen the swing-arm angle, and have a larger affect on the wheelbase.
But, I haven't figured out how to calculate that yet.

So, from what I can determine, the Rake and Trail will be changed by the same amount,
if you shorten the forks by 1", OR raise the rear ride height by by 1".

[Off Road]

1- Why the hell would I do this?
2- Was it useful?
3- Did I learn anything?

Sat down to answer those questions.
Started thinking about my bike.

The bike has excellent high speed stability, which probably takes away from it's cornering ability.
It would be nice to try a steeper rake and less trail

I have a shallow swing-arm angle, due to the FZR swing-arm.
The best option would be to shorten the dog-bones, to increase the swing-arm angle, and raise the rear ride height.
Hard to do because the FZR arm has a huge cross-brace that would hit the shock.

Option B. Steepen the rake by shortening the forks, by maybe 10 mm (raise the forks up 10 mm above the tree).

Ummm. Wait a minute.

As the bike sits right now, when I get on it, the front forks sag 20 mm,.........and the forks get shorter.

Duh. If I want my forks 10 mm shorter, when I'm riding the bike, then I could just increase the sag, from 20 mm to 30 mm.


So, maybe I did answer questions 2 and 3.
I'm learning new shit ever day, and it appears that some of it might come in handy.

Here's a interesting 5 minute read on fork length, sag, pre-load.

http://www.gtamotorcycle.com/vbforum/sh ... ng-Preload" onclick="window.open(this.href);return false;

[silverstrom]

Interesting where he says "Adding preload does not compress the spring, it just extends the fork. This is always true, except if the fork is topped out." That would be a good point to question everything else he has to say.

[Off Road]

Interesting where he says "Adding preload does not compress the spring, it just extends the fork. This is always true, except if the fork is topped out." That would be a good point to question everything else he has to say.

I don't know John. It kinda makes sense to me.

Let's say the rider sag on the bike is 2". If you add 1" of preload, it will reduce sag to 1".
Effectively, the forks are now 1" longer.
The front springs still have the same load on them, so their length won't change.
The additional fork length is due to the added preload spacer.

If you continue to crank in preload until the forks are topped out, and your left with zero rider sag.
In this case if you add another 1" of preload, the springs will have to compress, because the forks can't physically get any longer.


Or.....I could be wrong.

Here's a interesting 5 minute read on fork length, sag, pre-load.
http://www.gtamotorcycle.com/vbforum/sh ... ng-Preload" onclick="window.open(this.href);return false;

Oops. Sorry about that. I disn't actually read the whole thread, only the first couple of posts. My bad.
But, today it's cold outside, so I sat down and read all 3 pages.

Lots of information/opinions in that thread.
Some of it may be valid, some of the posts might just be internet stories.
Here's some of the comments, that I found most interesting.



- "Adjusting ride height and adjusting preload are two separate things!
And it's true that nobody directly adjusts "preload" with a target value for the "preload". One adjusts the "preload" in order to get the "sag" correct. Then one adjusts ride height front and rear to get the geometry of the bike correct so that it steers properly and has proper grip coming out of corners (mostly rear) and has sufficient cornering clearance.
Measure the sag, compare with target value, adjust preload, re-check. Ride the bike to observe steering feel to establish if ride-height changes are warranted. Then you adjust the RIDE HEIGHT - not the preload! - in order to get the steering response correct."



- "Best way I've found to describe the concept is that you want two thirds of the suspension available for braking and compression over bumps, BUT at the same time you want a third available for rebound or "getting light" over the tops of hills. So if it's set for static sag in this manner and it bottoms out under braking or on rough roads, you need stiffer springs because more "preload" will take away from that top third. And conversely, if you can't get that top third of static sag with rider without "preload" then you need softer springs."




- "One makes adjustments to spring preload to alter how much fork/shock travel is being used. The subsequent change in ride height is a byproduct of a preload change, not the objective of it. Competent suspension tuners will compensate for that change in ride height by sliding the forks up/down in the triples or by lengthening/shortening the shock."



- "The difference between the two is that by adjusting the spring preload you are changing the resting point of the suspension relative to the damper travel, but it's easier than changing linkages. So you will have more bump travel /less rebound travel or vice-versa, along with the ride height change."




Hmmm. I hadn't heard of this before.

- "Regarding the rear shock, turning the rings is analogous to turning the preload adjuster on the forks, . Of course it also increases ride height by extending the rear suspension and - here is the surprise - because of the progressive swingarm linkage on all modern bikes, will tend to make the rear suspension action *softer*. "




And this comment is probably my favourite.

- "Of course that adjustment changes other things too, so that's not a meaningful observation, as there are almost no adjustments that affect only one element of the suspension settings, in isolation of all the others."

[silverstrom]

The way I read it he is saying that cranking down the preload does not compress the spring. It certainly does on my F2 350. I have progressive springs with 1 inch spacers and EC preload adjusters. When I started cranking in preload the spring compressed. Maybe that doesn't happen with bigger fork and springs, but with spindly 35 mm forks and wispy thin springs it takes little effort to compress the spring. Anyway, in my simple mind preload means preloading the spring, and that can only happen by compressing it, otherwise it does nothing. My only argument is him telling guys that don't know any better that the preload does not compress the spring. It does. And it's an unweighted measurement, not topped out.

Seems others think this way too.
http://www.strappe.com/suspension.html" onclick="window.open(this.href);return false;

Preload: the amount the spring is compressed when it's not supporting any weight. Some preload is necessary just to keep the spring in place when the suspension is fully extended. More preload is used to increase ride height and increase the apparent spring rate.

I'm likely just looking at it the wrong way.

[RZResurection]

The way I read it he is saying that cranking down the preload does not compress the spring. It certainly does on my F2 350. I have progressive springs with 1 inch spacers and EC preload adjusters. When I started cranking in preload the spring compressed. Maybe that doesn't happen with bigger fork and springs, but with spindly 35 mm forks and wispy thin springs it takes little effort to compress the spring. Anyway, in my simple mind preload means preloading the spring, and that can only happen by compressing it, otherwise it does nothing. My only argument is him telling guys that don't know any better that the preload does not compress the spring. It does. And it's an unweighted measurement, not topped out.

Seems others think this way too.
http://www.strappe.com/suspension.html" onclick="window.open(this.href);return false;

Preload: the amount the spring is compressed when it's not supporting any weight. Some preload is necessary just to keep the spring in place when the suspension is fully extended. More preload is used to increase ride height and increase the apparent spring rate.

I'm likely just looking at it the wrong way.

I can't speak to the new style USD forks as far as spring compression operation goes. I was looking up some YouTube videos regarding seal replacement. I did notice that the videos showed the technician using a wrench to remove the top cap. When the cap was removed, the spring did not rifle out as I had expected. To me that says that it's a different set up than my RZ forks. So, maybe the quote in question could possibly be correct.

With regards to the RZ forks, if I remember, the springs have some compression on them while fully extended - which speaks to your quote above. I remember having to compress the top cap and insert the circlip to hold the cap. The spring is compressed some at full extension. Now, if you add spacers to the top of the springs, you compress the springs more.

I agree with you John. As far as I'm concerned, the word 'pre-load' clearly describes the action. In the old RZ forks, this is done thru compression of the spring.

I didn't read the article, but that's my $0.02

[RZResurection]

My final thoughts.
......Oh, and the 2002/03 R1 trees, wouldn't be my first choice.

:(

I have my handy rake and trail adjuster. Just have to install it. This will overcome the short coming of the 02-03, small offset.

Everyone should have one.

[Off Road]

I have my handy rake and trail adjuster. ....Everyone should have one.

Damn, I really wish my Ohlins came with one of them.
My only option is shorter dogbones, and that sounds expensive.

[Off Road]

Preload: the amount the spring is compressed when it's not supporting any weight. .....

More preload is used to increase ride height and increase the apparent spring rate.

Interesting where he says "Adding preload does not compress the spring, it just extends the fork. This is always true, except if the fork is topped out."

Hey John and Jeff.
I think it's just the way the statements are worded, and whether or not you think about the forks with weight on them.

When the forks are topped out, and you're trying to re-install the forks caps, you are definitely compressing the spring and applying some preload.
And it can be a real pain sometimes.

When the bikes on it's wheels with you aboard, the weight overcomes the preload, the springs compress more, and the bike sags.
At this time, if you add more preload, you are not adding any additional weight to the forks, so the springs will not compress any further.
Instead, the added preload spacer will make the forks longer, decrease the sag, and increase the ride height.

[cookie]

My only option is shorter dogbones, and that sounds expensive.

I took a couple of pairs in to my machinist and had them shortened. Didn't cost much at all IIRC, maybe $40 a set?

[RuZty]

When the bikes on it's wheels with you aboard, the weight overcomes the preload, the springs compress more, and the bike sags.
At this time, if you add more preload, you are not adding any additional weight to the forks, so the springs will not compress any further.
Instead, the added preload spacer will make the forks longer, decrease the sag, and increase the ride height.

That's the key concept right there. If you add preload it will compress the spring at full droop because the fork has no travel left. When you are on the bike it extends the fork, giving you back travel that wasn't there before you compressed the fork from fully extended by sitting on it. (ie. the compressed spring length is the same but the spacer increases its effective length, up to the limit of topping out the fork). You need to separate static compression (preload) and dynamic compression (gravity) for it to make sense.

[RZResurection]

My only option is shorter dogbones, and that sounds expensive.

I took a couple of pairs in to my machinist and had them shortened. Didn't cost much at all IIRC, maybe $40 a set?

This may not be a cheap, but rigging a couple of these will give you some adjustability.

This was the best picture I could find. You get the idea.