Suspension - Event Date: January/01/1900 - January/01/1900
Location: Dayton, OH
Online Status: Offline
Calendar Event: Suspension
Posted: January/21/2003 at 6:42pm
Sus-pen-sion n: the system of devices (as springs) supporting the upper part of a vehicle on the axles.
OK, that should be the appropriate starting point for the average motorcycle drag racer. Contrary to popular belief, a shock is not something to be removed from the machine and replaced with a bar of tubing in an attempt to save weight! This device is very useful in traversing the 1320, especially where D.O.T. street tires and no bars are involved.
I must confess, as a veteran motorcycle drag racer, that my suspension knowledge is rather limited, compared to say, a road racer. But I should be able to convert what I do know into useful information.
First and foremost, you must have properly-functioning front and rear suspension. When you witness a hard launching Unlimited bike "squat," the energy release that occurs, which would normally lift the front wheel, is partially dissipated through the rear shock and converted to traction. Secondly, when you apply this much horsepower to a D.O.T. tire that was originally developed for a stock machine and it starts to spin (over bumps in the track, etc.), the tire does not want to stop.
The most critical aspect of the rear suspension appears to be the balance of spring pre-load/compression dampening and rebound dampening. Spring pre-load and compression dampening determine the rate of time relevant to the amount of force applied that it takes for the bike to squat. If a bike doesn't squat properly, it has a tendency to wheelie or spin the tire.
The wheelies are partially due to the fact that the center of gravity (CG) is higher for a bike that does not squat. As a general rule: for each inch that the chassis is lowered, approximately one tenth of a second improvement in a quarter mile elapsed time can result. This applies to the static or stationary position. General safety regulations require a 2" minimum ground clearance. I generally set up Unlimited machines at 4" to compensate for squat. Dynamically, when the machine squats on take off, you get the improved traction, but also a lower CG to offset wheelies. Additionally, you receive the added benefit of the bike raising while moving down the track, which results in more traction on the top end. Typically, machines with a taller chassis set-up leave poorly, but hook better up top. Of course, we are trying to achieve the best of both worlds!
If a bike does squat but returns abruptly, this is known as "pogoing." When a bike pogos, it plants the tire and then unloads it, which results in a spin or the chassis raises, which creates a wheelie situation. Both are undesirable. To offset this problem, you must slow the shock's rate of return with an increase in rebound dampening. I have found that most stock shocks do not have enough rebound dampening, due in part to the increased leverage of the long swing-arms that most Unlimited bikes use. I personally have had the best luck with a Fox Twin Clicker TM shock. They are very adjustable and since every bike responds differently to many variables, this comes in handy for fine-tuning.
Selecting a spring rate for your shock can be a slightly confusing task. Engine power/torque has a large effect on the squat. During the launch, as the top of the drive chain tightens, the rear end gets sucked up more with a larger, more powerful engine than it does with a smaller one. Normally, I set my bikes up with the same suspension geometry as a '91-'92 GSX-R1100 and use a 550 lb. spring. This works well for most applications and represents a good starting point for nearly all.
The front end set-up is not as critical for drag racing as the rear end, but it does play a major role. If the spring pre-load is too soft, a "slide hammer" effect can be noticed on take off. This arrangement allows the front end to raise quickly, then yanks the wheel off of the ground when the forks internal stops are reached.
If the pre-load is too hard, the front end can bounce around over bumps or bounce around after the gear shifts. This upsets the chassis, leading to rear wheel spin or wheelies. I convert most of my front ends to air over oil and adjust the pressure as required. A properly functioning suspension will also make the machine safer and more pleasant in the shut-down area. Notice should also be taken to install a quality steering stabilizer - I use the Street & Competition brand. Unlimited pilots occasionally have to land the machines after a gangster wheel stand. A road race-style stabilizer can help save your hide (leathers, that is)!
The good news is that nearly everyone is at the same point when it comes to suspension - ground zero! So what are you waiting for? Get a shock and start experimenting! It is easy to be an expert when no one else is trying to understand.
Until next time,
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