Rainy Day Suspension Basics... Understanding understeer.

[exwestracer]

Awhile back, I had a request for a “rainy day” piece on roll understeer. For various reasons (it’s not raining!), I haven’t got around to putting anything together until now. I figured why not try to include some of the basic reasons behind understeer in general.

Disclaimer: These are VERY basic ideas, derived from Isaac Newton, and a lot of years of my own racing experience. There are a number of cases where things work exactly the OPPOSITE, as well as other causes for understeer. YOUR MILEAGE MAY VARY.

A simple definition: As my friend (and longtime stock car racer) Doug likes to put it,” Understeer means you get to see the wreck happen”. The car comes into the corner, the driver turns the steering wheel what seems to be a reasonable amount, but the car uses more road than anticipated. Hopefully, there is more road to be used (if not...see Doug’s quote above). Reasons for this happening fall into 2 basic categories: Traction understeer, and geometric understeer.

Traction understeer means that, for whatever reason, the lateral traction available from the steer tires is not greater than the forward inertia of the vehicle. The car wants to keep going straight more than our capability to turn it. This often happens when we enter at too high a speed, or “over-cook” a corner. The same problem can be caused by braking too late or staying on the brakes too far into the corner. We are simply asking too much of the finite amount of traction available from the front tires. This is assuming that the front suspension is tuned (and working) correctly. A dangerous assumption!

I won’t go back into the weight transfer and traction ideas I covered in the “Let’s hit the bars” piece; except to say once again that if the car is lifting the inside wheel, we aren’t getting ANY traction help from that tire. That idea is not an absolute… The tire doesn’t have to be completely off the ground to have a serious reduction in traction capability. Less traction= more tendency to understeer, it’s that simple.

Another common cause for traction understeer is too stiff (or too soft) springs in the front. This has to do with the motion of the chassis relative to the tire sidewall. If the springs are too stiff, the tire becomes the effective springing medium. Since we very seldom deal with a perfectly smooth road surface, and there is no shock absorber inside the tire, too much sidewall bounce can lead to a loss of traction. The amount of spring rate that a given tire will tolerate is directly related to the tire construction. Modern ultra-low profile tires seem to put up with a lot more spring than the 50 or 60 series tires we were running on 20 years ago. Too stiff a front anti-roll bar will add a significant portion of the inside spring rate to the outside corner, effectively acting like a much stiffer spring on that side.

On the other hand, if the springs are too soft we risk bottoming out the suspension somewhere (either the shock, or metal to metal contact). When this happens, the entire weight on that corner becomes unsprung weight; usually punishing the tire beyond its limits.

Geometric understeer can be caused by a number of suspension inputs. When dealing with a modified production chassis, one of the most common problems we run into is bump steer. Bump steer can come from poor stock steering geometry, or from suspension bushing deflection, or both. If the steering geometry causes toe out as the suspension compresses, we aren’t getting as much tire angle as we think when turning the steering wheel. On a “front steer” vehicle, control arm bushing deflection will allow the lower ball joint to move toward the centerline of the chassis. Since there is no rubber in the steering linkage, this tends to “unwind” the steering arm and knuckle, again reducing tire angle. Both of these causes are usually addressed fairly early in the suspension modification process.

Geometric understeer can also come from the rear suspension, depending on design, and the amount of body roll allowed in the system. If the car uses a link type rear suspension, and the lower links run downhill to the chassis (common on very low cars); body roll will cause the axle to twist in the same direction as the front wheels are being steered. Instead of rotating as desired, the car will “crab” toward the inside of the road. This can be a desirable effect if understood and managed correctly. Many IRS vehicles actually have slight roll understeer designed into the suspension system. Again, the amount of this effect is related directly to the amount of body roll. If the car is locked down flat with a big rear bar, there isn’t enough motion available to make much of a difference.

Let's mull that over for a bit. Fire away!

[Nessumsar]

Great right up as usual Ray!

How much of an effect does bumpsteer play on general handling characteristics? At what amount of toe change is there too much bumpsteer (generally speaking)? Do we want bumpsteer to toe-in or toe-out on bump/rebound?

[Rod]

ohhh understeer!! thats a nail biter!! ha ha ha

Good job Ray

[j-rho]

Never heard of those types of understeer - the first one sounds like you could call it "You're driving it wrong understeer" - which isn't really the car's fault. :)

When I look out at the bevy of P-T cars, most have typical nose-heavy static weight distribution (something like 57% front), but with rear tires that are 50-90mm wider than the fronts. To me, that common pitfall contributes to chronic understeer more than anything else.

Sure the geometry is bad on most of these things, but you can make up for it a lot with lots of static negative camber, so the outside tire is happy at terminal roll. Nobody seems to be doing that either, fearing they'll wear their tires out.

Spring rate by itself doesn't tell you much, at least not when people are running different chassis. Wheel rate, or ride frequency, is what really matters. A 1400lb. spring on the front of my Camaro (stock subframe) is a lower (softer) ride frequency than a 450lb. spring on the front of my last strut-suspended race car. People freak out when they hear big spring rates but without knowing the motion ratio, the spring rate itself is practically meaningless.

In short: get bigger front tires, don't be afraid to run big front spring rates

[79-TA]

In the case of bump steer, I think it helps to clarify that if the compressed wheel is gaining toe out, the drooping wheel is gaining toe in. In a corner, both wheels are then effectively steering the car out of the turn. Or at least that's how I like to describe it.




Sure the geometry is bad on most of these things, but you can make up for it a lot with lots of static negative camber, so the outside tire is happy at terminal roll. Nobody seems to be doing that either, fearing they'll wear their tires out.


Well, in the case of pro-touring, these are cars that are meant to be able to put on some highway miles. If we were just autocrossing or even road racing, we could get away with it. But yes, more people could afford to make the adjustment when at the track.

I'm disappointed by how few people (and after market suppliers in some cases) look to improve the camber gain curve as part of the solution. Bolting on pricey control arms (that hopefully add a bit of caster) to a completely stock-height upright is just insane to me. Sometimes it seems that certain suspension kits try to eliminate body roll as much as possible just for the sake of limiting suspension movement.

[79-TA]

Hey Ray, how does roll understeer work in a car with leaf springs? I can visualize it easily in the case of a 4 link, but not leaf springs.

[Henesian]

Ray, all you're doing for me is reinforcing the idea that I've built my car to handle correctly(Which it does at its current level of smog legal performance) :).

[exwestracer]

Hey Ray, how does roll understeer work in a car with leaf springs? I can visualize it easily in the case of a 4 link, but not leaf springs.

Brett, it depends on the spring angle, arch, and axle mounting. If the spring is clamped directly to the perch with u-bolts, we'd like to think there's not much possibility of the housing twisting very far. If, however, the spring is attached with clamp blocks and rubber pads, it's much more likely that there will be some roll steer going on.

The front spring eye is the "locator" for the housing, so imagine a solid link there instead. Is it running uphill or downhill to the chassis? If the spring eye is above the perch, the housing would tend to roll oversteer. The flatter the front half of the spring is (no arch), the more it will act like a solid link in terms of geometry.

[exwestracer]

So, after watching LeMans this weekend, I'm noticing a definite trend toward cars being built around VERY large front tires. I believe it was the Patron Acura team that pioneered this in ALMS a few years back... They decided to run rear tires on the front as well (hey, the rules just said what tires they COULD use, not where). Now pretty much all the prototype teams have gone that route. IIRC, Ron Schwarz is using some pretty big rubber on the front of his full frame Javelin P/T build.

Maybe that will be the next trend, and it's a good solution. Make the car turn better at higher speeds by simply putting more steer rubber on the road. THis is going to require a complete re-think of front suspension and frame geometry. Once again, the Camaro guys may have an advantage, because the front half of the frame unbolts. Will we start seeing higher ride heights and 300 rubber out front in the future?

[Nessumsar]

So, after watching LeMans this weekend, I'm noticing a definite trend toward cars being built around VERY large front tires. I believe it was the Patron Acura team that pioneered this in ALMS a few years back... They decided to run rear tires on the front as well (hey, the rules just said what tires they COULD use, not where). Now pretty much all the prototype teams have gone that route. IIRC, Ron Schwarz is using some pretty big rubber on the front of his full frame Javelin P/T build.

Maybe that will be the next trend, and it's a good solution. Make the car turn better at higher speeds by simply putting more steer rubber on the road. THis is going to require a complete re-think of front suspension and frame geometry. Once again, the Camaro guys may have an advantage, because the front half of the frame unbolts. Will we start seeing higher ride heights and 300 rubber out front in the future?

This was my train of thought when we worked on my Nova, put big rubber all around. I'm still no where near my 315's full potential because I'm fighting crappy shocks and a lot of bumpsteer. I'm hoping to find a killer deal on some used "race" shocks; as I can't imagine the transformation that would take place going from my $35 KYBs...

[exwestracer]

This was my train of thought when we worked on my Nova, put big rubber all around. I'm still no where near my 315's full potential because I'm fighting crappy shocks and a lot of bumpsteer. I'm hoping to find a killer deal on some used "race" shocks; as I can't imagine the transformation that would take place going from my $35 KYBs...

Sorry, Jon. I'd forgotten you were running those big fronts as well.

[Nessumsar]

No need to be sorry Ray.

Could you expand a bit more on the pros/cons of bumpsteer?

[AutoX_a_Truck?]

Ray, I always read through your Rainy Day Suspension threads. Thanks for sharing your knowledge and experience.


Will we start seeing higher ride heights and 300 rubber out front in the future?

I would think for the 200 treadwear events you would be hard pressed to run too much tire on the front of a typical nose heavy pro-touring car.

Now that I have fenders on my truck, the optical illusion of the front 315s being as wide or wider than the 335 rears is gone so I would run a 335 on the front too, but the Ecsta XS is no longer an option and PS2s are too expensive at $400 a pop. My front suspension is less than ideal, but my truck pushes like a dump truck with a 295/40/17 KDW on the front compared to the 315/35/17 Ecsta XS although the Kumho is a much softer tire. At least with my setup I think the disadvanages of slightly more wheel/tire are greatly outweighed by the increased grip.

[dontlifttoshift]

Tyler, what's a fender?

Large by wide tires in the back with just large tires in the front....can't you tune the oversteer/understeer with the rear ARB during steady state cornering and then have the available traction to the rear to power out of the corner?

I agree, I think you will see a push (HA!) towards bigger front tires and more balanced tire packages. The 275/335 crowd seems to be running pretty big ARB rates in the rear to get the car to rotate on autocross.

[AutoX_a_Truck?]

Tyler, what's a fender?

"A body panel of such vehicle" and apparently they are not important on 1940 and older trucks

In my case a fender turns out to be these four pieces of metal that add about 80 pounds of weight, make it much more difficult to do alignments, check tire temps and tune/adjust pressures and the front shocks but hey rules are rules, and I actually kind of like my truck more now that it has "all body panels of such vehicle"

I do miss my Kumhos though

[j-rho]

Negative camber by itself isn't that bad on tires. I have tens of thousands of street miles on cars with over 2 degrees static negative camber, and experienced pretty even wear once some auto-x/track time was mixed in. It's when you combine measurable toe values with that camber, that you start to chew them up on the street. Keep toe at zero (which is fine for most performance applications) and a ton of negative camber won't kill you.

If the front end is redesigned properly the ride height doesn't have to go up any with wider tires, which can be just as short as the narrower ones.

With the amount of $ being spent on some of these builds, with full-on aftermarket subframes and whatnot, it boggles my mind that people aren't working harder to get more front tire under their rides, many of which are carrying 1000+lbs/corner up front. When getting around corners matters, the greatest geometry in the world with skinny tires will be no match for a ho-hum but well-executed suspension blessed with wide tires.

[Bryce]

295 is just not wide enough anymore. HAHA.

I have a 245 front tire, with 740 and 712 pounds on the front corners. I need more tire.

[exwestracer]

Negative camber by itself isn't that bad on tires. I have tens of thousands of street miles on cars with over 2 degrees static negative camber, and experienced pretty even wear once some auto-x/track time was mixed in. It's when you combine measurable toe values with that camber, that you start to chew them up on the street. Keep toe at zero (which is fine for most performance applications) and a ton of negative camber won't kill you.

If the front end is redesigned properly the ride height doesn't have to go up any with wider tires, which can be just as short as the narrower ones.

With the amount of $ being spent on some of these builds, with full-on aftermarket subframes and whatnot, it boggles my mind that people aren't working harder to get more front tire under their rides, many of which are carrying 1000+lbs/corner up front. When getting around corners matters, the greatest geometry in the world with skinny tires will be no match for a ho-hum but well-executed suspension blessed with wide tires.

Good point Jason, and I think caster and camber GAIN play a big part in tire wear as well. We tend to get pretty aggressive with these numbers as we're dealing with limited travel these days. If you've never watched a video of what a tire does when you crank it into a hard corner, you'll be amazed. That tire is squirming ALL OVER the place.

Keep in mind that the stiffness of the sidewall and transition area are very important in how a tire wears with aggressive alignment settings.

[j-rho]

Good point Jason, and I think caster and camber GAIN play a big part in tire wear as well. We tend to get pretty aggressive with these numbers as we're dealing with limited travel these days. If you've never watched a video of what a tire does when you crank it into a hard corner, you'll be amazed. That tire is squirming ALL OVER the place.

Keep in mind that the stiffness of the sidewall and transition area are very important in how a tire wears with aggressive alignment settings.
Sure, but above we're talking about people worried too much static negative camber will wear out their tires in normal street driving (inside edge dies first). On the street you spend most of your time going straight, not really loading up the tires or suspension. In that situation having some negative camber is ok, as long as toe is at or near zero.

Managing wear within the microcosm of the track/auto-x, where you're carefully minding pressures, frequently checking wear patterns and rotating tires, shouldn't be that hard either. Just have to know what adjustments to make.

[Norm Peterson]

Large by wide tires in the back with just large tires in the front....can't you tune the oversteer/understeer with the rear ARB during steady state cornering and then have the available traction to the rear to power out of the corner?
To some extent, you can do that, at least with a setup that's way off to begin with. But as you get into diminishing returns, no additional amount of rear roll stiffness can make an undersized front tire gain grip that it never had to begin with.

I *think* you'd have less room between the push caused by insufficient basic front tire grip and going way loose from too much throttle on corner exit.


Norm

[exwestracer]

To some extent, you can do that, at least with a setup that's way off to begin with. But as you get into diminishing returns, no additional amount of rear roll stiffness can make an undersized front tire gain grip that it never had to begin with.

I *think* you'd have less room between the push caused by insufficient basic front tire grip and going way loose from too much throttle on corner exit.


Norm

Good point as always, Norm.

And, as a general rule; if we have a problem at the front (lack of grip), we need to FIX it at the front (find more grip) rather than giving up rear traction. Balance adjustments with bars are supposed to be very minute tweaks, not crutches...

[Norm Peterson]

Never heard of those types of understeer - the first one sounds like you could call it "You're driving it wrong understeer" - which isn't really the car's fault. :)
There are quite a few effects that when you add them all together give you something that you collectively call either "understeer" or "oversteer" depending on what the end sum total is. You might have heard of "understeer budget", which sums the effects happening at the front, the effects happening at the rear, and finds a difference value that can be either + or -. It's convenient to identify each of these effects by what causes them (i.e. weight distribution, bushing and bracket compliances, roll camber, roll steer/toe steer, etc.), and at the OE level where they have actual numbers for all these things it gives some direction for how to proceed with the suspension tuning.



When I look out at the bevy of P-T cars, most have typical nose-heavy static weight distribution (something like 57% front), but with rear tires that are 50-90mm wider than the fronts. To me, that common pitfall contributes to chronic understeer more than anything else.
Even with good geometry and appropriate alignment settings, fitting your competition car to that sort of appearance-based formula is definitely leaving something on the table.

Not so long ago, one of the FWD GM cars was actually produced with front tires one size larger than the rear tires. That wasn't by accident or by the benefit of oversight by the bean-counters and management . . .


Norm

[diesel25lrs]

Negative camber by itself isn't that bad on tires. I have tens of thousands of street miles on cars with over 2 degrees static negative camber, and experienced pretty even wear once some auto-x/track time was mixed in. It's when you combine measurable toe values with that camber, that you start to chew them up on the street. Keep toe at zero (which is fine for most performance applications) and a ton of negative camber won't kill you.

If the front end is redesigned properly the ride height doesn't have to go up any with wider tires, which can be just as short as the narrower ones.

With the amount of $ being spent on some of these builds, with full-on aftermarket subframes and whatnot, it boggles my mind that people aren't working harder to get more front tire under their rides, many of which are carrying 1000+lbs/corner up front. When getting around corners matters, the greatest geometry in the world with skinny tires will be no match for a ho-hum but well-executed suspension blessed with wide tires.

Jason,
I'm just trying to educate myself as it's all pretty new to me buy why 0 on the toe? I know that my recommended toe is 3/16. Also-would you say that -1.5* would be a good setup for street and occasional autoX as far as tire wear is concerned?

[Bryce]

If you run zero toe or even toe out your turn in will be quicker.

Also your toe spec will affect ackermann.