Motorcycle suspension

[holdfastgreg]

Looking over disc brake setups for my BSA and have decided to go with a Honda front end for numerous reasons (mainly part availability.)  Questions is the CB550 fork is about 2" shorter than the CB750 - would this be better suited for higher speeds?  I've tried to hunt down information pertaining to suspension travel and speed but have come up with very little (most of it is drag racing applied.)  My rational behind smaller front forks is to keep my CG lower while also keeping me out of the air per se.  If anyone can chime in with suggestions I'd appreciate it! 

[wobblywalrus]

Greg, One year I lowered the triple clamps on my fork tubes one inch on my Triumph.  (The tops of the fork tubes stuck up one inch past the top of the upper triple clamp when I was done.)  The setup gave me an occasional and terrible speed wobble.  What I did would be similar to shortening the forks.

It is important to pay attention to trail.  A reduction in trail makes the bike less stable, and an increase does the opposite.

What model BSA do you own?

[Stainless1]

WW, the stiffness of your flexible flyer probably had more to do with the wobble than the 1 inch of fork shrink. 

I would ensure your frame is adequate for the speed you desire and the swing arm and steering head bearings are tight.

[754]

I think the 500/550 fork is at most an inch shorter, but the top tree, may have more down offset.

 I can imagine why anyone tunning on the salt would not slam their bike as much as posible to decrease wind resistance.

 If lowering front puts trail into a direction you do not want, lowering the back, should get it back to what it was..

 if you can find a 76 or later 750 Automatic fork or, 77/78 k fork, they are probably stiffer, longer lower legs and greater choice of fork brace, plus you can lower the tubes more than on early 750/550s.. and the tubes are hardchromed..

[holdfastgreg]

Thanks for the input.

The bike is a dry frame BSA A65.  I had no information to go and well I wish I had some go reads about this stuff.  I'll end up with cb750 suspension none the less.

[wobblywalrus]

Greg, this fall we set the Triumph on a platform with the suspension sag it has when I am on it.  I took a foto of the front of the bike.  Then we took some straps and compressed the suspension 1.5 inches.  Then I took another photo.  Both pictures were enlarged, adjusted so they were the same scale, and printed.  They were compared.  The frontal area reduction by lowering was the 4 inch width of the front tire x 1.5 inches, or 6 square inches, total.  This tire frontal area has a rounded and streamlined shape, so its reduction does not significantly improve the aero.

Really fast bikes will benefit from this frontal area decrease.  Bikes with the speeds of my Triumph (less than 150 mph) will barely notice it.  Lowering the Triumph, based on the calculations, is not worth the trouble.  It remains at its original height.

Good luck with your A65.  They are a much better bike than popular opinion would suggest.  In many ways, they are better than the Triumphs of the same era. 

[holdfastgreg]

Greg, this fall we set the Triumph on a platform with the suspension sag it has when I am on it.  I took a foto of the front of the bike.  Then we took some straps and compressed the suspension 1.5 inches.  Then I took another photo.  Both pictures were enlarged, adjusted so they were the same scale, and printed.  They were compared.  The frontal area reduction by lowering was the 4 inch width of the front tire x 1.5 inches, or 6 square inches, total.  This tire frontal area has a rounded and streamlined shape, so its reduction does not significantly improve the aero.

Really fast bikes will benefit from this frontal area decrease.  Bikes with the speeds of my Triumph (less than 150 mph) will barely notice it.  Lowering the Triumph, based on the calculations, is not worth the trouble.  It remains at its original height.

Good luck with your A65.  They are a much better bike than popular opinion would suggest.  In many ways, they are better than the Triumphs of the same era. 

Thanks for the input!
My dilemma stands at this - BSA's have a screwy triple tree.  I'm either going to be forced into a Honda triple tree and have custom bearings made or I'm going to have to rebore the triple tree clamps for the BSA to fit the 35mm honda struts?  I think I'm going to opt for the later, to hell will custom bearings....

[754]

The Honda stem , machine or grind off the weld, push stem out. Them either mod a Beezer stem, or whip up a new one. When I do them its usually reasonable..

 Re ..lowering;
 I switched my 750 from a Hooker 4-1 back to my old turbo high-pipe.. an unequal length 4-1, with a megaphone. This allowed me to lower the bike another 3 inches or more. 3 tenths gain first time out..!

 I cant see how lowering cant help it..

[fredvance]

Lower is better/faster,

[Constant Kinetics]

In drag racing a lower bike tends to hook better and ride more stable at higher speeds. I won't be able to make it to Bonneville 'till 2011 though, so i'm not sure if it has the same advantages on the salt.