Frame Geometry

hear is my new ride height.29 3/8" this is 2 1/8" higher than stock.1 1/2" higher than I had it set up.
With my old ride height I had 27 degrees of rake.what is the stock rake?

If you raise the rear and drop the front the amounts you're discussing, you're going to end up with a bike that's unstable when braking hard, unstable when accelerating hard, and is probably going to feel like the front is tucking in the corners.

I'd start by making careful measurements of bike's geometry before mods. Get a $10 magnetic angle finder at the hardware store and use it to measure the stock fork rake and the stock swingarm angle. Measure from the floor to a fixed point on the front and rear of the chassis so you can see how much your changes will be altering the chassis. See where the rear axle lines up with the countershaft/swingarm pivot. Measure the fork offset (measure from the center of the steering stem to the center of the fork legs.) Measure the diameter of your stock wheels and tires.

Once you have all those numbers, a good chassis geometry tool is here:

www.rbracing-rsr.com/advchoppercalc.html

Useful for determining trail, which is a PITA to measure accurately. It also lets you punch in changes and see how, for instance, shorter forks will affect the other chassis numbers.

I'd be hesitant to end up rake of less than 24 degrees and trail of less than 90mm on a racebike. On a streetbike, I'd probably won't those numbers even a bit more conservative than that -- around 25-26 degrees rake with about 100mm of trail.

After you've measured everything stock, then start bolting up the new parts and start measuring there. My guess is you're going to want to end up with shorter shocks to keep the swingarm angle in check. The front is going to be tough -- you're not going to want to drop the front end too much or you're going to run into cornering clearance issues; it's not hard to drag hard parts on these bikes with modern tires. I haven't seen fork-tube extenders or "slugs" used for decades, and personally I wouldn't use anything like that myself -- pretty much defeats the purpose of going to a stouter front end. But assuming you could find a pair to fit the 45mm tubes of the ZXR front end, you'd then have to fabricate a new rod for the rebound damping adjusters.

Jeff (AHRMA 120) and I have run into the same issue with the front ends on our AHRMA Vintage Superbikes. We think we have a solution, which basically entials grafting parts from two different sets of forks together, but it won't be easy, and probably won't be cheap.

Good luck, and let us know how your project progesses.

750turbo.com/suspension.htm
this should clear up some misconceptions, and start some arguments too!

Post edited by: Fossil, at: 2006/12/27 15:56

Post edited by: Fossil, at: 2006/12/27 16:05

Fos . Thanks ,Its all as clear as mud now.LOL.:side:

If I move the lower mount back 4" this will get me back to stock hight and put my wheel axle 1 5/8" bellow the drive to swingarm axle line.(what do you call this line?) How does this shock angle look(27 degrees)?Is there a min. max.?

Post edited by: mark1122, at: 2006/12/27 20:10

mark1122 wrote:

Fossil that red KZ looks sweet but there is obviously a clearance issue there.That was a nice project post you put together.Thanks.

I saw this yesterday and it took a bit to sink-in....
that link was not my project, that's some of the info I'm using for my own build.

Fossil wrote:

750turbo.com/suspension.htm
this should clear up some misconceptions, and start some arguments too!

Post edited by: Fossil, at: 2006/12/27 15:56<br><br>Post edited by: Fossil, at: 2006/12/27 16:05

Wow, there's some, uh, "interesting" theories contained in that missive. Curious as to what the writer's qualifications are, and why many of his tenants run counter to those of 99% of the suspension professionals.

Keep in mind free advice (mine or anyone else's) is worth exactly what you pay for it. I've formulated my opinions from racing for more than 30 years, still racing fast enough to hold an AMA Pro Expert roadracing license and qualify for AMA Formula Extreme races (at age 44). In my racing "career," I've nearly always served as my own mechanic and set-up my own bikes. I've been fortunate enough to work with and learn from some of the best suspension guys in the business, from Factory Ohlins guru Jon Cornwell to "Suspension Tuner of the Stars" Mike "Thermosman" Fitzgerald and everyone in between. Still doesn't mean I know squat, but at least you know my background.

There's no shortage of good chassis setup information available on the internet, in books, and magazines. Consider the source and look at anything written that seems outside the mainstream (such as "when you lower the front you have to *raise* the rear!) with a critical eye. If you really want to do a deep dive, Tony Foale has a great book on chassis design (expensive, but worth it), hosts seminars throughout the country, and has various software programs available on this website ( www.tonyfoale.com )

Keep in mind I look at these things from a roadracers point of view. That will align basically with a street sportrider, but could be drastically different from a dragracer or a streetracer/cruiser.

mark1122 wrote:

If I move the lower mount back 4" this will get me back to stock hight and put my wheel axle 1 5/8" bellow the drive to swingarm axle line.(what do you call this line?) How does this shock angle look(27 degrees)?Is there a min. max.?

Mark:

Ideally, the axle is in line with the swingarm pivot and the countershaft. As you can't easily alter the swingarm pivot location, your final setup may not be perfect -- as I've said before, everything in chassis design and setup is a compromise, particularly when you're working with a 30-year-old motorcycle. What you do at the rear will also be dictated by what you do at the front, and in what areas you're willing to compromise (ground clearance, rake, trail, swingarm angle, etc.) Dictating your swingarm angle by the length of the shocks you happen to have on hand really isn't the ideal situation. You'd much rather get the entire chassis set to as close to ideal as possible, then obtain the proper length shock. But we all play the cards we're dealt, and 1 5/8' is better than 4".

I wouldn't lose a lot of sleep over the shock angle. Those OEM shocks are fairly low-tech and, while the mounting angle does change the leverage ratio and hence the piston speed, the damping circuits of the shocks aren't sophsticated enough that it will make a difference, and the springs may need swapped out anyway since the shocks are set up for a different bike.

Consider the source and look at anything written that seems outside the mainstream (such as "when you lower the front you have to *raise* the rear!) with a critical eye

I have to admit that I never really understood that statement myself. To me it's the same as saying "Ok, you lowered the front now you have to lower the front some more".
I just figured I was to dumb to understand it

I've come across this info on 3 different sites re: modifying the Unitrac GPZs, I'm assuming it's valid. On the 2 sites I linked to, there are 2 different builders doing virtually the same modifications on their own bikes. "Lower the front and lower it some more" might just be the reason for raising the rear - to further reduce trail while maintaining clearance(?). I've already raised the back end of mine a couple of inches when I had the stock forks and the change was like night and day. When I install the GSXR front end and 17" wheel I guess I'll find out just how valid these recommendations are. I'm not necessarily "defending" the modifications, just offering them up for consideration.

What about this swingarm droop? As you raise the back the swingarm droop becomes grater.How much weight do we put into each peice?Gets confusing.:side:

Fossil wrote:

I've come across this info on 3 different sites re: modifying the Unitrac GPZs, I'm assuming it's valid.

Hey, three guys on the internet agree on something, it must be true! :laugh:

Seriously, do whatever works for you. If you can easily adjust the front or rear ride height of your bike, give it a whirl. There's a old saying among suspension tuners: "You don't know what "good" is until you've ridden "good." The corollary is you don't know bad until you've ridden bad. Experimentation is a good thing.

But when Mark is looking at spending hard-earned money and making difficult and expensive-to-reverse changes to his motorcycle, I don't feel bad about suggesting he look beyond what a couple of forum "experts" espouse -- particularly when at least some of it runs counter to what well-known chassis engineers and suspension tuners recommend.

As I said, there's a *lot* of good information out there on the Internet that's well-grounded in the actual physics of motorcycle dynamics. It takes more time and effort to learn about the science behind the topic, rather than simply trying to take what a couple guys thinks works "bitchin'" on their bikes and attempting to transpose that over you your own completely different motorcycle. But, in my opinion at least, the end results will be well worth the effort.