Inclinations.

[cibachrome]

Just what is an "ideal camber curve" ? One that doesn't jack the rear, one that doesn't cause the tire tread to 'snap-through' near peak Fy, one that doesn't cause unfriendly wheelhouse contact, etc ?
It means you need to know something about the tire(s), even the replacement baloney skins. Radials aren't known for having much camber stiffness, race tires really aren't 'radials', and some tires actually have a NEGATIVE camber stiffness. I'm toying around with some of the FSAE 'race-car' tires and there are some surprizes there. Without the tire data and the suspension characteristics, this becomes just handwaving.

Keep in mind there is another tire on the other side of the car that becomes a player in this too. Takes 2 to Tango.

 

[Isdtbower]

I am not a student of suspension. I have browsed it to at least know what I don't know, and to try and understand what most are talking about. But, I see a ton of things in Video. Especially slow motion. It is then important to talk to the drivers, and show the video to get their feel. It seems every suspension is a compromise even WITHIN an intended environment of use. It seems there are so many interactions that the scholars come up with, which may be ignored, that 95% of the time they miss the sweet spot. Video will show all of them, including tire movement which may lead you to an even greater camber curve than anticipated.

It would be interesting if the FSAE program includes looking at video from past builds. From that "information," the "ideal" camber curve (to the ground) would generally drop out. Then, it is deciding what is important in the straight line and turning. With IFS/IRS, camber to the ground seems most effected by body roll and the single tire hits. It goes on, but I think, the point I am trying to make is: Think about what you want the tire to do before you even think about suspension geometry. Rear suspension is relatively easy as generally 3D. But it gets real challenging when adding the turning equation ...and the camber/caster gain curve associated with that.

When I had a current build IFS "engineered," it required simulation and not normal CAD. I specified exactly the camber curve at straight and turning(39*.) It was then up to him to work the geometry, and then compromise on camber/caster where it hurt the least. Where it hurts the least in our environment (offroad racing) seems to be straight and at ride height. A few degrees of negative camber at ride height is not noticeable but helps when the suspension is at the extremes of both movement and forces.

We have a saying that, in rough offroad racing, the race is won in the rocks and tight turns but lost in the open straights. Suspension geometry allows precise direction and turning, while shock tuning and HP allows speed. Really loosing is crashing at speed or bicycling in a turn.

From video, we can see differences in body lean and effects on suspension. Interestingly, we study IFS camber curves with body parallel to ground and at 10* body roll (from roll center) and max turning (36-39*). In the rear, we only look at camber curves to 5* body roll as front jacking usually picks up the tire under speed. At low speed, body roll can go to 20-30* but the slip forces are not there...And offroad tires usually carry the tread into the sidewall significantly. (As a FYI, you can carry the tread too far into the sidewall to where it affects aero. We have seen 75hp "saved/lost" in changing sidewall tread profile on a single roller dyno.)

There are more knowledgeable contributors who will chime in. I listen for more of their thinking than to their "numbers" as each car has a different purpose.

Always learning. Others have been here before. Rocket science only works in well controlled conditions.

 

[SwinnyGG]

GregLocock, you are being paged, please pick up the red courtesy phone.

Jokes aside..

Think about what you want the tire to do before you even think about suspension geometry.

Despite some disagreements with some other stuff in that post.. this is the basic approach.

I'm toying around with some of the FSAE 'race-car' tires and there are some surprizes there. Without the tire data

Are you working on an FSAE car design? If so, have you purchased access to FSAE tire consortium data?

There was a comprehensive data set available on the most common tires in use for FSAE when I was designing FSAE car suspensions.. granted that was coming up on 15 years ago, so I have no idea if that project has been kept alive and current.

 

[cibachrome]

I have 40 + years in the Vehicle Dynamics field, I am a honorary member of the TTC and regularly submit software, comments, critiques and answer to student questions on the TTC form. I also have considerable experience with tires (testing, modeling, statistical analysis and usage in handling simulations) vehicle handling testing and simulation and synthesis via specification. The TTC is very alive and well. If you are in the Detroit area on Saturday September 30th, Doug Milliken will give a presentation on the FSAE tire Test Consortium at Lawrence College.

 

[Isdtbower]

Cibachrome: Apologies. Driven IFS is relatively new to offroad racing. There are a lot of builds with geometry that seem "too simple" with no camber or even caster gain. We are all learning, but I don't want to encourage the home builder to start just a $30-50k offroad IFS clip without considering advanced geometry that he might not see on the course. The little additional notes that Millikan makes, do make a difference with lots of articulation....not so much on pavement.