Car physics: Toe in?

Hello car physicists! I have question about optimum steering geometry for maximum lateral force in steady state cornering. My question is about the inner forward wheel and what our steering angle of that should be. On heavy cornering with accelerations, like g > 1, we will, on most cars I guess, have quite a bit of load transfer which I presume would give the outer wheels the most to say about slip angles, lateral force etc. The total cornering is of course the sum of inner wheels and the outer wheels. Anyhow, what I'm aiming at, is that the outer wheels should prety much go at the required slip angles for the required lateral acceleration. This leaves the inner wheels going att the established vehichle attitude plus, for the front, what steering angle we _happen_ to have. The question is, shouldn't we set up our steering geometry such that the steering angle for the inner wheel is making as much lateral force as possible out of it's situation? For most (all?) tires low load seems to give maximum lateral force to be reached at a lower slip angle than with high load - which should then sugest less steering on the inner wheel -> toe in. Is that your conclusion as well? This might be bashing in open doors as everyone seems to be running with toe in anyway (tough the have not told me why). -- Anders Karlsson

Sten Anders Karlsson <[email hidden]> wrote in message news:alv5u0$6sv$[email hidden]...

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> Hello car physicists! > The question is, shouldn't we set up our steering geometry such that > the steering angle for the inner wheel is making as much lateral > force as possible out of it's situation? > > For most (all?) tires low load seems to give maximum lateral force to be > reached at a lower slip angle than with high load - which should then > sugest less steering on the inner wheel -> toe in. Is that > your conclusion as well?

Sounds reasonable, but is sometimes impractical IRL. First off, note that toe-in is only really significant at near-zero steering angles since beyond that, the Ackermann geometry (or lack thereof) takes over. For road cars, the need to avoid tyre scrub at low speeds and large steer angles means that a fair amount of Ackermann (toe-out-on-turns) geometry is required. With race cars, I'm given to understand that low or even negative Ackermann may be used to give an amount of toe-in at larger lock values (I once tried some increased-Ackermann steering bellcranks on my R/C car, and didn't like them) but packaging constraints may limit this... Jonny

For whatever reason, a fair amount of toe-out in front seems to be more the rule for road racing. Generally it is said to help the car turn in, and on that note remember that in road racing you spend very little time in pure cornering, most of the time is spent under diagonal loading - decelerating while turning in, accelerating while exiting. So whatever you do with the front wheel geometry is going to have much more of an affect on turn-in than it will elsewhere in the corner. It does seem though, like if you already have Ackerman steering to properly account for the turn geometry, then the harder you push the car and the more it yaws outward, the less toe you would want, consequently adding some static toe-in in front should help. But that's not what people do, and in playing around with it in various sims, it does seem that adding some toe-out can make a dramatic difference in a car's turn-in behavior. In fact, the few times I have tried front toe-in in a setup the results have been horrible, the front end just would not bite. The closest I can come to an explanation is this: if a tire is lightly loaded vertically, it will require more slip angle to generate the same amount of side force as a heavily loaded tire (hope I got that right, I don't have my reference books handy). In that case the only way to get the inside tire to make a useful contribution to turn-in is to turn it more into the corner, i.e. use toe out. The fact that it may compromise ultimate cornering power is somewhat irrelevant because you spend relatively little time in pure cornering. Not 100% sure that makes sense, but that's the best I can do right now. ;o) "Sten Anders Karlsson" <[email hidden]> wrote in message news:alv5u0$6sv$[email hidden]...

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> Hello car physicists! > > I have question about optimum steering geometry for maximum lateral > force in steady state cornering. > > My question is about the inner forward wheel and what our steering > angle of that should be. > > On heavy cornering with accelerations, like g > 1, we will, on most > cars I guess, have quite a bit of load transfer which I presume would > give the outer wheels the most to say about slip angles, lateral force > etc. The total cornering is of course the sum of inner wheels and the > outer wheels. Anyhow, what I'm aiming at, is that the outer wheels > should prety much go at the required slip angles for the required > lateral acceleration. > > This leaves the inner wheels going att the established vehichle > attitude plus, for the front, what steering angle we _happen_ to > have. > > The question is, shouldn't we set up our steering geometry such that > the steering angle for the inner wheel is making as much lateral > force as possible out of it's situation? > > For most (all?) tires low load seems to give maximum lateral force to

be

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> reached at a lower slip angle than with high load - which should then > sugest less steering on the inner wheel -> toe in. Is that > your conclusion as well? > > This might be bashing in open doors as everyone seems to be running > with toe in anyway (tough the have not told me why). > > -- Anders Karlsson

Another effect that can come into play with toe-out is the increased induced drag on the inside tire on turn-in, which is an oversteer moment. "Haqsau" <[email hidden]> wrote in message news:alvm0m$1fmbo$[email hidden]...

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> For whatever reason, a fair amount of toe-out in front seems to be more > the rule for road racing. Generally it is said to help the car turn in, > and on that note remember that in road racing you spend very little time > in pure cornering, most of the time is spent under diagonal loading - > decelerating while turning in, accelerating while exiting. So whatever > you do with the front wheel geometry is going to have much more of an > affect on turn-in than it will elsewhere in the corner. It does seem > though, like if you already have Ackerman steering to properly account > for the turn geometry, then the harder you push the car and the more it > yaws outward, the less toe you would want, consequently adding some > static toe-in in front should help. But that's not what people do, and > in playing around with it in various sims, it does seem that adding some > toe-out can make a dramatic difference in a car's turn-in behavior. In > fact, the few times I have tried front toe-in in a setup the results > have been horrible, the front end just would not bite. The closest I > can come to an explanation is this: if a tire is lightly loaded > vertically, it will require more slip angle to generate the same amount > of side force as a heavily loaded tire (hope I got that right, I don't > have my reference books handy). In that case the only way to get the > inside tire to make a useful contribution to turn-in is to turn it more > into the corner, i.e. use toe out. The fact that it may compromise > ultimate cornering power is somewhat irrelevant because you spend > relatively little time in pure cornering. > > Not 100% sure that makes sense, but that's the best I can do right now. > ;o) > > > > "Sten Anders Karlsson" <[email hidden]> wrote in message > news:alv5u0$6sv$[email hidden]... > > Hello car physicists! > > > > I have question about optimum steering geometry for maximum lateral > > force in steady state cornering. > > > > My question is about the inner forward wheel and what our steering > > angle of that should be. > > > > On heavy cornering with accelerations, like g > 1, we will, on most > > cars I guess, have quite a bit of load transfer which I presume would > > give the outer wheels the most to say about slip angles, lateral force > > etc. The total cornering is of course the sum of inner wheels and the > > outer wheels. Anyhow, what I'm aiming at, is that the outer wheels > > should prety much go at the required slip angles for the required > > lateral acceleration. > > > > This leaves the inner wheels going att the established vehichle > > attitude plus, for the front, what steering angle we _happen_ to > > have. > > > > The question is, shouldn't we set up our steering geometry such that > > the steering angle for the inner wheel is making as much lateral > > force as possible out of it's situation? > > > > For most (all?) tires low load seems to give maximum lateral force to > be > > reached at a lower slip angle than with high load - which should then > > sugest less steering on the inner wheel -> toe in. Is that > > your conclusion as well? > > > > This might be bashing in open doors as everyone seems to be running > > with toe in anyway (tough the have not told me why). > > > > -- Anders Karlsson >

> I have question about optimum steering geometry for maximum lateral > force in steady state cornering. > > My question is about the inner forward wheel and what our steering > angle of that should be. The other posters have mostly answered all this but what hasn't been said is that on road cars at least some of the toe in/out is there to allow for compliance in the streering/suspension, in other words it reduces on the move. Paul

Partially true. Suspension compliance on a typical RWD car would normally cause the fronts to toe out more and the rears to toe in more (under power). So static front toe-in is normally used to counter suspension compliance, but AFAIK static front toe-out is strictly for handling and static rear toe-in is strictly for stability. "Paul Laidlaw" <[email hidden]> wrote in message news:am5jju$2qfi4$[email hidden]... > > The other posters have mostly answered all this but what hasn't beensaid > is that on road cars at least some of the toe in/out is there to allowfor > compliance in the streering/suspension, in other words it reduces onthe

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> move. > > > Paul > > >

"Haqsau" <[email hidden]> wrote in message news:am5qnu$2vlij$[email hidden]... > Partially true. Suspension compliance on a typical RWD car would > normally cause the fronts to toe out more and the rears to toe in more > (under power). So static front toe-in is normally used to counter > suspension compliance, but AFAIK static front toe-out is strictly for > handling and static rear toe-in is strictly for stability. I thought cars with positive offset steering (RWD) toed in on the move and cars with negative offset (FWD) toed out? Am I missing something? I wonder what toe setting (proper) citoens have? They have zero offset steering I believe. Although I guess compliance in the steering could work the other way to the compliance in the steering. But yes, I fully agree that it has an effect on handling. Anyway, I did say some :) > > The other posters have mostly answered all this but what hasn't beensaid > > is that on road cars at least some of the toe in/out is there to allowfor > > compliance in the streering/suspension, in other words it reduces on the > > move. Paul

It is always possible I am totally and completely wrong!!! IIRC positive offset means that the steering axis, when projected through the ground, falls inboard of the center of the contact patch. If that is true, then the drag force on the front wheels, which pulls them rearward when in motion, should cause them to toe out. In fact even with zero offset I think you will get dynamic toe out just due to the compliance in the lower control arm bushings. OTOH, I could be completely full of it. ;o) "Paul Laidlaw" <[email hidden]> wrote in message news:[email hidden]... > > I thought cars with positive offset steering (RWD) toed in on the move > and cars with negative offset (FWD) toed out? Am I missing something? > I wonder what toe setting (proper) citoens have? They have zero offset > steering I believe. Although I guess compliance in the steering couldwork

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> the > other way to the compliance in the steering. > > But yes, I fully agree that it has an effect on handling. > > Anyway, I did say some :) > > Paul > >

> "Paul Laidlaw" <[email hidden]> wrote in message > news:[email hidden]... > > > > I thought cars with positive offset steering (RWD) toed in on the move > > and cars with negative offset (FWD) toed out? Am I missing something? Haqsau <[email hidden]> wrote in message news:am8lnm$3p8ml$[email hidden]...

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> It is always possible I am totally and completely wrong!!! IIRC > positive offset means that the steering axis, when projected through the > ground, falls inboard of the center of the contact patch. If that is > true, then the drag force on the front wheels, which pulls them rearward > when in motion, should cause them to toe out. In fact even with zero > offset I think you will get dynamic toe out just due to the compliance > in the lower control arm bushings. OTOH, I could be completely full of > it. ;o)

Both right! RWD cars generally run +ve offset, so wheel drag causes toe-out on the move. FWD cars run -ve offset, and the tractive effort generated by the front tyres causes toe-out... Jonny