Motorcycle Design for Handling (Continued)
by:shift1313

Rake and Trail

I found this good pic of a crf which should help with rake, trail and offset:

As you can see from the pic, your Rake angle is the angle your forks are from vertical. Your trail is an imaginary line drawn from your steering axis down to the ground. The trail measurement is taken from this point back to the contact patch of the tire. With this point in front of the tire keeps your wheel going straight, or wanting to on its own. If it were behind the tire contact your wheel would try to turn itself around(think of a shopping cart wheel).

If you draw a line straight down from the steering axis(no rake) to the ground you know that the wheel will "trail" behind you as you push the cart. The wheel will always stay behind this point because its whole purpose is to "right" the wheel. The offset is defined as the distance from the center of your steering axis to the center of your fork tubes. changing this will change your trail without changing your rake. In some cases(mostly choppers) your forks arent permendicular to your steering axis(raked out) but this is really never the case on a sport bike because it makes the bike turn very very slowly. Rake angle is the biggest thing you can do to change the handling of your bike. Since most of us dont build our bikes from scratch we have to work with whatever frame we have as a starting point.

Depending on your bike your rake angle will usually be anywhere between 26 degrees down to 24 degrees. If your like me and start out with a big rake angle you have your work cut out for you. If you have the money, or the ability, you can change your rake angle by making a custom set of tripple clamps. Basically reverse what choppers do and make your forks and steering stem not parallel. The only problem with this is you are drastically changing your trail. A much easier way to change your rake is to raise or lower your front end in combination with raising or lowering your rear end. Raising your rear end by changing your linkage will essentially pivot your whole bike around your front axle giving you a steeper rake angle. This works good for small changes but there is a point where this will negatively affect your handling, ill go over that in the next post. As long as you dont have major ground clearance problems dropping your front end is really the best way to get an effective change in rake. Ofcourse every action has a reaction. Lowering your front end will change more then just your rake. Not always a bad thing but it will also effect your weight bias, your lean angle ground clearance and your overall attitude of the bike. In the end ill mention a few things about this as a whole.

Every change you make to your Rake will also change your trail. The steeper your rake angle gets, the smaller your trail gets. Think of the trail as what stabilizes your front end. A larger trail number will make your bike stable at speed but make it turn slower or resist you more, the opposite is true of a smaller trail. If you really break it down and think about what is happening when you steer it will make sense. Since your contact patch and your steering axis are at different points on the ground your not actually turning around your contact patch. This is probably the most complicated thing about designing a motorcycle front end or really thinking about your steering in general and whats going on. If you turn your steering right and left while standing next to the bike your bike will move right or left. This is because your contact patch is trying to rotate about the steering axis. This cant happen since they are at different points on the ground so the whole bike moves in the direction you are turning. This is also a reason why counter steering works. When you turn your steering you actually move the contact patch away from the direction the motorcycle is traveling forcing a change in the direction. note: it doesnt work at slow speeds because you also need the gyroscopic effect of the front wheel to aid this. You can also see a big change in trail when you hit a pot hole or a curb. Because your trail is the difference between the projection of your steering axis and the tire contact patch, when your contact patch hits a curb and is in front of your steering axis your front end wants to kick to the side. Again think of the shopping cart wheel. If you spin the wheel 180 degrees while pushing the cart its going to try and "right" itself by rotating back around.

Now that we know exactly what trail and rake are and how they affect your handling ill try to relate it to front end swaps.

As mentioned earlier lowering your front end really makes a big change in rake. This is very important to note. When swapping a front end make sure you know how long your stock setup is and how long your replacement is. Its always better to get a longer front end because you can "raise" the forks in the tripple clamps to compensate. If you have a smaller front end you really have no room for adjustments. This is something also to consider when going from an 18" rim to a 17" rim and different front end. There are ways around these problems like changing fork tubes, for example zx7 vs zx9 forks. Some years had the same forks just with different length tubes. Or you could always pay to have a custom set made. Its usually cheaper to find a front end that suits your needs.

If you look at bikes they generally have somewhat conservative rake and trail numbers because they are not typically designed to race in street trim. You will usually hear of a stock bike getting different tripples with adjustable offset(usually by the use of eccentric bushings) for race applications. "raising" the forks in the tripples to get a steeper rake angle then adjusting your trail with the offset is common practice. On my bike i dropped the front end 2" over stock and raised the rear to get a 22.9degree rake with a 3" trail. This suits my riding style and use just fine. I very rarely get over 100mph but i love to turn and drag knee so a well handling bike over a high speed stable one was my choice. Even with a 3" trail the bike is very stable up until 120mph(havnt pushed it over this yet).

Another thing to mention is the weight bias. As stated before lowering your front end changes your weight bias. This lowers your center of gravity and also will shift your weight forward on the bike. On my bike I chose to run clip ons under the top tripple clamp to set me over the front end. Alot of people choose to run bars which will set you farther back. There are advantages and disadvantages to a forward bias bike. Because your steering axis is up front having more weight up there will make it stick better in a turn and also make it more responsive to input letting your rear end track quicker. Since a bike "rolls" around its steering axis shifting the bike around the imaginary point on the ground it would seem beneficial to have the weight up front. Not everyone likes a bike to handle that quick like a gp race bike. Having more weight to the rear will neutralize the handling. It will make your front suspension more responsive to bumps in the road not having to account for the extra weight but you will loose some quickness. A 50/50 weight split is usually used unless the bike is purpose built.

I feel i should mention one more then when thinking about front ends. When your forks compress going into a turn your rake angle gets substantially steeper and also your trail diminishes. Typical sport bike suspension when fully compressed will change the rake angle by as much as 4 degrees. This is just something to consider.

Continued on next page