This is an article I wrote for WOO in August of 2008.
Cross Wedge Discussion
Remember
that the right front tire takes a lot of abuse. It has to turn the
vehicle. When it is loaded.. it has to take the weight of the vehicle
in the turn as well as try to turn it. Since the tire only has limited
grip, our goal it to remove some of the loading to the tire so that it
can do it's job more effectively. That is.. turn the tractor. We'll
look here how to lessen the load on the right front.
Let’s take a chassis that is equally bias... front to rear … left to right and 325 lb tractor
If you weight each corner you would see.
LF RF
81.25 81.25
LR RR
81.25 81.25
Each tire is supporting an equal amount of weight.
This
would be at rest. In motion there would be forces of acceleration
adding weight to the right rear tires. This is called squat. Since we
don’t run suspensions in our tractors the squat would be purely taking
up by the rear tires. And since you can not build anti-squat into our
chassis’s because they are not sprung, we will just assume an equal
loading on all four tires.
Now... as we enter the corner. Since
most of us do not run front brakes I will not deal with that part of
the circle of traction that deals with braking a vehicle as well as
turning. We will also not deal with the squat that is placed on the
tire because of the shifting of weight under braking. We will, though,
deal with the transfer of weight from the left side to the right side
of the tractor because that is very important.
Since I have
already posted about weight transfer in an earlier post, lets look at
how the weight effects the vertical loading of the tires.
If 25 lbs of lateral transfer is found through our calculations the tire loading would be in the center of the turn would be.
LF RF
56.25 106.25
LR RR
56.25 106.25
Now the tractor is quite uneven in the weight distribution from left to right.
The
tractor would be tipsy as 31% more weight has to be carried by the
right side tires. You would have to slow the tractor down to control
through the corner. But the tractor would be neutral, neither under
steer nor oversteer. But would be a handful to drive and would probably
want to understeer since the left rear is on the ground.
Notice that the left rear is still on the ground as the loading is still the same as the left front.
Now lets use caster and KPI to add some wedge to the chassis.
Lets
assume that when the left front is turned to the left it uses a force
of 15 lbs. This 15 lbs would be taken by the left front and the right
rear, and then conversely 15 lbs would come off the right front and
left rear.
The loading would then look like.
LF RF
71.25 91.25
LR RR
41.25 121.25
Now
if we look at the loading we can see a big change. The RR is taking the
most amount of load and the weight has come off the Right front.
The
left rear has had a considerable amount of loading removed and
depending on the degree of settings in the caster and KPI is probably
in the air. If not it is see much less force being put on it than the
right rear so then it would want to slide.
By looking at the
loaded biases you can see that the rear tire is taking more force than
the right front. The chassis would then be loose or oversteer. The
right front has more ability to turn the tractor.
Now... you
find that the tractor is still tight on some tracks. You can further
increase the cross weight by adding shims to raise and lower the left
front or right front spindles. That is called preload. We will talk
about pre-loading the chassis and its effect on corner weights in the
next installment.
When people tell you that you don't need KPI and caster to go fast .. now you know why you do.
Preload the chassis
Here are our baseline weights from before.
LF RF
81.25 81.25
LR RR
81.25 81.25
Lets
take our baseline weights and now drop the LF spindle down. This
effectively wedges the chassis at rest. Assume that 10 lbs is applied
to the chassis in wedge. Like before any weight we put on the LF also
gets put onto the RR.
Our weights would now be.
LF RF
91.25 71.25
LR RR
71.25 91.25
If
we look at the weights you can now see that the right front has less
weight on it already and the right rear is loaded more than in the
baseline. Effectively we have pre-loaded the chassis. We have moved
weight without having to apply a force to the chassis.
Now... we go into the corner. Using the same 25 lb of lateral transfer we would see the following loading.
LF RF
66.25 96.25
LR RR
56.25 116.25
As
we see here, the rear is doing more work than the front so the chassis
would be loose but a lot tighter than the last example.
Now lets
apply the Caster and KPI forces to the weights. Remember that these
will be shared by the LF and RR just like the shims.
LF RF
81.25 81.25
LR RR
41.25 131.25
Now
if we look at the loading... we see that the RR is taking a
considerable amount of force. This would be a VERY loose set-up.
This would be useful though if you have chassis flex. The loading here is in a perfect world were you chassis doesn’t move.
My
chassis needs a considerable amount of pre-load to be able to turn
well. But at Woodstock I was loose like everyone else and one way to
fix that without lowering your air pressures until they are flat is to
remove some of the shims and raise my spindles up. This removes weight
from the right rear and adds more to the right front and tightens the
chassis. I can then run my baseline air pressure setting and tune from
there.
We have not discussed air pressures and the effect they have on tire loading and how the effect handling.
A tire is basically a variable rate spring. The
more air you add to it the higher the spring rate will be. Now since we
are not talking about a sprung vehicles, we don’t need to concern
ourselves with Spring Load, sprung and unsprung weight.
We are
also not going to get into the fact that as you load a tire past a
certain point the effective grip of the tire will go down not up.
Your
tire can support a certain amount of load at a certain air pressure. As
you increase the load on that tire you need to increase the air
pressure to overcome the force pushing down on it. You can increase the
air pressure beyond the need placed on the tire by the force to reduce
grip and loosen one end or the other, and by dropping the air pressure
to the required pressure you can regain the grip.
As we increase
the air pressure in one end of the chassis, we are forcing the other
end to do more work. With turning left, this would be done with the
right side tires.
So.. as we add more air to the right rear, it
causes the front tires to do more work. This removes grip from the
rear. Loosening the chassis.
Now lets go the other way. You
chassis is loose and you need to tighten it up, you would put more air
in the right front. This causes the rear to have more grip and the
front to have less. Therefore it would want to push the front tire.
Left
side tires pressures do help with grip, but the left front usually has
more air to compensate for the extra force applied to it through the
wedge (or jacking effect) it gives to the chassis. By increasing the
air pressure in the left front you can actually wedge the chassis by
adding more stagger than the right front. This has it’s limits. When
you increase the air pressure you have less contact area to work with.
That’s why we use mechanical forces to apply wedge to the chassis.
The
left rear air pressure is mostly used to increase or decrease forward
bite. But by having it low you can tilt the chassis on that side and
could possibly have less lateral transfer of weight on the rightside.
But mostly I would tune for forward bite off the turn.
There are
limits you can go with air pressure. Too low and the tire will cup and
you would have limited available traction at that corner. If you go too
high you also lose contact area. If you run your right rear too high it
shows that you have a chassis issue that is effecting the turn.
You
should have someone watch you to look to see why the tractor doesn’t
want to turn. Most likely you tractor doesn’t have enough KPI or Caster
to wedge to chassis. Using too much air pressure to loosen the chassis
will certainly hurt forward bite.
Now a tire will work the best when the
loading is perpendicular to the ground. In other words the tire is the
most effective when it is at a Zero Camber Angle.
But your
thinking... I see tires tilled it when at rest when I visit the track.
That is true. Some are on purpose, some are just guesses.
You
have to think about what is already in your front end. We already have
10 to 15 deg of KPI built into the spindle plus we have possibly 15
degrees of negative camber built it. This is in a typical front end
design.
If the tire was flat at rest, when we turned the tire to
the left you would actually see that only the outside of the tire would
be touching the ground.
As mentioned earlier, maximum traction is when the tire is perpendicular to the ground.
So this is where the adjustable front suspension comes into play.
To
get zero camber angle with the tire turned you need to decide how much
you would like the tire to raise and set it to that. The easiest thing
to do is to place a 2 x 4 flat on the ground under your inside rear
tire and crank the wheel to the left. You then adjust the heim joints
on both sides so that the front tires are flat. Now when you go into
the corner, and wedge the chassis you will have maximum tire contact
patch.
Remove the 2 x 4 then crank your wheel to the left. You
should have the inside wheel in the air and both of your front tires
flat.
But..... depending on front air pressure setting.. you
also have to deal with lateral transfer of weight adding and removing a
few degrees of negative or positive camber when the tractor is turning.
Also if you drive heavily with the front right you will see a camber
gain or loss in the chassis.
The only thing I would say would be to look at every picture Jenni takes of your tractor and see what it is doing.
Also
if your tractor pushes.. then you will be driving heavily off the right
front. I would adjust to correct for the push and not use chamber to
try and correct the push.
If you are in the faster classes, a
lower center of gravity with a low roll center will minimize the effect
of chamber loss do to weight transfer. Also a wider track width can
also help in this effect.
Just because Cup cars use a large amount of negative camber or split camber settings.. doesn't mean it will work here..
Cup
cars have suspensions and must compensate for bump steer, dive angles,
and serious roll angles that cause camber loss or gain over the bumps
and on the brakes.
Do not use a cup car as a base for what you think your front axle should look like at rest.
But
if you aren't running an adjustable front end and it runs well..
imagine how well it would run if you could put the correct geometry
into the front axle design?
