Motorcycle Geometry 101

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Motorcycle Geometry 101
Article by: MoreRoadRacing

The factors of what makes a bike turn....

There are many contributing factors affecting how a bike feels and how it handles and what it will do when you're: on the gas... on the brakes... in a turn... or going down a straight. I will define a few terms and try to demystify what geometry is all about.

Rake is the angle most people know about, it is the relationship of the front forks to the ground.

Trail is the distance the contact patch of the tire trails behind the point projected through the Steering Axis to the ground. It acts a lot like the front wheel of a shopping cart.

Wheel base is the distance from one Axle to the other.

Rake, Trail and Wheel base are in most magazine articles but they are only half of the story... The following measurements are equally or even more important factors affecting stability and traction of a motorcycle.

Steering head to Swing arm pivot is a measurement most people seldom hear about yet, it is a very important number.

Swing arm angle is pretty self explanatory and is more important them most people can imagine.

Axle height is affected by the tires actual diameter it also affects Rake, Trail and Swing arm angles

Lowering the front or rear of a bike by as little as 5 mm can be the difference between a setting that works and one that wont. How often do you hear a race was won after they finally got the bike suspension or chassis figured / sorted out? All of a sudden the lap times come down and from there it is history.

When you raise the back of the bike it will reduce the Rake and Trail. It will make the bike turn in faster but how will it affect rear wheel Traction? and Stability? But, how was the bike raised in the rear? If you lengthened the rear shock you also increased the swing arm angle and shortened the wheel base... On the other hand if you put on a taller tire like the Dunlop 195 then you have reduced the swing arm angle and did not change the wheel base...

What happens if the front end is Lowered I.E. the forks are moved so more of them stick out above the triple clamps. This reduces the Rake, Trail, Wheel base and it also reduces the swing arm angle... A smaller front tire will do the same although it will not noticeably affect the wheel base.

So what does it all mean?

Rake: By itself doesn't tell us very much, it affects how fast a bike is able to turn in but may not affect it as much as the trail number can... A combination of Rake and Trail are what we need to balance...

Trail: Has a bigger affect on how fast a bike is able to steer, it will also affect how well a bike can finish a turn or how stable it will be. If your bike tends to run wide in a turn, feels unstable mid turn or does a head shake in the straight, maybe turns in real slow, sluggishly or maybe just feels a little vague... Then you need to look at your trail number... A trail number of just 5 mm longer or shorter can make a huge difference in what the bike is doing...

Wheel base: Well the shorter it is the quicker a bike can turn and the longer it is the more stable it will be... It's pretty much that simple...

Steering head to Swing arm pivot is a measurement most people seldom hear about yet, it is a very important number.

Swing arm angle is pretty self explanatory and is more important them most people can imagine.

Axle height is affected by the tires actual diameter it also affects Rake, Trail and Swing arm angles

Lowering the front or rear of a bike by as little as 5 mm can be the difference between a setting that works and one that wont. How often do you hear a race was won after they finally got the bike suspension or chassis figured / sorted out? All of a sudden the lap times come down and from there it is history.

When you raise the back of the bike it will reduce the Rake and Trail. It will make the bike turn in faster but how will it affect rear wheel Traction? and Stability? But, how was the bike raised in the rear? If you lengthened the rear shock you also increased the swing arm angle and shortened the wheel base... On the other hand if you put on a taller tire like the Dunlop 195 then you have reduced the swing arm angle and did not change the wheel base...

What happens if the front end is Lowered I.E. the forks are moved so more of them stick out above the triple clamps. This reduces the Rake, Trail, Wheel base and it also reduces the swing arm angle... A smaller front tire will do the same although it will not noticeably affect the wheel base.

So what does it all mean?

Rake: By itself doesn't tell us very much, it affects how fast a bike is able to turn in but may not affect it as much as the trail number can... A combination of Rake and Trail are what we need to balance...

Trail: Has a bigger affect on how fast a bike is able to steer, it will also affect how well a bike can finish a turn or how stable it will be. If your bike tends to run wide in a turn, feels unstable mid turn or does a head shake in the straight, maybe turns in real slow, sluggishly or maybe just feels a little vague... Then you need to look at your trail number... A trail number of just 5 mm longer or shorter can make a huge difference in what the bike is doing...

Wheel base: Well the shorter it is the quicker a bike can turn and the longer it is the more stable it will be... It's pretty much that simple...

Steering head to Swing arm pivot: This is a determining factor of what the Trail number should be, it also has a relationship to wheel base and thus affects cornering ability.

Swing arm angle: Is a very important factor too, more important in liter bikes than on 600's but as 600's gain more and more power it will become equally important. Swing Arm angle affects how well the rear wheel hooks up in a straight line and during a turn exit. If the Swing Arm angle is wrong it can have adverse affects on the suspension too.

Most modern sport bikes have pretty conservative geometry and can be optimized for better track and street performance, this can be achieved quickly and easily with accurate frame measurement information. Without such information considerable time can be consumed making gains in one area just to give it up somewhere else and in some cases uncontrollable high speed head shakes may develop, so extreme caution should be exercised when making geometry changes.

Stay tuned for Geometry 201 How to measure your bike and what to do with the numbers...

Charles More
More Road Racing

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