Motorcycle Design for Handling
by:shift1313

First things first, the most common swap out.
Rims and Tires
Most everyone will know about this but just incase ill start with it.

More important than rims and tires really is the tire profile. If your working with old bikes like me you are usually going from 18" rims to 17" rims. In some cases like the cbr900 your going from a 16" front rim up to a 17" rim. Tires make a huge difference in handling obviously.

If you go from a 120/80x18 front down to a 120/50 or something with less side wall you are effectively flattening out the tire, and ontop of that you are lowering your front end and increasing your rake angle. The most common mistake you will see is someone who runs say a 4.5" rim on the rear and trys to stuff a 170 or 180 section tire on it. Putting a wider tire on a thin rim will cause sever distortion to the contact patch area when riding. You will find that your bike will seem twitchy and not want to stay straight and will fall into the turns. This sounds good but because of the distortion you dont have a nice round tire to lean over you basically have flat spots like a very over exaggerated triangle. If you ever look at the difference between and old dunlop 207 and a 207 gp you can tell. Here is the 207gp profile on the right.

You'll notice the front is round but the rear has a round peak but huge flat spots. This is because most race bikes spend 99% of their time leaned over. Works great on the track but it isnt the most desireable street setup. Also if you try to stuff wider tires on thin rims the distortion may lead to tire failure.

General tire/rim combos
17"x4.5" 150-160 section tire.
17"x5.5" 170-180 section tire.
17"x6" 180-190 section tire.

You will also notice a difference if you change brands of tires. I think in general the metzler 180 tires are thinner then 180mm and the dunlops are actually wider then 180(might be the other way around, I dont remember). This makes a big difference on old bikes where chain/tire clearance is usually minimal at best when upgrading to wider rubber. I prefer to run metzler rennsport tires because of their cross section. They have a really nice rounded tire thats slightly steeper then other tires. This makes for a nice smooth lean and also a good transition from leaning from one side to the other. One thing you notice with distorted tires is they have drastic changes in the rolling radius when you are leaned over. For example: You are going down a straight fast, coming up to a long sweeping corner. You start to lean into the corner and your tire will feel like you fell off of a curb because of the big change in rolling radius(if you measure your tire from the farthest point out to your axle this is your rolling radius. when you are turning you measure from your side wall to the axle this is your rolling radius, this could be a huge difference). Because of the big change as you go into the curve your rear suspension will compress unless you are hard on the brakes and trail braking. If your suspension doesnt have enough compression dampening or your spring is too soft for you it will cause your rear end to dance a bit. This may or may not result in a high side if you are on the gas and not ready for it. So you may think stuffing a wider tire on that rim will make your bike handle better but usually without the proper rim combination you are making your bike handle worse.

I guess the next most common swap is the front end. I guess ill really just talk about front end geometry and how it affects handling.

Most fork swaps take old conventional forks and replace them with USD forks. Almost every 750cc+ bike has been running USD front ends for awhile and they are very easy to come by. The biggest benefit of USD over conventional forks really comes down to sprung vs unsprung weight. Not to mention since they are mainly used on larger bikes the cartridges and springs are usually stiffer and more adjustable. Sprung vs unprung weight really helps how well your suspension and braking works.

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