How much toe change during suspension travel is allowable?

[ETUVBOB]

Hi all,

I am currently working on setting up a front suspension for a light duty utilty vehicle the vehicle weighs about 1700 lbs and is rated to travel at a max velocity of 35 mph on mostly paved surface with some off road capabilities.

in designing front suspension i am wondering about steering geometry error. I have the suspension modeled in a 3d cad software where i am able to put the suspension through jounce and rebound and see how the camber, toe etc is changed.

I am wondering how much toe change error is allowable in the application described above? Right now when the suspension is at full jounce (4in of travel) the angle of the spindle with respenct to the center line of the vehicle is changing about 0.6 degrees and when the suspension is in full rebound (2in of travel from ride height) the toe changes about 1.5 degrees both changes are toe out. Do these numbers seem dangerous or is it a safe set up?

Any advice on this topic would be greatly appreciated

Thanks

ETUVBOB

 

[GregLocock]

This is a front suspension?

I'd try and get it more symmetrical, putting a knuckle in it like that near the nominal ride height will make the steering very sensitive to axle load.

1.5 degrees is equivalent to about 30 degrees at the steering wheel - do you really want that much steering when one wheel drops into a pothole? I wouldn't.

1.5 deg in 2 inches is 30 deg/m, very much towards the high end of values I have seen, if you look at modern sedans you'll see toe curves more like 4-5 deg/m, very linear at the front (toe-out in jounce, toe-in in rebound), often rather curvy at the back, with the sort of shape you are describing, but less exaggerated.

if this doesn't answer your questions, could you describe the suspension a bit more, and tell us what the other axle looks like as well. Is the vehicle 4wd or 2wd, if so which is the driven axle?

BTW 2 inches rebound travel is not very much, I'd be looking at 4 inches minimum for off road.

Cheers

Greg Locock

 

[ETUVBOB]

this is a 2wd vehicle the rear axle is the drive axle. It is an electric vehicle. The rear drive axle is similar to an electric golf cart rear axle. The front suspension is a double control arm independent with a rack and pinion steering set up.

At this point preformance of the suspension is not a concern. The main concern is the drivabilty of the vehicle. I guess the last two statments contradict each other but what i am saying is that it is not a race machine the vehicle will be used in a parking lot or service road enviroment most of the time. The controller is set so that the vehicle will not exceed 35 mph. The plan is to use this vehicle for transportating people around resorts or to use the vehicle for delivery of supplies on military bases or college campus.

what i got from your reply was that this suspension is not safe and should be revised. Is the suspension set up in such a way that the vehicle will be uncontrollable in a rough parking lot terrain?

 

[GregLocock]

I'm not saying it won't be safe, but it will be excessively sensitive to vertical inputs, hence tiring to drive. 35 mph is plenty fast enough to kill people, so you do have to pay some attention to this.

If you have the freedom to do so then I would recommend changing the rack/tie rod geometry (or the suspension hardpoints) so that you get a more linear curve of toe against suspension travel, and with smaller absolute values. The biggest single change you could probably make would be to move the outer tie rod ball joint outwards (if the rack is behind the axle), which will linearise the curve somewhat and reduce the extreme toe changes seen.

If this is all a bit hard to do in your CAD package then if you look in the FAQ there is a link to a free program that will analyse this stuff for you, called wishbone.bas. Having said that on re-reading your original note it sounds like you are happy with your current approach

Incidentally when optimising hardpoint locations we use a Taguchi DoE approach, this is often more productive than random tinkering!

Cheers

Greg Locock