UTAS build thread

[BillCobb]

Your self induced steering oscillation is probably an aligning torque camber driven phenomenon. Since caster is a steer-camber coupling mechanism, the Fx force driver can cause this high 'Q' vibration.

Just because your scrub radius is identified geometrically at some layout position, it doesn't mean that its 40mm under actual operating conditions. There is a load, pressure, camber and rim width sensitive Mx contribution to this dimension, too. I believe I posted some sample Mx/Fz function plots on the TTC Forum. If not, I can do so.
The reality is that all the TTC data is nice an tidy in a 2 axis plot but not actually relevant to a rolling tire on a flexible (and compliant) multi-bar suspension platform. You need to incorporate it into a dynamic or quasi-static model where the multi-surface tire characteristics bloom. Sure, in most all cases superposition works, but the Fz, alpha and gamma drivers need to be there. I'm pretty sure I posted such a 4D tire surface Matlab spline Simulink model of a tire test machine on the TTC site. Feed that with some open loop inputs from a simulated static braking test and comment on what you find. There could be +- 50 mm of dynamic S.R. variation and you should know by now what that means to your '40 mm scrub radius' label.

That's my positive feedback for now....

[MCoach]

Funny that you mention steer camber effects...

There could be +- 50 mm of dynamic S.R. variation and you should know by now what that means to your '40 mm scrub radius' label.

And I would absolutely believe that. When the tire is allowed to shift the entire car because of floppy sidewalls, I am (after this year) well aware that 40m scrub radius can easily become 100mm or more while running. However this deflection can also aid in covering some goof ups like not enough or too much camber gain because the soft tires will do what they want and stand up on the track. There's some decent pictures from several competitions. Some cars on 10s have the wheel cambered several degrees and the tire is essentially straight up.

As for our fix, we reduced camber gain (longer FVVSA) by two increments, I don't think we adjusted caster, increased tire pressure by about 1.5-2.5psi, and stiffened the front springs to compensate for the roll center/camber gain reduction. The rear was rebalanced with respect to total lateral load distribution (TLLTD). This got us most of the way there and allowed us to hang on to the thing. The icing on the cake was some ackermann shim and it went completely away. We had a chassis tube added which left us with goofy steer arm geometry which may have had something to do with the last bit there...

We may have video around of the phenomena. I'll check this weekend.
I know that Auckland runs the same wheels, so I'm curious if they have run into this issue as well. I'd love to hear their input. Maybe they were too smart to fall into this pitfall. :P

Thanks for the input Bill.

[Jay Lawrence]

Bill you mentioned the use of negative caster earlier. I asked a question on this a while back on another thread but I don't think I got an answer: wouldn't negative caster tend to attempt to rotate the tyre in the turned direction, rather than provide some resistance (i.e. steering feel, all else being equal), thereby creating an eerie unloading as you negotiate a corner?

[BillCobb]

Yes it would. Pure and simple front lateral force oversteer. But wait, it's easier to steer. Its more nimble. The oversteer effect may not be that disasterous if your front cornering compliance budget is managed well or if your ground speeds are below the Critical Speed (and that was the reason you are needing to do more maneuvering anyways). BTW this is a good example of the benefit of having separate front and rear tire and wheel and pressure designations.

And finally, an oversteering car is not necessarily unstable in open loop control. It's only if the amount of total vehicle oversteer is greater than the Ackerman Gradient at a designated speed.

My '58 Chevy Bel Air and my 1973 Corvette had manual steering. Now go figure the rest. The Stingray was a lot of fun on X-way exit ramps until I put that dammed heavy duty rear spring in it....

Oh and I almost forgot: Go out into your garage, sit in your car seat, engine on, transmission in Park (You ain't going anywhere). Now turn the steering wheel 45 degrees to the right and then 45 degrees to the left. When you turned to the right (That should be clockwise) which way did the left front fender go??? Right or left? A simple answer will do.

[mech5496]

I am (after this year) well aware that 40m scrub radius can easily become 100mm or more while running.

I believe it would get to zero or negative (loaded side) easier than you could get 100 (unloaded side).

Yes it would.

What if running negative caster but positive trail?

[CWA]

Interesting, MCoach. Would be great if you can find a video. I wonder if you have a picture of the chassis tube and your ackermann layout too?

Jay, when Bill described the use of negative caster, I had understood that this was to achieve a negative mechanical caster trail, to reduce the effective total trail, by 'eating into' the pneumatic trail. So the total trail is lessened, but will still be positive/conventional, and so still provide stability/self-centring torque, and not what you are describing Jay. The total effective trail value will be less than the pneumatic trail, not greater than it, as is the case with conventional caster. Bill, correct me if I've misunderstood.

"Oh and I almost forgot: Go out into your garage, sit in your car seat, engine on, transmission in Park (You ain't going anywhere). Now turn the steering wheel 45 degrees to the right and then 45 degrees to the left. When you turned to the right (That should be clockwise) which way did the left front fender go??? Right or left? A simple answer will do."

When steering right (c/w) the left fender goes right (and down) - indicating some positive (conventional) mechanical trail (and some steer jacking)..

[DougMilliken]

... Under threshold braking the car would enter a "death wobble", typically not seen on four wheeled vehicles. ...

There are many possible oscillators in automotive suspension/steering/chassis. Olley wrote about some of these and we combined his several monographs into one chapter in "Chassis Design" (SAE R-206) -- Chapter 6 Oscillations of the Unsprung, about 70 pages. His examples seem like a good way to start thinking about these problems -- although you haven't said enough to say with certainty that he discusses your specific problem.

[BillCobb]

Since all these dimensional factors are the same order of magnitude, just imagine what can be speculated on. Oh, and did someone forget to mention caster offset (Not to be confused with caster angle) ? Just be sure to reset the outer tierod ball height if you are evaluating caster changes on a car because the ride and roll steer parameter changes may be more influential to the evaluation that just a pure caster change.

The reason I suggested you steer the car and watch the fenders is because not all cars act the way yours did. Some fenders will head left when steered to the right. I have a front trailer hitch on one of them and lining up a trailer coupler can still drive me nuts if I need to hook up to push it somewhere.

[mdavis]

We may have video around of the phenomena. I'll check this weekend.
I know that Auckland runs the same wheels, so I'm curious if they have run into this issue as well. I'd love to hear their input. Maybe they were too smart to fall into this pitfall. :P

Thanks for the input Bill.

USF and Cincinnati also run those wheels. I know Auckland was running the red label (heaviest and stiffest of the dwt wheels) while usf and Cincinnati run blue labels (thinner and lighter). This may also have something to do with the shimmy. Apparently Douglas does make 5" inner/2" outer blue labels roughly once a year, but all of the wheels are paid for before they are produced and they end up shipping to Europe. You may be able to get in on that order if you have access to your money year round (our team did not) and can afford to pay Douglas and wait months for wheels.

That said, we ran less sr in 2013 and didn't have any issue with shimmy under braking. We had a lot of other front end issues, namely mid corner, but nothing like you describe.