Car Physics - Side Bite Part 2

The other thread broke down into a discussion of kart setup and weight transfer. I'd like to concentrate on the side bite issue. Paul Laidlaw pointed out that lateral force v. vertical load is not linear (I was thinking of slip angle curves at low levels of slip). But the force v. load curves I have seen (Gillespie) don't go to zero. The bottom of the curve starts at 200 lbs vertical load. It is approximately linear up to about 400 lbs. Most karts weight less than 400 lbs total. The most load any kart tire is ever likely to see is 200 lbs. Unfortunately, I have not been able to find any actual kart tire data. Still, I think we can assume a linear load v. lateral force curve up to 200 lbs. If that's true, then there isn't any inherent advantage to balancing the left/right load, is there? The key to getting the most grip is to have the right compound on the right side given the load while cornering. PD

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> Paul Laidlaw pointed out that lateral force v. vertical load is not > linear (I was thinking of slip angle curves at low levels of slip). > But the force v. load curves I have seen (Gillespie) don't go to zero. > The bottom of the curve starts at 200 lbs vertical load. It is > approximately linear up to about 400 lbs. Most karts weight less than > 400 lbs total. The most load any kart tire is ever likely to see is > 200 lbs. Unfortunately, I have not been able to find any actual kart > tire data. Still, I think we can assume a linear load v. lateral > force curve up to 200 lbs.

I doubt it's linear, a kart tyre is designed for a kart after all, it doesn't need to take car loads, but it may well be close enough to linear to make any advantage gained from limiting weight transfer relatively unimportant given the other options. Paul

There are two mechanisms by which tires generate grip. One is molecular adhesion, where the tires literally grip the track like glue. The other is hysteresis, where there is an energy loss in the tire as it deforms while sliding over the surface of the track. On dry dirt tracks, there probably isn't a lot of adhesion happening. The grip is likely coming mostly from hysteresis. When a dirt track gets tacky, and adhesion becomes more of a factor, the kart could end up with too much grip. This would be a good explanation for "side bite". Has anyone ever seen a chart for hysteresis tire grip v. vertical load? Or, for that matter, what about a molecular adhesion grip v. vertical load chart? PD

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>There are two mechanisms by which tires generate grip. One is >molecular adhesion, where the tires literally grip the track like >glue. The other is hysteresis, where there is an energy loss in the >tire as it deforms while sliding over the surface of the track. > >On dry dirt tracks, there probably isn't a lot of adhesion happening. >The grip is likely coming mostly from hysteresis. When a dirt track >gets tacky, and adhesion becomes more of a factor, the kart could end >up with too much grip. This would be a good explanation for "side >bite". > >Has anyone ever seen a chart for hysteresis tire grip v. vertical >load? Or, for that matter, what about a molecular adhesion grip v. >vertical load chart? > >PD >

One SAE paper I read on tire heating, sliding velocity, and grip talked about there also being a molecular adhesion portion to grip as you said. It was, however, considered so small that it could be disregarded, even on asphalt. That leaves basically "force" vs. slip angle or slip ratio; I've never seen or would expect to see data where the two forces could be seperated. The force is caused by the distortion in the tire resulting from a portion of the contact patch being unable to move for whatever reason as you already know. What causes it to stick? What causes anything to stick? The nature of friction is still a bit of a mystery to scientists, but interpenetration of the rubber with surface irregularities in the pavement appears to be a noteworthy factor in getting unusually high coefficients of grip. This is especially noticeable in RC foam tires, which operate the same way as full sized air filled tires do as far as load sensitivity and so forth is concerned. Softer tires don't increase grip on smooth surfaces after a point, according to an engineer I talked with. Run it on a rough surface however, and you can keep going softer and improve grip at the same time. There are several sources where charts of tire grip vs. vertical load are published. Milliken's "Race Car Vehicle Dynamics" of course springs to mind first. Indeed, all data I've seen (which isn't much) shows a definite drop in friction/grip coefficient with load. If you are able to alter under/oversteer noticeably by adjusting the front/rear stiffnesses in whatever four wheeled vehicle you're talking about, whether it's a go-kart, Indycar, or even an RC racer on foam tires, then you are observing the effects of this drop off. Without load sensitivity, most of chassis tuning would do nothing. Todd Wasson Racing Software http://PerformanceSimulations.com My car sim http://performancesimulations.com/scnshot4.htm

J. Todd Wasson wrote: > One SAE paper I read on tire heating, sliding velocity, and grip talked about > there also being a molecular adhesion portion to grip as you said. It was, > however, considered so small that it could be disregarded, even on asphalt. Really? In Gillespie, he says that molecular adhesion account for the majority of grip on a dry road. On wet roads, there is almost no adhesion, and whatever grip is available comes mainly from hysteresis. > Indeed, all data I've seen (which isn't much) shows a definite drop in > friction/grip coefficient with load. If you are able to alter under/oversteer > noticeably by adjusting the front/rear stiffnesses in whatever four wheeled > vehicle you're talking about, whether it's a go-kart, Indycar, or even an RC > racer on foam tires, then you are observing the effects of this drop off. > Without load sensitivity, most of chassis tuning would do nothing. One other general point about kart setups. It is easy to set up a kart so that the left rear tire will lift going around a corner. Before I started racing, I would have assumed that when that happened, the kart would spin, or at least be loose. The opposite is true. For oval karts, if you are lifting the LR wheel, the kart will be tight/ pushing. The only explanation I can come up with is that karts typically run 55% or more rear weight bias. So, even with all the rear weight on the RR, it still has more grip than the front end combined. But, I would expect that the more you keep the LR planted on the ground, then total grip at the rear would increase. But it doesn't. Pat Dotson

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> >> One SAE paper I read on tire heating, sliding velocity, and grip talked >about >> there also being a molecular adhesion portion to grip as you said. It was, >> however, considered so small that it could be disregarded, even on asphalt. > > > >Really? In Gillespie, he says that molecular adhesion account for >the majority of grip on a dry road. On wet roads, there is almost >no adhesion, and whatever grip is available comes mainly from >hysteresis. >

Hmm... The paper didn't actually use the phrase "molecular adhesion," but rather "adhesion," so perhaps Gillespie is using a different definition than I'm thinking of. Here's an exerpt from the paper: (This paper was concerning development of a numerical tire model specifically dealing with the relationships between tire heating, sliding velocity, and grip. The key here is that the focus of this is in the *sliding* portion of the print.) "The development of the contact model is based on studies of mechanical and thermal effects in the contact interface. This investigation is conducted on the microscale, on which two different friction mechanisms can be distinguished. Hysteretic friction results from the internal damping of rubber in case of periodic excitation. Therefore, the loss modulus is the decisive characteristic of rubber for this friction component. As the energy dissipation takes place in layers of the rubber block adjacent to the contact surface, hysteretic friction is a bulk effect." "The second part of rubber friction is adhesive friction. It is a consequence of interfacial bonds that are made up between the rubber and the friction surface on a molecular scale. In case of a relative motion of the two friction partners, these bonds are stretched and finally ruptured. The stored bond enery is dissipated. Adhesive friction is the minor part of rubber friction. Hence, the characteristics of hysteretic friction are the basis for the macroscopic contact model that will be briefly described in the following." Later on the paper says it leaves out the adhesive friction analysis entirely due to its relative insignificance. It should be noted though that this is referring primarily to what is happening in the portion of the print that is sliding, which in the case of a tire operating at the limit, might very well be the majority of the contact patch or a significant portion of it at least. I haven't read Gillespie's books, so can't comment on the relationship between this and what he described. Perhaps he was referring to the non-sliding region of the contact patch, where perhaps the situation is reversed. I haven't thought or read about it enough yet to comment.

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>One other general point about kart setups. It is easy to >set up a kart so that the left rear tire will lift going >around a corner. Before I started racing, I would have >assumed that when that happened, the kart would spin, or >at least be loose. The opposite is true. For oval karts, >if you are lifting the LR wheel, the kart will be tight/ >pushing. >

As I mentioned before, I work a bit with an engineer and a couple champion 1/8th scale gas RC drivers that have discussed the same effect with me. These cars behave opposite to what you just described. They have a solid rear axle (and front one-way bearings for selective 4WD) and also benefit from lifting the rear wheel when increased oversteer is desired. Note though that "lifting the inside rear wheel" probably means "lowering the load on the inside rear wheel." They typically adjust this on the RC cars by altering caster and are knowingly altering the inside rear load, or "lifting" it in varying amounts. Increasing caster causes more inside rear lift, and the car responds with increased oversteer on throttle (and subsequent understeer off throttle due to higher aligning torque fighting the servos and possibly bending the suspension a bit.) This sounds to be the exact opposite of what you've described with go-karts, which itself is the opposite of what I've heard other people say. I'm a bit confused :-) From other's descriptions, it seemed that leaning outside the corner or altering the setup in a way that increased inside rear tire lift caused an increase in oversteer on throttle, the same effect that we see in RC cars. I'll take your word for it though :-)

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>The only explanation I can come up with is that karts >typically run 55% or more rear weight bias. So, even >with all the rear weight on the RR, it still has more >grip than the front end combined. But, I would expect >that the more you keep the LR planted on the ground, >then total grip at the rear would increase. But it >doesn't. >

Locked axles are a little tricky to make generalizations about because there a couple of things happening like you described. First of all, the solid axle is trying to straighten the car, causing understeer of course. The car, generally speaking, should probably have increased understeer with less load transfer from this effect as you described. On the other hand, this forward/rearward force that's being developed (causing understeer) uses up some of the friction circle, leaving less side force. So whether reducing lateral load transfer (less wheel lift) at the rear causes understeer or oversteer depends on which one of these two effects dominates. I'd think that this would depend to a large degree on the rear track width / center of gravity height ratio. It could theoretically go either way. With the extremely low CG RC cars, inside rear lift is generally (always?) an oversteer effect on-throttle. I'd imagine the CG is in proportion much higher on a go-kart, so perhaps that would explain why the opposite might be happening. I'm no expert, mind you, but that's my take on it right now :-P Todd Wasson Racing Software http://PerformanceSimulations.com My car sim http://performancesimulations.com/scnshot4.htm

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> As I mentioned before, I work a bit with an engineer and a couple champion > 1/8th scale gas RC drivers that have discussed the same effect with me. These > cars behave opposite to what you just described. They have a solid rear axle > (and front one-way bearings for selective 4WD) and also benefit from lifting > the rear wheel when increased oversteer is desired. Note though that "lifting > the inside rear wheel" probably means "lowering the load on the inside rear > wheel." > > They typically adjust this on the RC cars by altering caster and are knowingly > altering the inside rear load, or "lifting" it in varying amounts. Increasing > caster causes more inside rear lift, and the car responds with increased > oversteer on throttle (and subsequent understeer off throttle due to higher > aligning torque fighting the servos and possibly bending the suspension a bit.)

Which is also one way of adjusting it on karts, you can also adjust the amount of rear axle flex which will affect camber on the loaded wheel, I imagine on an RC car (I assume a stiff axle?) you will create positve camber by unloading the inside wheel, this may explain your oversteer? > Locked axles are a little tricky to make generalizations about because there a > couple of things happening like you described. First of all, the solid axle is > trying to straighten the car, causing understeer of course. The car, generally > speaking, should probably have increased understeer with less load transfer > from this effect as you described. It only causes understeer under braking (rear wheels only) or on a trailing throttle, under power it improves turn in. I notice people really struggle to get hairpins right on karts at first because the effect of throttle/braking whilst turning is not the same as your average road car.

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> On the other hand, this forward/rearward force that's being developed (causing > understeer) uses up some of the friction circle, leaving less side force. So > whether reducing lateral load transfer (less wheel lift) at the rear causes > understeer or oversteer depends on which one of these two effects dominates. > I'd think that this would depend to a large degree on the rear track width / > center of gravity height ratio. It could theoretically go either way. With > the extremely low CG RC cars, inside rear lift is generally (always?) an > oversteer effect on-throttle. I'd imagine the CG is in proportion much higher > on a go-kart, so perhaps that would explain why the opposite might be > happening.

Well, that does happen on a kart but that is because the forward force vector is in effect offset from the centre of the kart (to the more loaded rear wheel), it isn't the effect we were (or me anyway!) originally talking about. I'm not sure I'm explaining myself very well. Paul

J. Todd Wasson wrote:

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>>Really? In Gillespie, he says that molecular adhesion account for >>the majority of grip on a dry road. On wet roads, there is almost >>no adhesion, and whatever grip is available comes mainly from >>hysteresis. > > Hmm... The paper didn't actually use the phrase "molecular adhesion," but > rather "adhesion," so perhaps Gillespie is using a different definition than > I'm thinking of. >

(clip) > > It should be noted though that this is referring primarily to what is happening > in the portion of the print that is sliding, which in the case of a tire > operating at the limit, might very well be the majority of the contact patch or > a significant portion of it at least. Gillespie's book isn't really about racing, so his comments might apply to conditions at well under the limits of the tire. The section on this subject is very short. I don't have RCVD, what do they say? > They typically adjust this on the RC cars by altering caster and are knowingly > altering the inside rear load, or "lifting" it in varying amounts. Increasing...>

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> This sounds to be the exact opposite of what you've described with go-karts, > which itself is the opposite of what I've heard other people say. I'm a bit > confused :-) From other's descriptions, it seemed that leaning outside the > corner or altering the setup in a way that increased inside rear tire lift > caused an increase in oversteer on throttle, the same effect that we see in RC > cars. > > I'll take your word for it though :-)

Caster is used on sprint and oval kart in the same way you describe above. Oval karts operate differently than sprint karts, mainly because of rear tire stagger. With no stagger, I'm sure that lifting the IR is faster. But, with stagger, you don't have to lift the wheel because the tires aren't fighting against each other. Also, in the particular form of kart racing I do, our motors are only producing about 9HP. Getting on the throttle doesn't have a lot of affect :)

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> Locked axles are a little tricky to make generalizations about because there a > couple of things happening like you described. First of all, the solid axle is > trying to straighten the car, causing understeer of course. The car, generally > speaking, should probably have increased understeer with less load transfer > from this effect as you described. > > On the other hand, this forward/rearward force that's being developed (causing > understeer) uses up some of the friction circle, leaving less side force. So > whether reducing lateral load transfer (less wheel lift) at the rear causes > understeer or oversteer depends on which one of these two effects dominates. > I'd think that this would depend to a large degree on the rear track width / > center of gravity height ratio. It could theoretically go either way. With > the extremely low CG RC cars, inside rear lift is generally (always?) an > oversteer effect on-throttle. I'd imagine the CG is in proportion much higher > on a go-kart, so perhaps that would explain why the opposite might be > happening.

CG to track width ratio for oval karts is about 3:10 (12" vcg, 40" track width). Not sure how that compares to other vehicles. PD

"Pat Dotson" <[email hidden]> skrev i melding news:[email hidden]...

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> The only explanation I can come up with is that karts > typically run 55% or more rear weight bias. So, even > with all the rear weight on the RR, it still has more > grip than the front end combined. But, I would expect > that the more you keep the LR planted on the ground, > then total grip at the rear would increase. But it > doesn't.

That phenomenon is quite similair to putting one tire onto the grass, I remember I had a rather fun discussion with a certain mr. Cassidy here in the days of N2 because it did exactly the opposite. It would spin into the turn if you had the inside front on the grass... I would expect the vehicle behaviour to vary in such a situation depending on the rear suspension being stiff/solid axle/LSD/independent? Or would it? Matt Knutsen

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>> As I mentioned before, I work a bit with an engineer and a couple = >champion >> 1/8th scale gas RC drivers that have discussed the same effect with = >me. These >> cars behave opposite to what you just described. They have a solid = >rear axle >> (and front one-way bearings for selective 4WD) and also benefit from = >lifting >> the rear wheel when increased oversteer is desired. Note though that = >"lifting >> the inside rear wheel" probably means "lowering the load on the inside = >rear >> wheel." =20 >>=20 >> They typically adjust this on the RC cars by altering caster and are = >knowingly >> altering the inside rear load, or "lifting" it in varying amounts. = >Increasing >> caster causes more inside rear lift, and the car responds with = >increased >> oversteer on throttle (and subsequent understeer off throttle due to = >higher >> aligning torque fighting the servos and possibly bending the = >suspension a bit.) > >Which is also one way of adjusting it on karts, you can also adjust the = >amount of >rear axle flex which will affect camber on the loaded wheel, I imagine >on an RC car (I assume a stiff axle?) you will create positve camber by >unloading the inside wheel, this may explain your oversteer?

Interesting about the rear axle flex adjustment. The RC cars have independant front and rear double wishbone suspension systems, so there isn't a similar adjustment for them. They have two halfshafts for a driveshaft that rotate at the same speed, so it acts like a solid axle in that regard. I suppose you're right about the positive camber coming from the caster adjustment. However, the simulator is ignoring camber right now and the oversteer effect is still there.

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>> Locked axles are a little tricky to make generalizations about because = >there a >> couple of things happening like you described. First of all, the = >solid axle is >> trying to straighten the car, causing understeer of course. The car, = >generally >> speaking, should probably have increased understeer with less load = >transfer >> from this effect as you described. =20 > >It only causes understeer under braking (rear wheels only) or on a = >trailing >throttle, under power it improves turn in. I notice people really = >struggle >to get hairpins right on karts at first because the effect of = >throttle/braking >whilst turning is not the same as your average road car. >

Oh, ok. So you're seeing the same effect on the karts as we see in RC.

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>> On the other hand, this forward/rearward force that's being developed = >(causing >> understeer) uses up some of the friction circle, leaving less side = >force. So >> whether reducing lateral load transfer (less wheel lift) at the rear = >causes >> understeer or oversteer depends on which one of these two effects = >dominates.=20 >> I'd think that this would depend to a large degree on the rear track = >width / >> center of gravity height ratio. It could theoretically go either way. = > With >> the extremely low CG RC cars, inside rear lift is generally (always?) = >an >> oversteer effect on-throttle. I'd imagine the CG is in proportion = >much higher >> on a go-kart, so perhaps that would explain why the opposite might be >> happening. > >Well, that does happen on a kart but that is because the forward force >vector is in effect offset from the centre of the kart (to the more = >loaded >rear wheel), it isn't the effect we were (or me anyway!) originally = >talking about.

Oh, ok. I didn't read the other thread, so maybe I'm off on a different tangent :-) Todd Wasson Racing Software http://PerformanceSimulations.com My car sim http://performancesimulations.com/scnshot4.htm

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> >> It should be noted though that this is referring primarily to what is >happening >> in the portion of the print that is sliding, which in the case of a tire >> operating at the limit, might very well be the majority of the contact >patch or >> a significant portion of it at least. > > >Gillespie's book isn't really about racing, so his comments >might apply to conditions at well under the limits of the >tire. The section on this subject is very short. I don't >have RCVD, what do they say? > >

I don't recall reading anything about this in RCVD. Perhaps Gillespie's comments are referring to behavior well before the limit. I don't now.

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>> They typically adjust this on the RC cars by altering caster and are >knowingly >> altering the inside rear load, or "lifting" it in varying amounts. >Increasing >...> >> This sounds to be the exact opposite of what you've described with >go-karts, >> which itself is the opposite of what I've heard other people say. I'm a >bit >> confused :-) From other's descriptions, it seemed that leaning outside the >> corner or altering the setup in a way that increased inside rear tire lift >> caused an increase in oversteer on throttle, the same effect that we see in >RC >> cars. >> >> I'll take your word for it though :-) > > >Caster is used on sprint and oval kart in the same way >you describe above. > >Oval karts operate differently than sprint karts, mainly >because of rear tire stagger. With no stagger, I'm sure >that lifting the IR is faster. But, with stagger, you >don't have to lift the wheel because the tires aren't >fighting against each other. Also, in the particular >form of kart racing I do, our motors are only producing >about 9HP. Getting on the throttle doesn't have a lot >of affect :) >

Good point :-)

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>> I'd think that this would depend to a large degree on the rear track width >/ >> center of gravity height ratio. It could theoretically go either way. >With >> the extremely low CG RC cars, inside rear lift is generally (always?) an >> oversteer effect on-throttle. I'd imagine the CG is in proportion much >higher >> on a go-kart, so perhaps that would explain why the opposite might be >> happening. > > >CG to track width ratio for oval karts is about 3:10 >(12" vcg, 40" track width). Not sure how that compares >to other vehicles. > >PD

Production cars are typically in the 20-25" vcg, 50-60" track width range, I think, so the CG is a bit higher in general in comparison to the track width. The RC cars are about 20-25mm vcg with approximately 265mm track width, which is a very low ratio. (Amazingly enough these cars can flip sideways when traction is really good :-)) Todd Wasson Racing Software http://PerformanceSimulations.com My car sim http://performancesimulations.com/scnshot4.htm

J. Todd Wasson <[email hidden]> wrote in message news:[email hidden]... > > Interesting about the rear axle flex adjustment. > > The RC cars have independant front and rear double wishbone suspensionsystems, > so there isn't a similar adjustment for them. They have two halfshaftsfor a > driveshaft that rotate at the same speed, so it acts like a solid axle inthat > regard. >Every RC I have ever had from 12 scale electric to 1/8 scale gas had a diff. They would be terrible without it. Dave

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>> The RC cars have independant front and rear double wishbone suspension >systems, >> so there isn't a similar adjustment for them. They have two halfshafts >for a >> driveshaft that rotate at the same speed, so it acts like a solid axle in >that >> regard. >> >Every RC I have ever had from 12 scale electric to 1/8 scale gas had a diff. >They would be terrible without it. > >Dave >

The solid axle rears I'm referring to are on the 4WD cars with one-ways driving the front wheels. Todd Wasson Racing Software http://PerformanceSimulations.com My car sim http://performancesimulations.com/scnshot4.htm

J. Todd Wasson <[email hidden]> wrote in message news:[email hidden]... > >Every RC I have ever had from 12 scale electric to 1/8 scale gas had adiff. > >They would be terrible without it. > > > >Dave > > > > The solid axle rears I'm referring to are on the 4WD cars with one-waysdriving

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> the front wheels. > > > > Todd Wasson > Racing Software > http://PerformanceSimulations.com > My car sim > http://performancesimulations.com/scnshot4.htm > >

The 1/8 scale gas car I had came with a rear diff and front diff. It did have one way bearings to only allow the fronts to drive when the rears rpm ellisped the fronts. Dave

>The 1/8 scale gas car I had came with a rear diff and front diff. It did >have one way bearings to only allow the fronts to drive when the rears rpm >ellisped the fronts. Oh, ok. Yes, there are a lot of ways to do things. You can stick one-ways and diffs just about anywhere you want, I suppose. Todd Wasson Racing Software http://PerformanceSimulations.com My car sim http://performancesimulations.com/scnshot4.htm

Doug Milliken directed me to section 2.9 of RCVD, which is (I think) at least partly the explanation of side bite. Section 2.9 deals with induced drag caused by tire deformation (slip angle). According to my rough calculations, 10 to 15% of the HP available on a stock 4-cycle kart with a good conventional setup will be needed to overcome the induced drag of the tires in a corner. As the kart comes closer to bicycling (100% of the total weight transferred to the right side of the kart), that induced drag might take up 30% of available HP. With too much side bite, I think the kart is transferring more weight onto the right side than is needed for the tires to generate sufficient grip. That should allow for faster corner speeds, but a kart doesn't have enough inertia or HP to overcome the induced drag and take advantage of the extra grip. Conventional wisdom in karting is that too much grip will slow you down. People do lots of weird things to kart tires to "take bite out of them". In all the changes and adjustments that you see, what they are really doing is increasing the cornering stiffness of the tire, which reduces slip angle. I know that none of this has much to do with simulators, but it would be nice to get some feedback to see if my thoughts are making any sense. Pat Dotson [email hidden] (Pat Dotson) wrote in message news:<[email hidden]>...

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> The other thread broke down into a discussion of kart setup and weight > transfer. I'd like to concentrate on the side bite issue. > > Paul Laidlaw pointed out that lateral force v. vertical load is not > linear (I was thinking of slip angle curves at low levels of slip). > But the force v. load curves I have seen (Gillespie) don't go to zero. > The bottom of the curve starts at 200 lbs vertical load. It is > approximately linear up to about 400 lbs. Most karts weight less than > 400 lbs total. The most load any kart tire is ever likely to see is > 200 lbs. Unfortunately, I have not been able to find any actual kart > tire data. Still, I think we can assume a linear load v. lateral > force curve up to 200 lbs. > > If that's true, then there isn't any inherent advantage to balancing > the left/right load, is there? The key to getting the most grip is to > have the right compound on the right side given the load while > cornering.