Camber Curves of Front vs. Rear Suspension 5

[mark512]

I haven't managed to find a lot about this online - there seems to be plenty of information regarding "setting up" a particular vehicle for a specific purpose (street, track, drifting, etc.) but these discussions are mostly limited to adjusting the static cambers of a particular car.

What I'm looking for is the more general case: in an arbitrary vehicle where the static cambers and camber gains of the front and rear suspensions can be arbitrarily chosen, what are the factors affecting their selection, but in particular, what affects the choice of variation between front and rear?

For example, it seems to me that if there is a vehicle with 50/50 static weight distribution, double wishbones and identical tires on all four corners, why not have the camber curves of the front and rear suspensions be identical? I'm aware that camber angle with respect to the road affects tire grip and camber thrust and therefore affects understeer/oversteer, but there also appears to be many other ways of adjusting this balance other than by having different wheel cambers.

Thanks in advance!

 

[gruntguru]

Ahhh - a closer look at the photo reveals driveshafts - I didn't realise it was AWD!

So a "lightweight" beam axle would need to mount the differential to the chassis and retain the half-shafts and CV joints shown on the IFS setup. Still do-able.

Anyways the main point of my post was the increased rollover tendency due to shortening of the outside wheel track.

je suis charlie

 

[Isdtbower]

"Shortening of the outside wheel track." Thanks for bring this up.

New, very expensive center sections ($20-30k), place series 30 and 934 size CV's back to back. A bull or ring gear rotate around those. Most common is about a 5-7" CV separation for Arm-to-frame strength, camber geometry, and traction control. With arms and axles longer, it is possible to use Camber to keep the wheel track similar thru articulation. This "theory" holds pretty well until you get to about 22-25* of axle droop or stuff. From there you definitely loose wheel track fast. Wheel travel above that threshold usually means a straight axle, which are found in the rear of most race trucks with articulation of up around 36". These have huge sway bars to control body roll. Or a double set of bars that allow articulation to a point and then kick in the big one. Actually, swinging sideways of a straight axle is a real problem addressed by designed long double triangle links.

There have also been pivoting designs similar to the Ford Twin Traction Beam (TTB). Which have not been popular in 4WD racing. There are a few willing to fabricate improvements to correct the extreme camber of the TTB design....but...

Robust racing centers are made by:

http://www.proformance.com.au/

,

http://www.weismann.net/sidewinders.html,

http://www.fortinracing.com/product88.html

Generally offroad only as tire wear on pavement becomes a huge issue.

 

[gruntguru]

The main problem is the outside contact patch moving towards the CG due to:
-

body roll moving the CG outwards especially in bump with RC migrating below ground level​

-

Tyre deflection​

Just try and imagine where the CG is relative to the contact patch in the above photo.

je suis charlie

 

[mark512]

Inspired by cibachrome's posts I decided to do some digging and number crunching myself. The data is from five "sporty" RWD cars whose camber curves I could find on the internet. They are the Nissan 350Z, the NA (1989-1997) Mazda Miata, the ND (2015-) Mazda Miata, the 964 (1989-1994) Porsche 911 and the Honda S2000.

I started with the camber curves (camber w.r.t car vs. suspension travel) and using the front and rear track widths of the respective cars I converted that into camber w.r.t road vs. roll angle:

To me, the slopes of all the traces look quite similar, so I decided to plot camber change vs. roll angle, essentially taking the above graph and vertically shifting the traces to start at 0:

From this graph, it does seem that all the slopes are in the same ballpark. Also, it's clear that that the front outside wheels are reaching positive camber faster than the rear outside wheels, which seems to correlate with the idea of the front end letting go before the rear. With this in mind, I made another plot, this time of the front outside wheel camber minus the rear outside wheel camber vs. roll angle to see how in these vehicles "front positive camber margin" changes with roll:

The Z is relatively constant with roll whereas the others seem to be more progressive.

I realize that for the NA Miata I probably have the wrong static camber numbers which is why it seems to be an outlier, so I guess we can ignore those curves...

 

[cibachrome]

Just for yucks, I rubbed my crystal lamp and came up with K&C measurements of most of the cars you have mentioned, in case there was some consistency in all the inconsistancy. Here is a table of a few of the salient parameters supposedly designed into these cars (I often have my doubts, based on the high variability we see in measurements of the same model year to year (especially with the numbers only car). Gees, go with net build and skip the slotted holes.

Keep in mind that they are all trying to maintain some level of total vehicle limit understeer, so the speed sensitive steering gain stays on this planet and that the painfully slow frequency response / ay response times of low understeer vehicles are not so truck-like. But, you can't really do this with camber simply because the tires they use don't have significant amounts of camber stiffness (its a radial thang). Meanwhile some are squeezing the turnip by putting in soft steering gear mounts ("toe compliance") or cranking the roll steer up. (You gotta have some roll though to get this job done). Meanwhile, most of their tires run out of Mz long before Fy, and this presents a quandry that even amuses the judge as well as juries. You see, all that toe compliance understeer magically becomes oversteer when Mrs. Mz flips her sign. Not a good way to go IMHO, but the magazines love these cars because only guys with large values of 2 balls can drive them when it counts.

Tell me what tires you want to mount up and I might be able to rub the lamp one more time. See attached.

 

[mark512]

But, you can't really do this with camber simply because the tires they use don't have significant amounts of camber stiffness (its a radial thang)

which sounds like a good thing to me, as it seems that none of the cars I looked at have anywhere near 100% camber compensation in roll, and you can get much more ideal suspension geometry without the requirement of insane camber gains in bump.

Tell me what tires you want to mount up and I might be able to rub the lamp one more time.

Funny...I'm actually in the process of constructing a tire measurement trailer (for lack of a better word, I couldn't find a standardized term for it) since I'm nowhere near, and couldn't afford time at, one of those rolling road tire test facilities. The measurements are not nearly as detailed or accurate as the rolling road units with precision 6-axis force/torque sensing, however Fx and Fy can be measured, and the load (Fz), camber and slip angle can be varied.

Anyhow, the tire size is 205/45R17 both front and rear. I haven't narrowed in on specific one(s) yet though.

 

[cibachrome]

Couple of observations: Don't have your exact size (only 205/50R17) but clear winner(s) are Conti BMW 3 Series and GDY, also BMW 3 Series. Data suggests you have a choice of high Mz stiffness vs. low Mz stiffness for about the same Fy stiffness. Low is EMT construction and high is their AL3 (Summer) tire.

I believe you would like the swift relaxation distance of Conti's for that "Oh what a Feeling" impression. This is a key ingredient of the BMW mystique, but you also need the steering guts to pass it through if you want the road feel.

 

[mark512]

Well I think we have a winner because the Contis are what I was definitely leaning toward. They're not cheap, but also not too expensive either. My daily driver is a VW Golf and I have family members with an Audi A5 and also Porsche Panamera. Contis are the OEM tires on all three and I've been pretty impressed with their performance, so they sounded like a safe bet to me.

 

[cibachrome]

The Conti's you want are the 'star' tires (OEM BMW). There is a dime sized 5 pointed star on the sidewall. A BMW dealer may not even have them but would be happy to substitute non-star tires to the unsuspecting. Try for some OEM take-offs: New owner wants to pimp out the car with new wheels needing a different tire. Offer to buy the take-offs. NFG in water, mud, or snow, but crispy on a winding dry road.

 

[mark512]

For steering setup, I actually have a thread from a couple of years ago bookmarked, it was regarding the E30 BMW 3-series:

http://www.eng-tips.com/viewthread.cfm?qid=340743

I remember that the BMW "star" tires were mentioned, and it indeed was you who mentioned them. I'll definitely look into that!