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Camber Curves of Front vs. Rear Suspension

Camber Curves of Front vs. Rear Suspension

(OP)
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!

RE: Camber Curves of Front vs. Rear Suspension

Are you talking about production carx or track cars?fwd or rwd or awd? Roughly speaking a lot of received wisdom is based on crossplies which are far more sensitive to camber than radials.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Camber Curves of Front vs. Rear Suspension

(OP)
In this case more of a production car, and RWD, but not excessively powerful (in the order of 0.5 G maximum acceleration.)

And yes, I've noticed that with a lot of the older literature they're assuming you're using bias-ply tires, drum brakes, etc.

RE: Camber Curves of Front vs. Rear Suspension

In the ideal case forr a production car the static camber setting is optimised for minimum tire wear. Camber gsins are often an outcome of suspension architecture and desired RCH. The front suspension also has a limitation that camber gain affects bump steer, so if your rack is in the wrong place you may have to modify the camber gain to get the desired bump steer. There is also an upper limit to camber gain as gyroscopic precession causes feedback in the steering.

Since the most common front suspension has a fairly awful camber curve I think it is safe to assume that camber gain is not an especially crucial curve, at least for linear range. For the driven axle the ideal camber gain is sufficient to keep the wheels square to the road, for traction.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Camber Curves of Front vs. Rear Suspension

(OP)
That's the feeling that I was getting - that camber gain is more of a "design output" than a "design input" as in the better approach might be to consider RCH, RC migration, bump steer, steering geometry (KPI, scrub), packaging constraints, etc. first and given these, if the resulting solution's camber gain seems reasonable, then that's it.

"For the driven axle the ideal camber gain is sufficient to keep the wheels square to the road, for traction" - that's also what I've been looking at, and with regards to static camber in the alignment specs for performance RWD cars, it seems like there always is more negative static camber called for in the rear than the front.

RE: Camber Curves of Front vs. Rear Suspension

The C5+ Corvettes are double wishbone all around and they don't spec more negative camber in the rear than the front. Just the opposite.

RE: Camber Curves of Front vs. Rear Suspension

(OP)
Cibachrome, thanks for the data! So if I understand this correctly, this is degrees of camber change with respect to the road, of the outside wheel per degree of roll?

It makes sense to me that the trucks have a rear value of near 0 deg/deg because they probably have live axles, and the front values are nearly all positive as the shallow camber curves of typical front suspensions (as I understand, and as Greg pointed out) make the front wheel lose negative camber with roll.

What I'm having trouble understanding is the fact that practically all of the rear suspensions (that aren't live axles) seem to have a net negative camber gain with roll. With a track of 1500mm and a roll angle of 5 degrees, this corresponds to about 65mm of wheel travel on each side. That seems to imply a camber gain of in excess of 2 degrees per inch of travel, or 75 degrees per meter, whereas when I've looked at camber gain per unit of linear travel, I haven't yet seen anything in excess of 1 deg/in or 40 deg/m...

RE: Camber Curves of Front vs. Rear Suspension

... most of that 2 degrees per inch being accounted for by the body roll. Suspension with small camber change relative to the bodyshell means the camber will be close to that of the angle change of the bodyshell itself.

Picture what happens with pure trailing arms ... the camber equals the body roll.

RE: Camber Curves of Front vs. Rear Suspension

I think the main reason why irs rwd cars do not have 100% camber/roll compensation is that it implies a low rch, so you need a big rear sta bar to control roll. Heavy, hard to package,

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Camber Curves of Front vs. Rear Suspension

IRS RWD cars with high HP motors need to maintain REAR scrub radius as a result of tire Mx changes under Fx and Fy application. The best way to confirm this is to find tires of same size and compare Mx at low Fy-slip conditions. Run them on the car and they will talk to you. If you loose the static negative scrub because of the tire Mx shift to positive, its a new dance. Try wheel offset, too, if they package/fit, but this affects toe stiffness and that you won't like, either.

Same for FWD, high power motor front suspensions. Broken wrists from "Torque Induced Steer".

RE: Camber Curves of Front vs. Rear Suspension

Here's some data to contemplate:

Pneumatic scrub is a term used to represent tire Mx normalized by Fz. Hence it's a dynamic scrub radius modifier. It relates actual contact patch line of action with the geometric one. If you want to maintain your negative scrub radius for longitudinal criteria, you engage Mr. Purple or Mr. Red. If you want the car to be a handful, get get Mr. Green or Mr. Purple-Dashes, assuming your car already has a nominal but not gigantic amount of negative scrub at the spindle. Another easy science project can be reversing the tires on the rims to swap the signs of P-Scrub. But, be watchful of conicity and ply steer details before you grab that "ahaah" moment. Uni-directional tires may be players in this phenom.

RE: Camber Curves of Front vs. Rear Suspension

(OP)
Thanks again for the data! The conversation started with camber curves but has moved to the amount of negative scrub radius that an IRS on an RWD car would need - would you therefore consider keeping scrub radius negative under all circumstances to be more essential than having ideal camber curves?

RE: Camber Curves of Front vs. Rear Suspension

(OP)
I was doing some more rummaging around these forums and found this from '04:

http://www.eng-tips.com/viewthread.cfm?qid=98416

Quote (neutralsteer)


Is there a method for setting camber compensation? I suppose one it would be a strong function of the vehicle roll gradient (and the time distribution the car spends in corners or on straight aheads, although this would be quite area dependent). I'm not sure how to set a target value though. Any advice?

Quote (GregLocock)


Well, that is a damn fine question. In practice it is pretty much set by tyre wear considerations, but if we didn't have to worry about that it would be nice to keep just a little negative camber on the maximum corner.

So, with a vehicle roll gain R deg/g, it would seem reasonable to set the camber gain C=R+0.5 deg/g

In practice this does not happen. Typically C=~0.5*R

That last equation seems relatively consistent with the RWD independent suspension production car camber gains that I could find on the internet, approximately 1 degree of camber gain per inch of travel, and with typical track widths, an inch of travel corresponds to approximately 2 degrees of roll.

RE: Camber Curves of Front vs. Rear Suspension

To give you some idea on this from a design perspective for the front axle no alarm bells would ring if the camber gain fell anywhere in the range 20-30deg/m, whereas say bump steer would normally be maintained within 0.5 deg/m of target. I can't remember the equivalent bands for an IRS, as I've been working on live rear axles or twist beams for whst seems like a decade!

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Camber Curves of Front vs. Rear Suspension

Put on your seatbelt and pull it tight, because a topic to entertain the curious has risen that's always been a favorite of mine: "What should my 'Camber Curve' be "? If you have a clean sheet of paper and access to tire data that you believe, you can take a stab at this problem in an unusual way. When you get to play with a tire on a tire test machine, it becomes very clear that the Mx data channel gives you insight to the tread surface conditions, belt stability, tread life and Fy output. In manual control, you can hover the tester at inclination angles necessary to give the best heat distribution, the best sound ("frying bacon" in a race slick) and a high Fy at an array of pressures and Fz loads. Rim width gets into this, too.

You create a 3D function of Mx as a function of slip and inclination angle at a nominal Fz load. We want to take slices of this plot to determine where the Mx is zero for a slip angle or two in the range of Fz’s your vehicle is likely to generate at high Fy levels. The slices across slip angle are meaningful.
Next, we fit these Mx slices to another tire friendly (I'm a spline fan) function and calculate the zero crossings. Naturally, Matlab provides all the necessary service function, so this whole deal takes more statements to make the pretty plots than it does to do the math, once the data is fitted appropriately. Then we contemplate the graph of these inclination (camber) angles vs. the Fz loads that produced them at a few slip angles. A decent vehicle dynamics model of your car should already have given you approximate 4 wheel loads and a roll angle at your limit of control (whatever that means to you). In the case of my limited knowledge of the car you are dreaming about, all the wheel loads in the Fz range (inside, outside, front and rear) would be set in the middle of the slip zone contemplated.

Bingo, this ought to produce "Happy Tires" as I call them: tread hot and like chewing gum, temps uniform, and the tread band pealing of slowly instead of chunking, delaminating, marbling or smoking. (Sounds like a steak cooking, eh ?). (for track tires that is). For dirt tires, you need trough data, Camber Ridge data, or load wheel data (if anyone is crazy or rich enough to spend money running load wheels in muddy/dirty conditions).

But, words of caution: Your camber curve is important in roll for cornering maneuvers. That means some roll angle from suspension travel is necessary (to make use of your camber by roll curve). Roll implies suspension travel and that's the x variable in your camber curve. (I prefer it to be the Y axis, but there is convention, you see.) AND, (and this is a BIG and), the lateral camber compliance (from bending and soggy wheel bearings, etc.) plays into this construct as well as caster angle (caster is camber by steer, eh ?)

If you only optimize the front, your extra grip will make the car very loose. Your driver will complain. If you have the freedom to do the front and rear, you will get some extra Ayg's but stability may not be improved because there are other features of your chassis which are not conducive to this (Like weight distribution or TLLTD or a solid rear axle).

But, it's all better than the usual hand waving, smoke, mirrors, packaging constraints, lack of time excuses, blank stares and legacy explanations I've heard so much about over the years. This process makes for some awesome plots, too. Very impressive in a design review with Management who know nothing at all about the subject !

RE: Camber Curves of Front vs. Rear Suspension

Thanks. Great post for me.

The practical Camber/Caster bandwagon. Any ideas to make us think and measure is helpful. In my world with no testing facilities, it is real world of pictures and video.

This last weekend I showed a picture similar to the one above... to about 20 offroad racers, industry leaders, manufacturers, tuners, fabricators, suspension designers, media, etc. Only two picked up on Camber and secondly "pneumatic scrub." It made for great conversation, and I learned a lot of other tricks that you didn't know that you didn't know or see.

From your discussion, I see that the "car" above is being supported well into the sidewall...and not the tread. So I guess that helps the driver from rolling so easily.

Interesting: I recently saw on a prototype tire a longitudinal strip of full height "rubber" on the line between the sidewall and tread. I guess that is to keep the lugs together on roll or against something. The sidewall lugs were all placed to easily side slip....and maybe not make the "pneumatic scrub" worse. Someone is not talking to others. Basically the shock and tire guys are just trying to fix what the suspension designer missed....or ignored.


RE: Camber Curves of Front vs. Rear Suspension

So I guess that helps the driver from rolling so easily.
Yes - by "un-gripping" the tyre - on a race car.

Surely it would be preferable to keep the tyre upright and somehow reduce the massive migration of the outside wheel contact patch towards the front view CG? Can't help thinking a lightweight beam axle would do a better job.

je suis charlie

RE: Camber Curves of Front vs. Rear Suspension

"Surely it would be preferable to keep the tyre upright and somehow reduce the massive migration of the outside wheel contact patch towards the front view CG? Can't help thinking a lightweight beam axle would do a better job."

Not in the OP's question, but a beam axle does do a better job offroad until the speeds increase. The straight axle always/usually places the tread flat with the ground...no matter the body roll or terrain. There is also the benefit of turning tighter with a straight axle. (45*) And is generally stronger. Lightweight is not in the vocabulary as axles can be up to 500 lbs to prevent breaking. 40 spline axles (1.7") are becoming standard in rigs even weighing less than 3000lbs. Air pressure can be dropped to below 8 lbs and more with no detrimental effects on "offroading." Competition tires are filled with lead or water and weigh in at 300-500lbs ea. From competition you can go all the way up to OEM which do perfectly well for the intended environment.

At some point this changes to the advantage of a IFS suspension. (This is also debated in the offroad community) The faster you go the more critical steering becomes. Articulation of 15-25" in the front usually means full hydraulic steering which is not "perfect" for high speed, and has no centering. But there have been significant improvements in this lately. IFS has mechanical steering so it is precise....or as good as you can design it considering bumpsteer. There are other positives at speed (3' whoops at 80-100mph) but also the negatives such as this thread about Camber curves. Driven IFS or 4x4 IFS is relatively new to high end racers needing lots of articulation. Areas of design that might have been ignored are now as important as the traditional. Amazing how most say that 4x4 IFS is a "compromise" as a "legacy explanation". (I love OEM techi wording sometimes)

RE: Camber Curves of Front vs. Rear Suspension

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

RE: Camber Curves of Front vs. Rear Suspension

"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.

RE: Camber Curves of Front vs. Rear Suspension

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

RE: Camber Curves of Front vs. Rear Suspension

(OP)
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...

RE: Camber Curves of Front vs. Rear Suspension

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.

RE: Camber Curves of Front vs. Rear Suspension

(OP)

Quote (cibachrome)


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.

Quote (cibachrome)


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.

RE: Camber Curves of Front vs. Rear Suspension

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.

RE: Camber Curves of Front vs. Rear Suspension

(OP)
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.

RE: Camber Curves of Front vs. Rear Suspension

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.

RE: Camber Curves of Front vs. Rear Suspension

(OP)
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!

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