Wheel base and grip

[rapido]

How does altering your bikes wheel base affect grip? Ive recently fitted a new chain and sprockets to my K5 1000 track bike. Ive changed the gearing from 16,46 to 15,43. The bike was quite flighty before as i had to put the wheel as far in the swing arm as it would go to accommodate the large rear sprocket. It was quite unstable under heavy braking (felt like the back end wanted to come round) but I had loads of grip from the rear. Now with my new gearing and longer chain the rear wheel is as far back as it will go. I was at Donnington yesterday using the same tyres as usual (racetec k2 slicks) and suffered a big loss of rear end grip. I was getting slides coming out of foggy's the melboune loop and a very scary one round schwantz's. The track was dry and the conditions were good in the morning but I just couldnt be as hard with the throttle as usual. Any ideas?

[Ascalon]

I think, from reading about various developments, that it is not so much the wheel base overall, as the length of the swingarm.

Sports bike manufacturers have for some time been trying to make the longest possible swingarm for bikes while maintaining a short overall wheel base.

This has the characteristic of fast turn in, or agility, while maintaining grip levels for drive.

What you did was effectively reduce the length of the swingarm, as well as the wheel base, though I would not have thought that the range of adjustment available on the chain adjuster alone would have been sufficient for such dramatic results.

A

[David W]

I read that they used to run the GSXR race bikes with as long a wheelbase as possible.

Longer swingarm should have the effect of adding stability, and putting more weight over the front, but like it says above I wouldn't expect the standard adjustment to have that big an effect unless you've done other things? Are you running a bigger back tyre, more rear ride height/dropped the front?

[porter_jamie]

changing nothing else, moving the rear wheel backward reduces the weight on the rear contact patch and increases it on the front one, and vice versa. changing the gearing (bigger rear sprocket) can have a similar effect to moving the swinging arm pivot down, which effects the amount of squat/anti squat

[rapido]

I read that they used to run the GSXR race bikes with as long a wheelbase as possible.

Longer swingarm should have the effect of adding stability, and putting more weight over the front, but like it says above I wouldn't expect the standard adjustment to have that big an effect unless you've done other things? Are you running a bigger back tyre, more rear ride height/dropped the front?

The only change was the gearing. But the difference in drive was really noticeable. Ive ordered a 45 rear sprocket which ill try on track again on the 3rd on June and hope it feels a little better. What Jamie says in his reply sounds correct from my experiences. Ill measure difference in the chain adjustment tonight and let you know what it is.

[DocJohn]

*Good post, as always*

Mike is, of course, spot on.

There's a typical error made in people simply running the wheelbase as long as possible in an effort to improve stability; usually on road race bike, but this in itself doesn't work for the reasons Mille has pointed out.

Running the adjusters fully back, as stated, tips the bike forward, increases the leverage on the shock and raises ride height - most noteably on rapid and forceable loading; like hitting a bump.

Classic error in road racing is to lengthen chain, run a soft spring, decrease high speed compression and set lots of sag - but this is all counter productive - if you consider the longer swingarm acting on the shock; with a light, unpreloaded spring, the bike tends to "pack down" rapidly and dramatically, turning the bike into something akin to a hard-tail chopper!

If you want to make the bike more stable, in general terms, run the longer chain; harder spring (or slightly less sag as a poorer alternative), more high-speed compression and a (slightly) higher ride height on the forks.

2004-2007 fireblades, like the gsxr K5/6's, tended to run with fork caps flush to the yokes in superstock trim on the roads

-John

[littleblue72]

you might also struggle keeping the front end down, running 15-45. the gsxrs with their torque should be used more in the midrange than revved like a 600. i run 15-43 for almost all the circuits on my k6 thou. i tried shorter gearing but it wheelied everywhere

[rapido]

you might also struggle keeping the front end down, running 15-45. the gsxrs with their torque should be used more in the midrange than revved like a 600. i run 15-43 for almost all the circuits on my k6 thou. i tried shorter gearing but it wheelied everywhere

Very interesting reading and all makes perfect sense. Thanks guys.