Best Swingarm?

Shoe48 wrote:

Hey Mark ? Are you the Mark Hatten that raced Honda Singles awhile back ? Welcome aboard if you are ..

That's me. Been racing modern Ducatis for the past five or six years, but cut my roadracing teeth on a Honda Ascot.

Hatman wrote:

wireman wrote:

if you are going roadracing id reccomend going no longer than the stock arm;)

Actually, a longer arm is in the cards.

Other than the normal decrease weight/increase chassis rigidity/improve suspension/etc., my goals with this project are:

1) Increase cornering clearance by lifting the bike up (The old Muzzy-built Kawasaki Superbikes of this era were about 1.5 inches higher at the lower frame rails)

2) Quicken steering by decreasing both rake and trail.

3) Move weight bias forward

4) Decrease the wheelbase if at all possible

#1 can be accomplished a variety of ways. 17" wheels compound the issue. Jacking up the back with longer shocks is easy, but then you run into issues with an improper swingarm angle -- a swingarm with the axle below a line drawn from the countershaft through the swingarm pivot will rise and stiffen during acceleration, which causes problems (ie: loss of traction, potential highsides) at corner exits. Jacking up the rear end past this line decreases the wheelbase, but moves the weight bias even more to the rear. Increasing the front clearance can be done by decreasing the rake and/or by raising the front with longer fork.

#2 (at least the decreasing rake part) can be accomplished by dropping the front or raising the rear. However, simply dropping the front just adds to ground clearance issues, and simply raising the rear causes the issues described in the previous paragraph. The stock '82 GPz 750 has a rake of 27 degrees and a trail of 107mm. The 2007 GSXR 1000 (pretty much the open-class racebike of choice) has a rake of 23.8 degrees and a trail of 98mm. I'd be shooting for 25 degrees and about 100mm. Changing the trail can be done with different offset triple clamps or adjustable triple clamps.

#3 can be accomplished by moving the engine forward within the wheelbase. This can be done by moving the front wheel back (by decreaseing rake)or by increasing the swingarm length.

#4 can be accomplished by decreasing the front rake, shortening the swingarm (with the problems noted above), or shortening the stock frame (difficult at best)or using a custom frame.

So, my planned course of action is to decrease the front rake through either modifying the steering head angle, using custom triple clamps, or both. The resulting loss in wheelbase will then be made up with a longer swingarm. The longer swingarm will also raise the rear of the bike without moving the wheel axle beyond the countershaft/swingarm pivot line.

Those changes will accomplish goals #1-#3, but not #4. The above plan of action will result in the wheelbase staying about the same. That is a good tradeoff to accomplish three first three goals. The only way to accomplish all four would be to use a custom frame, which isn't legal in AHRMA Vintage Superbike.

I'll keep folks posted with details and pics as the project unfolds.

ok that makes sense if you are taking something out of the front id reccomend a good stabilizer with it though;)

Most race organizations require one (although AHRMA doesn't), and it's always a good idea regardless. Really though, if I can build it to the specs I outlined and keep the trail at around 100mm, it shouldn't need it. Of course, we won't know until it's on the racetrack -- theories don't win races!

Hatman wrote:

Shoe48 wrote:

Hey Mark ? Are you the Mark Hatten that raced Honda Singles awhile back ? Welcome aboard if you are ..

That's me. Been racing modern Ducatis for the past five or six years, but cut my roadracing teeth on a Honda Ascot.

I have owned my share of Honda Ascots and remember you from the Thumper Page awhile back .. I should take the time to meet you at Mid Ohio this year .. Welcome to the Board Mark ,, These guys are a lot smarter than they look ,,, :whistle: :whistle: ..

Hatman wrote:
a swingarm with the axle below a line drawn from the countershaft through the swingarm pivot will rise and stiffen during acceleration, which causes problems (ie: loss of traction, potential highsides) at corner exits. Jacking up the rear end past this line decreases the wheelbase, but moves the weight bias even more to the rear. Increasing the front clearance can be done by
Hatman.I am triing to set up my kz and need advice.Can you give me a maximum deveation for the wheel axle from this line? Like ,will I notice much diff If I am X mm. from center of this line? I will not be racing so I may have more leeway than you.It would be helpfull to have some #'s to keep me on the right path. THANKS

I don't know that there are any "magic numbers" -- like most things in chassis design and suspension setup, everything is a series of compromises.

From a physics standpoint, the chain tries to pull the wheel "forward" when accelerating. If the if the wheel axle is below the line drawn through the countershaft & swingarm pivot, the suspension compresses as the chain tries to pull the wheel forward. If it is below, it extends the suspension.

In a perfect world, the axle will not be below the line with no weight on either wheel. In the real world, it may be necessary (raising the rear to increase ground clearance, decrease the fork rake, etc.) At that point, it's a matter of choosing the lesser of evils.

In a roadrace situation, your rear suspension is always compressed when you are exiting a turn, so if the axle was slightly below the line in an unladen situation, it shouldn't be an issue. Where it could come into play is if the rear wheel breaks traction, the suspension tops out, the axle pivot moves past the centerline. Now the chain is working to stiffen the suspension, which would make saving that slide even more difficult. Not really a concern on the street, but something to keep in mind.

The other thing to keep an eye on is how much the chain is going to rub on the swingarm in the pivot area when you increase the swingarm angle.

Most chassis guys I have spoken to on the subject of swingarm angle go for 12.5°. And yes you do have to watch the chain run over top of the swingarm or it will cause to much squat under acceleration and this can make the front end push in turns
Regards Chris

Hatman wrote:

I don't know that there are any "magic numbers" -- like most things in chassis design and suspension setup, everything is a series of compromises.

From a physics standpoint, the chain tries to pull the wheel "forward" when accelerating. If the if the wheel axle is below the line drawn through the countershaft & swingarm pivot, the suspension compresses as the chain tries to pull the wheel forward. If it is below, it extends the suspension.

In a perfect world, the axle will not be below the line with no weight on either wheel. In the real world, it may be necessary (raising the rear to increase ground clearance, decrease the fork rake, etc.) At that point, it's a matter of choosing the lesser of evils.

In a roadrace situation, your rear suspension is always compressed when you are exiting a turn, so if the axle was slightly below the line in an unladen situation, it shouldn't be an issue. Where it could come into play is if the rear wheel breaks traction, the suspension tops out, the axle pivot moves past the centerline. Now the chain is working to stiffen the suspension, which would make saving that slide even more difficult. Not really a concern on the street, but something to keep in mind.

The other thing to keep an eye on is how much the chain is going to rub on the swingarm in the pivot area when you increase the swingarm angle.

Is this scorp from Giga? Welcome if so!

I thought you were going on a 1k- not a 750!

here is some things to consider- the ZX6 swing arm I got only had to be milled about a 1/8th inch each side to fit perfectly- - I welded on tabs for the dual shocks- I will try to take a pic- if I can 'member- PM me on giga if I forget.

You want to swap out them forks if legal- use the same bike forks as you get the swing arm for- because the today's forks are generally shorter than the stock Zs- you will get your decrease in rake and trail that you want- usually very close to instability- last one I saw the trail was the same as a Gixxer- same with rake- I would put the forks and swing arm/shocks on first- then measure- it almost never comes out as you first thought!