Self aligning torque and steering feel

[Tommy Cookers]

surely (with road tyres) the contact patch distorts such that the resultant force migrates to reduce the effective mechanical trail ?
ie about 50 mm of the mechanical trail can be 'lost' in this way
it has been said (of 1930-50s cars) that the SAT could and did go negative

EDITED (a lot)
IIRC a reduction in mech trail to 85 mm? more-or-less cancelled SAT when on the track (manual steering of course)
this reduction was a byproduct of lower profile tyres (the best value performance mod for trackdays)
because with the typical combination of steering axis angle and the amount of axle offset forward of steering axis
(designed IIRC to cause vehicle weight eg when parking to reduce steering 'weight' by part-opposing SAT ?)
a small reduction in tyre diameter caused a rather large reduction in trail

[olefud]

surely (with road tyres) the contact patch distorts such that the resultant force migrates to reduce the effective mechanical trail ?
ie about 50 mm of the mechanical trail can be 'lost' in this way
it has been said (of 1930-50s cars) that the SAT could and did go negative

IIRC a small reduction in mech trail to 85 mm caused a scary reduction in SAT (beyond the usual scary reduction)
reducing trail will greatly reduce the inbuilt steering-geometric components of vehicle weight that always tend to emulate SAT

At least during the thirties with solid front axle the cars displayed noticeable positive camber. This was to locate the kingpin axis within the tire contact patch. While SLA suspension has made kingpin axis location much easier, there are still examples of offset wheels causing some unsteady steering conditions.

[Erunanethiel]

So as far as I understand, you can use pneumatic trail to gain feel (hardening the steering feel?) when you run out of SAT.
But when you do that, again how are you going to know when the tires lost grip?

[Greg Locock]

"So as far as I understand, you can use pneumatic trail to gain feel (hardening the steering feel?) when you run out of SAT."

No.

SAT=Fy*pneumatic trail

As PT goes to zero so does SAT

[Erunanethiel]

"So as far as I understand, you can use pneumatic trail to gain feel (hardening the steering feel?) when you run out of SAT."

No.

SAT=Fy*pneumatic trail

As PT goes to zero so does SAT

I thought it was the other way around? Like when one is going up, other one is going down.

[Greg Locock]

Well, you were wrong.

Fy increases with slip angle to some point, which various people confusingly refer to as some sort of limit. thereafter it dies off a bit, generally settling at say 80% of the peak value.

PT start quite high, say at 60mm, and then drops to around 30mm for typical cornering efforts, to 0mm at the peak Fy.

SAT is the product of the two, so it starts at zero, because Fy is zero at zero slip, and ends at zero, because PT is zero, at the maximum lateral force peak.

I'm guessing it remains near zero thereafter - easily checked on a tire F&M plot.


Therefore according to the equation I gave earlier, SWT increases as you approach FyMax, but starts to drop sooner or later, depending on how much mechanical trail you have. For a race car it would seem wise to reduce MT so the driver can ride around FyMax confidently, for a road car we might use anything up to 30mm of MT to give a more linear feel to the car, such that the SWT is related to Fy directly. I have heard development engineers claim that less MT is good, for production cars, but in my limited opinion I can't claim that 15 mm seems any easier to drive hard than 28mm. There again I am pretty rubbish at evaluating limit handling in real life, if I don't crash it it must be OK.

Note that this ignores complexity like camber steer and ply steer because they are small beer.

[Erunanethiel]

Well, you were wrong.

Fy increases with slip angle to some point, which various people confusingly refer to as some sort of limit. thereafter it dies off a bit, generally settling at say 80% of the peak value.

PT start quite high, say at 60mm, and then drops to around 30mm for typical cornering efforts, to 0mm at the peak Fy.

SAT is the product of the two, so it starts at zero, because Fy is zero at zero slip, and ends at zero, because PT is zero, at the maximum lateral force peak.

I'm guessing it remains near zero thereafter - easily checked on a tire F&M plot.


Therefore according to the equation I gave earlier, SWT increases as you approach FyMax, but starts to drop sooner or later, depending on how much mechanical trail you have. For a race car it would seem wise to reduce MT so the driver can ride around FyMax confidently, for a road car we might use anything up to 30mm of MT to give a more linear feel to the car, such that the SWT is related to Fy directly. I have heard development engineers claim that less MT is good, for production cars, but in my limited opinion I can't claim that 15 mm seems any easier to drive hard than 28mm. There again I am pretty rubbish at evaluating limit handling in real life, if I don't crash it it must be OK.

Note that this ignores complexity like camber steer and ply steer because they are small beer.

Oh so you can choose how close you want the SWT to change when approaching to FyMax? Like I can even do it decrease at the same time I past max Fy by changing the PT?

[Greg Locock]

No you tune it by changing MT, PT will always be near zero at FyMax

[Erunanethiel]

No you tune it by changing MT, PT will always be near zero at FyMax

What is MT?

[WilO]

Mechanical trail.

[Erunanethiel]

Mechanical trail.

What is it and what does it have to do with pneumatic trail?

[Greg Locock]

it is the distance in side view along the ground between where the steering axis strikes the ground and the nominal centre of the contact patch. It is part of the suspension geometry.

[Erunanethiel]

it is the distance in side view along the ground between where the steering axis strikes the ground and the nominal centre of the contact patch. It is part of the suspension geometry.

So, if pneumatic trail and SAT, everything that gives you feedback runs out (or decreases) BEFORE you reach peak Fy, how are you going to know when the Fy is?

[Greg Locock]

Stop being lazy, use the equation I provided and the values you have been given, and work it out.

[-Felix-]

I know it's a complicated field and you'll need time to understand all effects and their influences between each other, but there are so many good books out there covering the basics. Grab some. And be patient, it doesn't come to you over night, not even weeks.