Steering Paradox

[Claude Rouelle]

Z,

Are you suggesting that if the more heavily loaded outer tyre reaches peak lateral force (peak Fy) at, say, 1 degree greater slip-angle than the inner tyre, then the steering should be anti-Ackermann (ie. the wheels should toe-in when steered away from straight ahead)???

Yes I do (camber is not taken into account in this simplified reasoning neither the influence that higher slip angle and the Fx increased "rolling resistance" on the inside wheel will have on the yaw moment. Our balance vs grip method which take the tire Fx and Mz into account shows the same trends.

That is what I believe in and that is what i have experienced on race tracks. For example when I race engineered F3 on Michelin tires (in 2 years we won more than 50 % of the races we competed in and both championships) where the tire models and data clearly showed that the more you loaded the tire the more slip angle you needed to get the most Fy, we always got quicker with anti-Ackerman geometry. In fact the front became "too good" (driver comment) and we had the work on the rear to use this additional front grip and get back to the same balance; same yaw moment, more grip. That was even more true on tight circuits like Pau or Monaco. Quantitatively, the amount of need anti-ackerman needed was a function of the track grip and a bit the driver style. I went racing with the same car in Germany where the mandatory tire was a different one and where the more you loaded the tire the less slip angle we needed; there a pro Ackerman worked better.

Maybe I was right but for the wrong reason(s) and I am always ready to listen but so far that is what my reasoning and experience is. A few successful Le Mans and F1 engineers I discussed this with also thinks this way, although if all of them agreed on the principle some of them also say that "it is not that simple..they are other factors starting with tire temperature and tire temperature distribution. These guys have extremely sophisticated tire thermal models. I agree and that is why using tire IR temperature sensors while testing different Ackerman will teach you a lot. But these engineers and I could all be wrong. What is your take?

Claude

Claude

[Z]

Claude, (and anyone else interested ),

For what follows, let's define "Ideal (or 100%) Ackermann" to mean that when the steering-wheel is turned away from straight ahead, lines drawn through both front axles always intersect the extended rear axle line at the SAME POINT. So for low speed, no-slip cornering this point is the "Instant Centre" for plan-view motion of the car body wrt ground.

Furthermore, note that for a typical wheelbase/track FSAE car to be able to corner "between kerbs" ~6m apart, with no tyre slip, the outer-front wheel must steer ~30 degrees, and the inner wheel ~45 degrees. This puts the car in the middle of the track at a FSAE hairpin, which in the rules has 9m OD and 0m ID (ie. outer-front wheel follows path ~6m dia., and inner-rear wheel ~3m dia.).

Mike Cook suggested above that based on typical tyre data the front wheels should steer ~1 degree less than required by "Ideal Ackermann" (Mike, correct me if I misunderstood). So at full-lock perhaps only 14 degrees of "dynamic toe-out" is required, rather than 15 degrees. However, given Mike's example of the Skid-Pad where only 1 deg toe-out was required rather than 2 deg, it might be interpreted (by lazy students!) that only half, or 50%, Ackermann is required.

Claude, in his post, seems to categorically recommend that the wheels should be toed-in when steered, giving anti-Ackermann (again, Claude, correct me if I misunderstood). This means that at full-lock there is MORE than 15 degrees difference between the "Ideal" and actual steer angles.

I say that "tyre curves" are largely irrelevant to FSAE steering geometry, and the maximum practical amount of pro-Ackermann (dynamic toe-out) should be sought. Here is why.

1. During high speed (ie. large radius) cornering, the front wheels are hardly steered at all. Therefore, their relative toe angles are determined mainly by the "static" setting, and only slightly modified by any Ackermann geometry. High speed sweepers are typical of high profile race series, such as F1, and the F3 mentioned by Claude. Here toe-in of the front wheels can reduce the "slip-angle drag" from the inner wheel, and allow more efficient use of the outer wheel, and so increase speeds. BUT! I am not aware of any really high speed sweepers in FSAE.

2. The rules of FSAE state that there will be (possibly many) hairpins as described above, on any AutoX and Endurance track. Assume that when going through these hairpins, the rear, and outer-front wheels have, say, 6 deg slip-angle for their peak Fy. Then with parallel-steer the inner-front wheel might be forced to run at well over MINUS 6 degrees. That is, it pushes the nose of the car OUTWARDS with all its available force! Admittedly, the more heavily loaded outer-wheel wins this fight, but still not good. Even worse with anti-Ackermann.

3. Tyre "Fy vs Slip-angle" curves don't have sharp "peaks". Typically they have broad, rounded tops. Some just gently reach a plateau and stay there (correct me if wrong for FSAE tyres). This means that when running at +/- 1 degree away from the "peak" the tyre has almost as much Fy as at the peak. However, the change in steer-angle makes a much bigger difference to the car-rearward component of Fy (ie. slip-angle drag, which increases 20% going from 5 to 6 degrees). Repeating this, the cornering (centripetal) force changes very little, but car-coordinate drag force (not tyre rolling drag!) changes a lot. This can be a problem in that it can upset the yaw balance between the two front wheels. However, I don't think this is a big issue on FSAE type tracks (see 5 below).

4. Most R&P steering linkages can NOT track the Ideal Ackermann curve to less than +/- 5 degrees (ie. they are ~5+ deg "wrong" at some steering position). This swamps any +/- ~1 deg effect of tyre "peak-Fys". So get the geometry right first...

5. And I haven't even got onto "transient" effects yet (ie. yaw acceleration), which is important in FSAE because of all the changes in direction. Briefly, more dynamic toe-out (pro-Ackermann) is better. And there is also the issue of yaw stability in steady-state cornering, where again it is better to have the inner-wheel "saturated", or past its peak (this perhaps only relevant to Skid Pad, because not much steady-state in AutoX or Enduro.)

Bottom line is that anti-Ackermann might suit "real" racecars on full-sized circuits, but that ain't FSAE.
~~~o0o~~~

Now, all the above probably sounds like a load of "theoretical" codswallop.

So I leave it to Carroll Smith, who I recall at his last FSAE event publicly saying to the teams something like "FSAE cars should have as much pro-Ackermann as you can get from a conventional steering linkage, because even that is not enough."

Z

[Claude Rouelle]

Claude, in his post, seems to categorically recommend that the wheels should be toed-in when steered, giving anti-Ackermann (again, Claude, correct me if I misunderstood).

NO I never said that! I said that Ackermann choice is mainly tire specific. Some tire (mainly race radial tire new generation) requires more anti-Ackerman and some like cross play require pro Ackerman. It depends on your tire.

More to come

Claude

[Moop]

Originally posted by Z:
Then with parallel-steer the inner-front wheel might be forced to run at well over MINUS 6 degrees.

I'm not seeing this - what makes you say this?

I can say for sure that in my simulations I've never seen anything like this happen, even with only about 5 deg ackermann at 25 deg inside wheel steer angle. At least, not anywhere to yaw moment = 0

[Mike Cook]

Like he mentioned, in a hairpin the inside might need to be steered 45deg and the outside 30deg. With parallel steer, they get steered the same. If the outside tire has 6 deg of slip, then the inside tire would have 45-30-6 = 9 degrees of slip in the wrong direction (i.e. pushing the car out of the corner).

[Z]

Thanks Mike, that's what I was getting at.

Students might try slowly pushing their car, with steering at full lock, around a hairpin with a sandy surface. If the steering has insufficient Ackermann, then both front wheels will "snow-plough", ie. slide with lots of toe-in.

Z

[Marvel]

Originally posted by Mike Cook:
Like he mentioned, in a hairpin the inside might need to be steered 45deg and the outside 30deg. With parallel steer, they get steered the same. If the outside tire has 6 deg of slip, then the inside tire would have 45-30-6 = 9 degrees of slip in the wrong direction (i.e. pushing the car out of the corner).

I am not sure but still...will that matter when (in the sharp corners), you try to completely unload your inner wheel or try to get some greater amount of lateral load transfer??

[Z]

Originally posted by Marvel:
... you try to completely unload your inner wheel...

Marvel,

Yes, that is one solution. Lift the inner-front-wheel and it doen't matter what Ackermann you have.

BUT! now you have lost one means of adjusting handling balance (under/oversteer). Furthermore, if you are running a "spool" differential, then typically you want to lift the inner-rear-wheel, which means similar vertical loads on both fronts.

Don't paint yourself into a corner.

Z

[The Stigg]

Hi Guys and especially Z,

here are my 2 cents on this topic:

Slip angle drag through kinematic steering design at the inside front tyre for FSAE is one of the most underestimated tuning factors I've ever seen.
We tested this over the last 3 years and it provided at least the biggest lap time improvement for us. BTW - by far more than any roll-center displacement optimization at all.

And here is why:

1. FSAE is about yaw acceleration. slip angle drag and therefore fx can be build up more than a 1th(depends on tyre) of a second faster than fy!!!!

2. The heat generated by this can have an immense impact on your laptimes especially in the first 2-5 laps.

That's why I would totally agree with Carroll!

Originally posted by Z:

So I leave it to Carroll Smith, who I recall at his last FSAE event publicly saying to the teams something like "FSAE cars should have as much pro-Ackermann as you can get from a conventional steering linkage, because even that is not enough."

[zh]

so finally, can we say that amount of ackerman percentage we choose depends only on tire graphs