2) Does a softer spring increase or decrease the dynamic
load on a tire?
3) Why does lowering cross weight on an oval car increase
oversteer?
Thanks!
--
Pat Dotson
2) Does a softer spring increase or decrease the dynamic
load on a tire?
3) Why does lowering cross weight on an oval car increase
oversteer?
Thanks!
--
Pat Dotson
Concerning cross weight, this is again due to load sensitivity.
Decreasing cross weight decreases the load on the right front and left
rear, and increases load on the left front and right rear. This will be
true even after the load transfer due to cornering. This means that
during cornering there will be less difference in load from left to
right in the front than in the rear, which will give the front better
grip than the rear, thus increasing oversteer.
Does a softer spring result in more or less *dynamic* weight
transfer to that tire?
I would always have said less. Now, I've been doing some
checking with my new racing kart, and lower cross weight
results in more tranfer to the RF. Viewing the chassis of
the kart as the suspension, less cross weight equals less
spring at the RF, yet load transfer to that corner increases.
Most of that weight is coming off of the LR tire, but I would
have expected the RR to get a lot more of the transferred
weight.
Any other thoughts?
Thanks,
Pat Dotson
> Tires normally grip more with more applied load, but the catch is that
> due to load sensitivity, a pair of tires grips best if the load on them
> is equal. You are undoubtedly familiar with this as the reason why
> anti-roll bars do what they do. With regards to cornering grip, steady
> state traction is not affected by spring stiffness, but most drivers
> don't spend much time in steady state cornering. Most cornering, even
> on ovals, is transient - decelerating in, accelerating out, and the rate
> of acceleration changes rapidly also. As a result the lateral and
> longitudinal loads are always changing, and under changing loads softer
> springs, bars, and dampers will decrease the dynamic load transfer on
> the tires. Therefore under rapidly changing loads the more compliant a
> suspension is, the better the tires will grip (until you get to the
> point where you have so much compliance that the suspension and tire
> geometry is being adversely affected).
> Concerning cross weight, this is again due to load sensitivity.
> Decreasing cross weight decreases the load on the right front and left
> rear, and increases load on the left front and right rear. This will be
> true even after the load transfer due to cornering. This means that
> during cornering there will be less difference in load from left to
> right in the front than in the rear, which will give the front better
> grip than the rear, thus increasing oversteer.
> > 1) Why does a tire grip better with softer springs?
> > 2) Does a softer spring increase or decrease the dynamic
> > load on a tire?
> > 3) Why does lowering cross weight on an oval car increase
> > oversteer?
> > Thanks!
> > --
> > Pat Dotson
I believe it would be less, unless there are other things going on.
That would be one of those "other things". You and your kart together
probably have a fairly high center of gravity, meaning that a little bit
of lean can transfer a lot of weight. Normally in a car the CG doesn't
move much when the car rolls, so we can ignore the effect. In your case
the CG may be moving quite a bit when the kart rolls, because it is so
much higher than the roll center. If a softer spring means that you
lean more, you will transfer more weight, both dynamically and
steady-state.
Yes. But not many. ;o)
Thanks,
Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com
My little car sim screenshots:
http://performancesimulations.com/scnshot4.htm
2) Does a softer spring increase or decrease the dynamic
load on a tire?
3) Why does lowering cross weight on an oval car increase
oversteer?
Thanks!
--
Pat Dotson
> Can someone repost the original question? My reader missed it.
> Thanks,
> Todd Wasson
> ---
> Performance Simulations
> Drag Racing and Top Speed Prediction
> Software
> http://PerformanceSimulations.Com
> My little car sim screenshots:
> http://performancesimulations.com/scnshot4.htm
> > Does a softer spring result in more or less *dynamic* weight
> > transfer to that tire?
> I believe it would be less, unless there are other things going on.
Assume a specific amount of total weight transfer from left
to right. Assume that the cross weight has been lowered from
before, so the RF corner is now softer relative to the RR
corner than it was before.
Will a higher or lower percentage of the total weight
transfer go onto the softer RF compared to before?
The VCG is 11.5". Pretty low. But as I tried to clarify
above, I'm not talking about total transfer. I'm talking
about the percentage of total transfer to the RF vs. the
percentage to the RR.
Thanks,
Pat Dotson
> Will a higher or lower percentage of the total weight
> transfer go onto the softer RF compared to before?
> > > the kart as the suspension, less cross weight equals less
> > > spring at the RF, yet load transfer to that corner increases.
This is all rather complicated as the "roll rate" at each end
is due to the entire chassis twisting, but the (height x
centrifugal force) applied to cause the roll is distributed,
I imagine, along the length of this torsion spring...
Jonny
For CG height, you scale the kart, then tip up the rear
of the kart, and record the new front wheel weights.
There are several programs that will calculate VCG from
this information.
> Pat, how are you measuring weight transfer, and how did you determine
> your CG height? If you decrease total roll stiffness, which would be
> the case with less cross weight, you get more roll angle. That being
> the case, I am not sure why you are saying that the total weight
> transfer is still the same. Total weight transfer is not just a
> function of cornering force, it's also a function of what happens to the
> CG during cornering. Sorry to be so dense, but I think maybe I am not
> understanding the problem correctly.
> > > > the kart as the suspension, less cross weight equals less
> > > > spring at the RF, yet load transfer to that corner increases.
> When you say "less spring", is that a lower rate or simply less
> preload? If the latter, I wouldn't expect it to be a ***
> effect in the total weight transfer.
You may be right, I may be trying to simplify thing
too much. Be that as it may, lower cross weight on
my kart chassis equals more transfer to the right
front, at least initially.
--
Pat Dotson
Weight transfer is just a matter of the CG being higher than the
ground, so accelerating laterally (turning) means a torque builds up
using the arm ground->CG (height) which provides the extra force
(=weight transfer; bad name if you ask me, since no weight actually
moves) at the outer wheels.
Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art : http://www.marketgraph.nl/gallery/
--
Pat Dotson
> On Thu, 02 May 2002 19:49:36 -0500, Pat Dotson
> >I'm assuming zero roll in a kart. There is no appreciable
> >chassis roll, nothing like a car at least. I think the
> >CG only moves laterally with weight transfer.
> The CG doesn't change with anything except a bit with fuel
> displacement, and ofcourse driver displacement (a good way to get more
> grip in a kart is to hang outwards when turning).
> Weight transfer is just a matter of the CG being higher than the
> ground, so accelerating laterally (turning) means a torque builds up
> using the arm ground->CG (height) which provides the extra force
> (=weight transfer; bad name if you ask me, since no weight actually
> moves) at the outer wheels.
> Ruud van Gaal
> Free car sim: http://www.racer.nl/
> Pencil art : http://www.marketgraph.nl/gallery/
I just gave this a shot with a reasonable C-5 Corvette model in my project.
With equal spring rates and roll stiffnesses all the way around (no ARBs), a
steady state left hand 80mph turn with the front wheels at approximately
optimum steering angle produced the following loads (I forget the lateral
acceleration, something like 1.1 g's on racing tires):
LF RF Difference
220 1550 1330
LR RR
190 1560 1370
Softening the right front spring by 10% produced the following:
LF RF Difference
265 1500 1235
LR RR
145 1610 1465
The "difference" reflects the weight transfer across each "axle." Softening
the right front spring resulted in less weight transfer at the front end and
more at the rear, just as the others described, which would give a tendancy
towards oversteer (in this left hand turn, anyway.)
This is steady state though. It's tough to pin down the dynamic loading
behaviour you're talking about here exactly without drawing out some graphs
(sorry, no time right now to do that :-))
Just for curiousities sake, the differences in load when softening the right
front spring were:
LF RF
+45 -50
LR RR
-45 +50
Also note though that I'm not sure that my CG height was correct, but the
behavior described by the others is still there.
I don't know much about karts though, I'm afraid. With those suspension
systems, maybe things might work a little differently. Haven't given karts
much thought yet :-(
Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com
My little car sim screenshots:
http://performancesimulations.com/scnshot4.htm