How to: Un-Tweak your suspension
4 steps to a better handling car
Do you have any Hot Tips?|
|Based on an article by DEREK BUONO
PAGE ONE (OF TWO)
The term tweak originated with pan-car racers, but it applies to all types of vehicles.
While tweak setting is relatively easy to understand and adjust on a pan car, on full-suspension vehicles it can
seem like a black art. A chassis is said to be tweaked when it is warped, or when the suspension settings
create the effect having of a warped chassis.
Tweak causes a car to corner better in one direction than the other, and in extreme cases, it can even cause the
car to pull to one side under acceleration. Sometimes, tweak is caused by weight bias; if one side of the chassis
is significantly heavier than the other side, the chassis will seem tweaked even if it is perfectly straight and
the suspension is set precisely. So how do you know if your full-suspension touring or off-road car is tweaked because
of a problem with the chassis, the suspension, or weight bias? And how do you fix it? Follow these steps:
Check the chassis for trueness
It's best to do this before you build your kit. Remove the front and rear suspension assemblies, and place the chassis on a flat, level surface such as a kitchen counter. Push down evenly on all four corners. If it rocks or one side lifts off the counter, it's warped. If your car has a top deck, loosen the screws; then press the chassis firmly down against the counter and retighten the screws. If that doesn't get the tweak out, it's best to replace the chassis, but you could try to tune other areas to compensate for it.
Right: Press down evenly on all four corners of the chassis to check whether it is
warped or whether the top deck needs to be readjusted.
Set all the droop screws so the suspension has equal downtravel.
This is a key element in getting a tweak-free car.
Set suspension droop
You can skip this step if your car does not use droop screws (also known as down-travel screws).
Disconnect the shocks so the arms hang freely; make sure they can swing through their range of motion
without binding. If everything operates properly, use the droop screws to set the maximum down-travel
so it is the same on both sides of the car. Associated offers an inexpensive molded gauge that makes the
job easy; or you can buy a deluxe aluminum version from Hudy. Just make sure your shocks are long enough
to allow the arms to reach maximum down-travel. That brings us to the next step.
Check the shocks' lengths
Shocks are your car's legs; if one of your own legs was longer than the other, you would
be lopsided. If your shocks are of different lengths, your car will be lopsided. Use calipers or a Trinity
Shock Dyno to make sure the left and right shocks are the same length and have the same amount of travel.
If they don't, you probably have the wrong number of internal spacers in one of them, or one of the shock
shafts may be out of spec. Another possibility: you really goofed up and used a rear shock body on the front
suspension, or vice versa. If the shocks are built properly, slight length discrepancies can be eliminated
by threading the shock eyelet in or out.
Check each shock length; they should all be the same.
The distance from shock mount to shock mount should be the same on both sides.
Checking for tweak
Now that all the shocks are the same length and the preload and droop are set, it's time to check the tweak.
The best way to gauge suspension tweak is to use a balance system like this one from Niftech (shown on a Hudy setting board). |
When using a balance system, set the gauge to level and place the car's front wheels on the stationary bar;
be sure to center the car precisely over the pivot post. If the car isn't tweaked, the pivoting bar the rear
wheels are on will remain level. If one side drops, that side of the car is heavier, and the car is tweaked.
To fix this, recheck the shock length and preload settings; increase the preload on the light side.
When you're satisfied with the adjustment, turn the car around and set the front suspension tweak.
A scale system measures the weight over each wheel and makes it even easier to set tweak. But be warned:
it's easy to become obsessed with trying to make the left and right scales read exactly the same, and it's
virtually impossible to do. Any change you make to one shock affects the weight distribution to the other three.
Get the numbers as close as you can, but remember that races aren't run on scales; they're run on the track.
And that brings us to the most practical tweak test, which is simply a figure-8 lap. Run a figure-8 with the
throttle steady and the same steering input from the transmitter when you turn left and right. If your car's
turning radius is different for the left-turn parts of the
figure-8 than for the right turns, it's tweaked. Likewise, if you know your steering servo's endpoints are set properly,
but your car spins out when turning in one direction and pushes in the other, it's tweaked.
A tweaked suspension may also cause an off-road car to pull to one side or fail to jump straight.