Toe Control

[Frank]

If you were driving formula SAE car, going around the skid pad

ie steady state (well relatively steady steering angle and turn radius anyhow..)

what angle of toe deflection (at the axle) is acceptable?

0.0-0.2 deg
0.2-0.4 deg
0.4-0.6 deg
0.6-0.8 deg

I'd really appreciate both "gut feeling" responses, and those from teams with experimental testing data

thanks in advance

Frank

[Kevin Hall]

I would say a total of 0.6 - 0.8 deg would be fair, allowing for some slop in your rod ends, etc. I think that is s driveable amount. Cutting that to zero is optimum, and less than the 0.8 deg is better too, but I don't think it would be scary, or seriously hindering of performance to have a little play. after all, we all know how hard it is to get steering free of any play.

Kevin Hall
University of Saskatchewan
'03-'04 Team Director

[Charlie]

Steering play is a totally different story, as it is driver-controllable. Small automatic corrections will be made through the steering wheel for small toe deflections. Rear toe deflection is not correctable and unstable.

To figure out an acceptable amount, see how accurate you set your toe. If the best you can get is 1 degree of accuracy with your setup methods, .5 degrees of toe deflection should be fine. If you want to set it more accurately, there isn't a point unles you can lower your deflection as well.

How are you measuring the deflection?

-Charlie Ping
Auburn University FSAE (http://eng.auburn.edu/organizations/SAE/AUFSAE)
5th Overall Detroit 2003
? Overall Aussie 2003. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Kevin Hall]

Frank

Are you meaning per wheel, or total? I was thinking of the front, and intend that to mean 0.6 - 0.8 deg total, so as Charlie said, 0.5 deg on a wheel should be fine. I am unsure as to how you would eliminate this 100% in our applications. High quality rod ends always offer less play, as do fasteners with shims, etc.

Next question.....how many guys do an actual alignment with a standard 4-wheel alignment computer? That would be the most accurate.

Kevin Hall
University of Saskatchewan
'03-'04 Team Director

[Frank]

our old car saw 100 Nm of rear wheel torque during skidpan (strain gauges / data aq)

we were measuring the rear toe deflection statically

we hard mounted the chassis to the ground

we did two tests

in the first test we applied a point moment to the upright and mounted lasers on the upright

we loaded (incrementally) and unloaded a few times to check repeatability

in the second test we did the same thing, this time loading the hub, and the lasers are mounted in the hollow stub shaft

we found you get an initial free play due to the bearings, and then a linear response from the upright toe link / rod end combination

for a 50 mm steering arm, and high quality 5/16 rod ends, we saw 0.55 deg of (linear) deflection for 100 Nm

we are concerned about the small steering arms, and are going to change to 100 mm, which should cut the linear deflection to below 0.3 deg

the bearing free play depended on what combo we were using, and ranged from immeasurable to a worst case of 0.25 deg (really sloppily mounted C3 clearance deep grooves at 40 mm CL spacing)

the measured cornering torque was actually from our previous car which had a lot of trail (app 60 mm) on the rear, our new car's rear trail is very small (app 20 mm) so hopefully the toque will be a bit less

could anyone with strain gauges on links possibly give me a ballpark figure of their rear steering torque encountered on the skid pan?

regards all

Frank

[frenzy]

Hello everybody,

I've just read "Pat's Column" on the Formula Student Germany page. He mentions problems with rear toe control on some of the cars.
I'm not sure if I've realized what went wrong on the examples:

http://www.formulastudent.de/uploads/RTEmagicC_image5_01.jpg.jpg
http://www.formulastudent.de/uploads/RTEmagicC_image6_02.jpg.jpg

I suppose that the locations where the wishbone and the link for toe control connect to the upright shouldn't be as close together?
It would be "easier" to withstand tire deflection if the link had a bigger lever arm.

(here is the complete post:
http://www.formulastudent.de/p...ts-column-january-3/ (http://www.formulastudent.de/public-relations/fsg-news/news-details/article/pats-column-january-3/) )

[HenningO]

Whoa, bringing back a 6 year old post (good thing though, I got to take part of Frank's data).

frenzy, you are right. The larger lever arm needs only to be bigger on the upright side though. Make sure though the lever arm itself is stiff enough http://fsae.com/groupee_common/emoticons/icon_smile.gif

A very good point that Frank made, which can be useful to remember, is the torque around the steering axis (which also exists in the rear) can be significantly lowered with appropriate suspension geometry. By keeping the mechanical/pneumatical trail and the scrub radius small, the torque produced by the longitudinal and lateral forces of the tire at the CP will not generate a very big (self-aligning) torque, thus less compliance.