Physics Questions - Weight Transfer, Tire Grip, etc.

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

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. "Pat Dotson" <[email hidden]> wrote in message news:[email hidden]...

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> 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

Great answers Hacksaw. It all goes right along with the way I understand it to work. But there is still something that has me confused: 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 Haqsau wrote:

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> > 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. > > "Pat Dotson" <[email hidden]> wrote in message > news:[email hidden]... > > 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

"Pat Dotson" <[email hidden]> wrote in message news:[email hidden]... > Great answers Hacksaw. It all goes right along with the > way I understand it to work. But there is still something > that has me confused: > > 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.

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> 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.

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. > Any other thoughts? Yes. But not many. ;o) > 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

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 "J. Todd Wasson" wrote: > > 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

Haqsau wrote: > "Pat Dotson" <[email hidden]> wrote in message > > > > 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. Let me be more clear... 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?

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> > 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. > > 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.

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

Pat Dotson <[email hidden]> wrote in message news:[email hidden]...

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> 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 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 dominant effect in the total weight transfer. 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

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.

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. 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. Haqsau wrote:

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> > 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.

Jonny Hodgson wrote:

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> Pat Dotson <[email hidden]> wrote in message > > > > > 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 dominant > effect in the total weight transfer.

I'm not sure. There is less preload with lower cross weight. I think there is less spring rate as well, since the front end of the chassis will be less resistant to deflection. I think chassis flex would equal a very progressive spring rate. > 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... 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

Okay another dumb question: are you in the kart when you are making these measurements? "Pat Dotson" <[email hidden]> wrote in message news:[email hidden]...

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> 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. > > 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. >

On Thu, 02 May 2002 19:49:36 -0500, Pat Dotson <[email hidden]> wrote: >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/

Ruud, you might be the man to answer this. In RACER, if you put a softer spring on one corner of the car, with no other changes. Will that corner see more or less dynamic loading than before? RACER puts out those numbers doesn't it? -- Pat Dotson Ruud van Gaal wrote: > > On Thu, 02 May 2002 19:49:36 -0500, Pat Dotson > <[email hidden]> wrote: > > >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/

>Ruud, you might be the man to answer this. In RACER, if you >put a softer spring on one corner of the car, with no >other changes. Will that corner see more or less dynamic >loading than before? RACER puts out those numbers doesn't >it? 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

On Fri, 03 May 2002 21:15:24 -0500, Pat Dotson <[email hidden]> wrote: >Ruud, you might be the man to answer this. In RACER, if you >put a softer spring on one corner of the car, with no >other changes. Will that corner see more or less dynamic >loading than before? RACER puts out those numbers doesn't >it? Yes, you could try and check the loads in Racer. With a softer spring on just one corner, the car would lean more into that corner, you'd say. Not necessarily meaning more load, just more suspension compression. Looking at Todd's experiment, it seems the diagonal takes more of the weight. As for dynamic loading, you'd think that with a very stiff spring you get very responsive loads; like from normal (~weight/4) load to a high load very quickly. In a soft spring, the car must lean first, and this takes time. So the average load will be less. Todd's experiment then would suggest the steady state load is also less, so dynamic load is then less. Just guessing, but with the softer spring like in Todd's example, the car leans more, but the weight of the left-rear wheel (and body) keeps the car from getting the load at the right-front from reaching the same value as with the stiffer spring (as with the stiffer spring you get less rotation in the car). Hope that doesn't clutter things up. ;-) Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

On Fri, 3 May 2002, Ruud van Gaal wrote: > 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). In a normal car with suspension, moving the CG toward the inside will reduce load differences between inner and outer tires, common example is left-weight offset on oval track cars. Working through the tire load sensitivity effects, the car will have more grip with the offset CG (assuming that other things are "equal"...which never actually happens!) Why should this work opposite (lean out) in a kart? Does it really work this way?? I have some ideas, but we haven't studied karts in detail so I'm curious to hear any theories. -- Doug Milliken

Doug Milliken wrote:

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> On Fri, 3 May 2002, Ruud van Gaal wrote: > > > 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). > > Why should this work opposite (lean out) in a kart? Does it really work > this way?? I have some ideas, but we haven't studied karts in detail so > I'm curious to hear any theories.

I've heard a theory that road course karts that have to turn left and right need to unload weight from the inside rear tire to make the kart turn. The driver leaning toward the outside of the turn isn't increasing grip, he is reducing drag on the inside rear wheel caused by the solid rear axle and lack of any stagger. Pat Dotson

Yep, The solid Axle is the reason. I used to race Karts for many years and once I learned that I akways let my body lean to the outsideit lets the inside rear wheel "spin" if the inside wheel gets a good bite the kart pushes off the turns... "Pat Dotson" <[email hidden]> wrote in message news:[email hidden]...

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| Doug Milliken wrote: | > On Fri, 3 May 2002, Ruud van Gaal wrote: | > | > > 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). | > | > Why should this work opposite (lean out) in a kart? Does it really work | > this way?? I have some ideas, but we haven't studied karts in detail so | > I'm curious to hear any theories. | | I've heard a theory that road course karts that have to | turn left and right need to unload weight from the inside | rear tire to make the kart turn. The driver leaning | toward the outside of the turn isn't increasing grip, | he is reducing drag on the inside rear wheel caused by | the solid rear axle and lack of any stagger. | | Pat Dotson |

On Sun, 05 May 2002 01:39:04 -0500, Pat Dotson <[email hidden]> wrote:

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>Doug Milliken wrote: >> On Fri, 3 May 2002, Ruud van Gaal wrote: >> >> > 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). >> >> Why should this work opposite (lean out) in a kart? Does it really work >> this way?? I have some ideas, but we haven't studied karts in detail so >> I'm curious to hear any theories. > >I've heard a theory that road course karts that have to >turn left and right need to unload weight from the inside >rear tire to make the kart turn. The driver leaning >toward the outside of the turn isn't increasing grip, >he is reducing drag on the inside rear wheel caused by >the solid rear axle and lack of any stagger.

Ah, that might explain it. I learned it with indoor karting; someone suggested it to me and it seemed strange at the time. But especially with slippery understeering karts it helped to get through the turn with less understeer. I then assumed it was upto the grip on the outerfront tires, but karts are funny with their overgrippy tires (depending on the state of the tires you really have to adjust how you drive, esp. at U-turns). Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

Does it sound right to say that a kart's roll center is at the outside of the tire contact patch? Seems like it is since there is no suspension. I really want to try to come up with a realistic handling kart in RACER! -- Pat Dotson Ruud van Gaal wrote: > > On Sun, 05 May 2002 01:39:04 -0500, Pat Dotson > <[email hidden]> wrote: > > >Doug Milliken wrote: > >> On Fri, 3 May 2002, Ruud van Gaal wrote: > >> > >> > 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). > >> > >> Why should this work opposite (lean out) in a kart? Does it really work > >> this way?? I have some ideas, but we haven't studied karts in detail so > >> I'm curious to hear any theories. > > > >I've heard a theory that road course karts that have to > >turn left and right need to unload weight from the inside > >rear tire to make the kart turn. The driver leaning > >toward the outside of the turn isn't increasing grip, > >he is reducing drag on the inside rear wheel caused by > >the solid rear axle and lack of any stagger. > > Ah, that might explain it. I learned it with indoor karting; someone > suggested it to me and it seemed strange at the time. But especially > with slippery understeering karts it helped to get through the turn > with less understeer. > > I then assumed it was upto the grip on the outerfront tires, but karts > are funny with their overgrippy tires (depending on the state of the > tires you really have to adjust how you drive, esp. at U-turns). > > Ruud van Gaal > Free car sim: http://www.racer.nl/ > Pencil art : http://www.marketgraph.nl/gallery/

On Mon, 06 May 2002 22:52:50 -0500, Pat Dotson <[email hidden]> wrote: >Does it sound right to say that a kart's roll center is >at the outside of the tire contact patch? Seems like >it is since there is no suspension. Hm, I have no idea. I guess you can see the framework as a very stiff suspension probably. At least the karts I drive have it seems some kinds of sturdy plastic that's being used to connect the wheels (with enough bumping you get extra camber because of the damage, but it doesn't break as such). You might see the rods connecting the wheels as the suspension, I guess. >I really want to try to come up with a realistic handling >kart in RACER! The very least you need to run Racer at 1000hz then (debug.ini's timing). I found that necessary (or better) for karts and F1 cars. Also note that the next version may use instant centers, but those are reasonably similar to roll centers (you deduce both from the rods). Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

Start with the easy situation of a rigid frame (not true for karts, but useful for thinking about things) on a flat road. Then the only suspension is the tires. For SMALL amounts of roll, the outside tire compresses (bump travel) and the inside tire extends (rebound travel). Since the tire spring rates are probably about the same (for SMALL deflections) the "ride travel" will be about equal (but opposite) on each side. So the "roll center" must be on the ground, in the middle of the kart. Of course this idea falls apart as soon as one wheel is (nearly) lifted off the ground, at which point the roll center must move to somewhere near the outside wheel (karts sometimes "bicycle"). And since the kart frame is flexible, one end (the rear) can roll more than the other. A "simple" vehicle is not necessarily simple to analyze... On Mon, 6 May 2002, Pat Dotson wrote:

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> Does it sound right to say that a kart's roll center is > at the outside of the tire contact patch? Seems like > it is since there is no suspension. > > I really want to try to come up with a realistic handling > kart in RACER! > > -- > Pat Dotson > > Ruud van Gaal wrote: > > > > On Sun, 05 May 2002 01:39:04 -0500, Pat Dotson > > <[email hidden]> wrote: > > > > >Doug Milliken wrote: > > >> On Fri, 3 May 2002, Ruud van Gaal wrote: > > >> > > >> > 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). > > >> > > >> Why should this work opposite (lean out) in a kart? Does it really work > > >> this way?? I have some ideas, but we haven't studied karts in detail so > > >> I'm curious to hear any theories. > > > > > >I've heard a theory that road course karts that have to > > >turn left and right need to unload weight from the inside > > >rear tire to make the kart turn. The driver leaning > > >toward the outside of the turn isn't increasing grip, > > >he is reducing drag on the inside rear wheel caused by > > >the solid rear axle and lack of any stagger. > > > > Ah, that might explain it. I learned it with indoor karting; someone > > suggested it to me and it seemed strange at the time. But especially > > with slippery understeering karts it helped to get through the turn > > with less understeer. > > > > I then assumed it was upto the grip on the outerfront tires, but karts > > are funny with their overgrippy tires (depending on the state of the > > tires you really have to adjust how you drive, esp. at U-turns). > > > > Ruud van Gaal > > Free car sim: http://www.racer.nl/ > > Pencil art : http://www.marketgraph.nl/gallery/ > >