You've seen them at the
track: drivers who bring all manner of balancing gadgets and setup tools
with them, and spend time before each run making sure that their cars
are set up perfectly. So you ask yourself, do I really need to go
through all of that to be competitive? There really is no simple answer
to that question; some drivers are good enough that a minimal amount of
effort can keep them running strong, while others would never hit the
track unless they felt fully prepared. If you are inclined to spend time
balancing your car, it won't be long before you hear about tweak, and
how it can adversely affect your car's performance. On the most basic
level, tweak can be thought of as an uneven weight distribution on a
car's suspension, and can lead to odd behavior and inconsistent
performance. Tweak can be caused by a variety of things, including a
twisted chassis, poor electronics placement, uneven tires, and
mismatched shock springs. Luckily, eliminating tweak is an easy process,
and we're here to show you the way.
Difficulty Meter and
Before you begin checking your car for tweak, you'll want to get a
few things out of the way. First and foremost, you want to make sure
that your car's chassis is as close to perfectly flat as it can be. If
you are building a new car, this is easy--just take the chassis and set
it on a flat surface. If any of the corners rise off of the surface and
you can rock the chassis back and forth, you're in trouble already!
Ideally, the chassis will be totally flat against the surface, and
you'll be good to go.
1. Next, you'll want to
consider the weight distribution of the electronics inside your car (for
a detailed breakdown on finding the perfect weight distribution, check
back in XRC #98, January 2004). If too much weight is concentrated in
one area of the chassis, it can produce the same tweaked effect. It is
probably not possible to achieve the perfect weight balance, but you can
certainly move things around to achieve the best possible combination.
2. Finally, make sure that
the wheels and tires you are using are the same size. If you race with
rubber tires, this probably won't be a problem, but with foam it can be
an issue. All the setup work in the world will not help your car if the
right front tire is 2mm wider than the left rear, and so on. If you have
or can borrow a tire truer, try to true your cars' tires down to the
same size before a run, and definitely before you check tweak. If you
want to be absolutely certain about your tweak settings, you can also
try using a set of aluminum setup wheels, available from companies like
Hudy, Integy, and others. They look trick and will take tire size out of
the equation for good.
Since tweak is dependant on a few other elements of chassis setup,
it is important to do things in the right order. If you set your car's
tweak before setting ride height, for example, you will only mess up the
balance after raising or lowering the car. Get used to doing things in a
particular order, and then make sure to follow it each and every time
you work on your car.
1 Set Ride Height • Most of the suspension adjustments
you make will be dependant on the car's ride height, so it is very
important to always check the ride height first. Sit the car on your
setup board or on a flat, level surface and check the ride height at
each of its four corners. Make adjustments to the shock collars where
necessary to achieve your desired ride height.
There are a few specific things that you can try to avoid to
help maintain a tweak-free car. These may not apply to every
chassis out there, but they should give you some general ideas:
Screw in the "X" • When you put
together the fundamental pieces of your car such as the
bulkheads, shock towers, and upper deck (if applicable), try to
always tighten the screws in an "X" pattern. This
prevents any one screw from being initially over-tightened and
subsequently twisting the chassis, shock tower, or deck. Once
you get used to using this pattern, you will probably do it
subconsciously whenever you build a car in the future. Begin by
screwing in all four screws about 80% of the way, but do not
tighten them. Tighten the upper left screw, then the bottom
right screw, then the bottom left screw, and finally the upper
right screw. This should produce a consistent and even result
Free the Belts
• If you are running one of the many belt-driven cars on the
market, this tip's for you. Never run the belts so tight that
they may be causing the chassis to flex. This should go without
saying, of course, but you'd be surprised at how many cars we've
seen with this problem, especially now that most kits offer
adjustable differential angles that control the belt tension.
The belt should always be loose enough that you can press down
on it by at least 1 cm before it goes taught. If it is too tight,
it can cause the chassis to flex, and this will almost certainly
mess up your car's tweak--and since it's an on-power effect, you
may never even notice it.
Wheels • As you subject a set of wheels to the rigors
of on-road racing, it's not at all uncommon for them to become
slightly bent or warped. This is not always something that you
notice right away, but it definitely has an effect on the car's
tweak. Likewise, if you are running foam tires,
the tires' diameter will change as they wear down, and this will
not only affect the tweak but the ride height and camber as well.
Before you check your car's tweak, make sure that the wheels are
not warped and if necessary, true the tires so that they are all
the same diameter.
2 Make Camber
& Toe Adjustments • With the car's ride height determined,
now is the time to do anything that will affect the angles of the wheels.
Use a camber gauge to measure the camber independently at each wheel,
and make any necessary adjustments. Do the same for toe-in at the front
of the car (rear toe will typically be controlled by suspension mounts
or wheel carriers). If you find that you need to make drastic changes to
camber, it's a good idea to re-check ride height afterwards just to make
sure that nothing changed.
If you are racing with rubber tires, remember
that different brands of tires may have slightly different
diameters, so if you switch from one to another you need to
re-set your ride height and droop. The difference may be small,
but for an accurate comparison of different tire brands and
compounds, you want to make sure everything else is exactly the
3 Set Droop
• Ah, droop--everybody's favorite! Now that the ride height is fixed
and the wheels are situated, it's time to make droop adjustments.
Covering all the different aspects of droop could make for an article in
itself (in fact, we just did one in XRC #115, June 2005) so we won't
revisit them here. If you're unsure how to measure droop properly, go
back and read that article. Yes, right now. All done? Good! Now, at one
corner of the car, lift the suspension up to its full extension while
holding the wheel against the surface, and use your ride height gauge to
take a measurement. The car's droop setting will be the difference
between that reading and the ride height. Make any necessary changes
using the car's droop screws or by changing the shock lengths, and then
repeat the process for each of the remaining corners.
DROOP QUESTION #97
Doesn't changing the shocks affect droop?
One of the most common reactions people have to seeing somebody
check tweak is, "doesn't that change the droop that is
already set?" It may seem odd at first, but droop actually
remains unaffected from a shock collar change. Thinking back,
you will remember that droop is the difference between the
suspension's maximum extension at that corner and the ride
height. Since the maximum extension occurs in an un-sprung
state, the shock collar is removed from the equation. If you had
to make drastic changes to un-tweak your car (and if you did,
something is probably wrong) you may have change the ride height
afterwards. To do this, make exactly the same adjustment on both
shocks on the same side to raise or lower the car. As long as
you make the same adjustment, the car will remain un-tweaked,
and as long as you return to the original ride height, the droop
will remain unaffected as well.
Okay, you made it this far, so here comes the challenging part. If
you've been racing on-road for any length of time, you've probably
noticed that the preceding three steps are things that you've always
done before hitting the track. In many cases, your car will work okay
without a tweak check, and many drivers simply don't bother with it. But
you'll also notice that every professional driver doesn't just check
tweak often, they do it before each and every run, and with good reason.
There is simply no better way to ensure consistent performance from your
car. If you're trying to feel the effect of a setup change, make
absolutely sure to check for tweak before you hit the track.
1. Keep it Flat
• Before even attempting a tweak check, make sure that you are working
on a perfectly flat and level surface. Begin by setting your tweak
station on the surface, but do not place the car on it. Remove the
pivoting side of the station (the one with the alignment bubble) and sit
it on the stationary side of the station. You want the bubble to be
perfectly centered. If it's not, try to find a better surface, or if all
else fails (and this is not recommended) use something to even out the
station. A good tweak station like the MIP one used here will allow you
to adjust its legs to level it out.
2. Orient the Car
• Place your touring car on the tweak station with the front of the
car on the pivot. Elevate the rear of the car using a 10mm standoff (if
you have a step-type ride height gauge, use the back of the gauge) so
that its wheels are not touching the surface of the station on that side.
Place the standoff at the center of the chassis, and make sure that the
car is as close to centered on the station as possible.
When you check and set tweak, keep people off of
the table! Many tracks have weak or wobbly pit tables, and if
somebody leans against one side of the table, it can throw off
your tweak station.
3. Un-Tweak It
• If there is any tweak in your car--and chances are that there
is--you'll see that the pivoting part of the tweak station is no longer
perfectly centered. To fix things, you'll need to make adjustments to
the shock collars on the pivoting side (for now, the front). On the side
that is further down, back off the collar a quarter of a turn at a time
(or conversely, screw in the side that is further up). Each time you
make an adjustment, re-check the alignment. You shouldn't have to make
huge adjustments to get things in balance; if you have to keep making
turns and the car does not seem to balance, there is probably something
else that needs attention.
Now that the front is aligned, remove the standoff that was underneath
the rear of the car. You don't need to turn the car around, since any
problems with the rear will still cause the front to move on the pivot.
Again, make changes to the shock collars to re-align the pivot, but this
time make the changes to the rear shocks. Keep making incremental
changes until the pivot is again centered.
4. Tweak Free!
• If everything went as planned, you should now have a tweak-free
touring car. Again, if you had to make drastic changes to get things to
balance out, chances are that something more serious is wrong with your
car. Take a look at our list of possibilities to see if one of them
might be the culprit.
It is worth checking the car's chassis for tweak
by placing it on a tweak station by itself with the wheels
removed. If the chassis comes up tweaked, sometimes all it takes
is twisting it a bit from side to side to get it back into form.
If it is off by a significant amount, you may need to loosen
things like the upper deck and bulkheads, get the chassis level,
and then re-attach them using an "X" pattern as you
fasten the screws. Once your chassis is completely level, you
can go ahead and confidently set tweak.
We know that it seems like a lot of work, but in the end, having a
consistent car pays off. Making random suspension changes and
downloading setups off of the internet may be the easy things to do, but
if your goal is to really learn about chassis tuning and feel the
changes that you are making, having a tweaked chassis will result in
nothing but negative progress. As always, if you put in the wrenching
time, you'll be happy with the results.