Frame Geometry

Have to agree with Hatman on this one. I've been following the thread and checked out the links. I know that the superbikes of today run a forward leaning stance from the factory i.e., the back end is raised in comparison to the front end. However, the bikes are designed from the jump with this suspension geometry in mind. Therefore, to get this type of setup to work properly, you would need to redesign the frame.

I also understand that different things work for different people and someone may have discovered something that we're all missing. My concern is that someone may end up getting hurt trying these "new" methods. The information is out there so don't be afraid to read everything you can get your hands on (and there's a ton of it).

I'm doing the same swap on my KZ100 except I used a GSXR front end. I mocked the frame and swinger up with struts to determine the proper shock height. Then I checked different manufacturers, Progressive, Work, Ohlins, to find the shocks I needed, then fabbed the upper mount. Lot's of work but worth it.

mark1122 wrote:

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:

BTW, upon re-reading this thread I realized I wasn't clear on one point -- the rear axle should be at or near the countershaft/swingarm line with the rider aboard. Since you'd normally shoot for 25mm to 30mm laden sag (sag with the rider aboard), be sure to factor that into your calculations.

this is the latest.I've droped the forks 3' to simulate new front ride hieght,and moved the rear lower shock back 3"(this is stock ride hieght at rear)I have 4 1/4 " under header. I dont see how I can get from 2 1/8" swingarm droop to 0" without loosing serious ride hieght?How did Jeff put 4" longer forks on his Z without creating serious droop?

Post edited by: mark1122, at: 2006/12/29 15:46

mark1122 wrote:

the swing arm axle should be in line with a line drew through the drive and the swing arm pivet. Mine is going to be way off(4 1/8"). My old set up was 1 11/16" below.Now what?

That is called anti-squat geometry. If all three points were in line, the back end would squat too much and the rear tire would spin at launch. You want the swingarm pivot to be above the line created by the two sprocket centers. In other words it should form a triangle when unladen. Typically, the three points should line up when the rear shock has used about 40 to 50% of it's travel.

Too much anti-squat will jack-up the back end at launch, too little will cause it to squat. If it squats too much, you'll just spin the rear tire. If it jacks up, you'll be more prone to wheelies.

The final amount you want is determined by the tire (much like everything else on the bike).

The problem is that you need to juggle all of the factors for the suspension before you build and it's easy to neglect one or another.

If you want to do it right, you want to start with this John Bradley book volume 1. This book gives good advice for starting points on spring rate, laden sag, unladen sag, anti-squat, etc. etc. When you alter one, you have re-check all of the others, but the book gives a good place to start and will tell you what factors to consider when altering things.
I followed his advice on the minor changes I did on my bike and the bike never handled better.
Volume 2 is out now also, but I haven't picked it up yet.

The book is not cheap, but will probably pay for itself, and definitely follow Hatman's advice and really consider the source of information on the net.

Also, if you don't mind spending a little money, Works will custom-make a shock for your needs based on the geometry you send them on paper. When I did that, the shocks they sent me matched what the book said I needed (I didn't mention the book when I ordered them). I was lucky enough to talk with one of their techs instead of sales staff, though.



Oh, and on the leaning shock question, theoretically, leaning it will make the rate more progressive, but will reduce the initial rate (and preload). Mounting it closer to the rear axle, you will regain what is lost by the angle, but then there may be an issue of the length of the shock being too short.

Post edited by: loudhvx, at: 2006/12/29 17:04

Yep, John Bradley and Tony Foale are probably the two "kingpins" when it comes to chassis design books. (Jeremy Burgess may know more, but he hasn't written a book yet!). I used Foale as a reference as he has a website with some good (and free) information, as well as some freeware chassis calulation programs. Neither books are exactly "easy" reads (unless you're an engineer, I guess) but both are extremely valuable, both as overall primers on chassis design and development, along with great reference books to have on the shelf.

I have Foale's book, but not Bradley's. Amazon has Bradley's book for $65, but the wait is 4-6 weeks. Foale's book is available for immediate shipment from his website.

Thanks for the input, loudhvx.

Thanks guys.That exactly what I needed to know.Now it makes sence.

Post edited by: mark1122, at: 2006/12/30 11:32

Hatman,here are some fork extentions.What do you think?
Fork extensions on eBay

Post edited by: steell, at: 2006/12/30 11:54

Max McAllister, who owns Traxxion Dynamics, certainly is well-known in the roadracing world. What he's offering is quite a bit different from a true "slug", which is an extension of the fork tube for conventional (right-side up) forks. Basically, a slug is a short piece of fork tube with male threads on one end and female on the other. The male end threads into where the cap currently is, and the cap then threads into the female end on the slug.

I haven't really seen them around since the old chopper days. It was a cheap way to get extended forks. The problem is you've now created a new weak point and a source for flex in your forks. I think there's a reason why they really haven't been around since the old chopper days decades ago.

Max's extensions are a different animal. Basically they're a new, taller fork cap that let's you slide the forks down into the top clamp up to 20mm. The connection between the new fork cap and the original fork leg is still contained within the top triple clamp, so flex/distortion won't be an issue. Also, you'll notice they're built around the rebound adjuster, so that doesn't have to be modified.

At $200 for a pair, it seems to be a pretty expensive way to gain such a small amount in fork length. Ideal for the roadrace bike for which they're designed, but not much use for your purpose.

Mark 1112,
First: your bike looks fantastic.
Second: There sure is a lot of theory discussing and conjecture on this topic and a lot of it sure sounds logical. However, I have a ZX9R inverted front end on my KZ and I run 17s on it and I installed a set of Ohlins shocks that were not made for the bike and it raised the ride height in the rear. All of this contributed to steepen the rake angle.

I'm in the middle of adding a mono-shock rear and even wider PMs and I've posted it in the projects under 'Project J' or something like that if you want to 'peep' on it (it's in progress):whistle: .

My point is this: there is no comparison between before and after. It's so much better the way it is now, and I think it's going to be better when I'm done. Admittedly, I'll be payinga LOT more attention to detail when I set-up the rear end; but, my point is this: the stock stuff is 19/18 with skinny (ie flexy) parts, and now it has sticky tires, light wheels, impressive brakes, and handles waaaaay better. It's still a flexy-flier, but I haven't addressed the frame, yet...but comparing where it came from to what it is, there is no comparison; it's that much better.

Bad Kaw wrote:

Mark 1112,
First: your bike looks fantastic.
Second: There sure is a lot of theory discussing and conjecture on this topic and a lot of it sure sounds logical. However, I have a ZX9R inverted front end on my KZ and I run 17s on it and I installed a set of Ohlins shocks that were not made for the bike and it raised the ride height in the rear. All of this contributed to steepen the rake angle.

I'm in the middle of adding a mono-shock rear and even wider PMs and I've posted it in the projects under 'Project J' or something like that if you want to 'peep' on it (it's in progress):whistle: .

My point is this: there is no comparison between before and after. It's so much better the way it is now, and I think it's going to be better when I'm done. Admittedly, I'll be payinga LOT more attention to detail when I set-up the rear end; but, my point is this: the stock stuff is 19/18 with skinny (ie flexy) parts, and now it has sticky tires, light wheels, impressive brakes, and handles waaaaay better. It's still a flexy-flier, but I haven't addressed the frame, yet...but comparing where it came from to what it is, there is no comparison; it's that much better.

I agree, 1112's bike does looks great.

Bad Kaw, how much shorter are your forks than stock, and how much did the Ohlins shocks raise the rear? Did you find you needed a steering damper? Thanks

loudhvx wrote:
If you want to do it right, you want to start with this John Bradley book volume 1.

BTW, if you're interested in Bradley's book, Michael Moore at Eurospares ( www.eurospares.com ) is the U.S. distributor. $80 plus $8.10 Priority Mail delivery, and they have them in stock. Michael also has some interesting chassis design & building info on his page, and hosts a couple of e-mail lists related to the topic.

Bradley's book is one that's been on my wish list for some time now. This discussion spurred that memory, so I ordered it Friday!

Bad Kaw wrote:

Mark 1112,
However, I have a ZX9R inverted front end on my KZ and I run 17s on it and I installed a set of Ohlins shocks that were not made for the bike and it raised the ride height in the rear. All of this contributed to steepen the rake angle.

Thanks I'll check out your new project j.
How long are your zx9 forks from axle center to top ?What is your new rake and trail?