5/17/2011

05-17-11 - Dialing Out Understeer

Almost all modern cars are tuned for understeer from the factory. Partly because of lawyers, but partly because that's what consumers want. Certain Porsches (ever since the 964) and BMW's (ever since the E46 M3) have been badly tweaked for understeer.

You can mostly dial it back out. I will describe some ways, in order from most preferred to least. Obviously the exact details depend on your model, blah blah blah. I'm just learning all this stuff, I'm no expert, so this sort of a log of my learnings so far.

In general when trying to get more oversteer you have to be aware of a few issues. Basically you're trying to increase grip in the front and decrease grip in the rear ; you don't want to go so far with decreasing grip in the rear that you decrease overall grip and get much slower. You also don't want to create lift-off oversteer or high speed mid-corner snap oversteer or any of those nasty gremlins.

1. Driving Technique. Go into corners fast, brake hard to load up the front, turn in with a bit of trail brake. This helps a lot. Now move on to :

2. Alignment (Camber & Toe). Basically more camber in front and less camber in rear will increase oversteer, because in a corner the tires are twisted sideways, so by giving the front the ideal camber they will have good contact while cornering, and the rear tires will be on edge and slip. Obviously there's a limit where more toe in front is too much, so the idea is to set the front tires to their ideal camber for maximum grip, and then set the rear somewhere lower to give them less grip. If you want a fun alignment you generally want zero front toe, and just enough rear toe to keep the car stable under braking and in high speed turns (zero rear toe is a bit too lively for anything but autocross use). Note that severe camber on your driven wheels can hurt straight line acceleration.

3. Higher rear tire pressure. This is sort of a temp/hack fix and real racers frown on it, but it is cheap and pretty harmless so it's easy to try. Many people get confused about how tire pressures affect handling, because if you search around the internet you will find some people saying "lower your pressure to decrease grip" and others say "raise your pressure you to decrease grip". The truth is *both* work. Basically there is an ideal pressure at which grip is maximum - changing pressure in either direction decreases grip. However, lower pressure also leads to tires moving on the rim, which is very bad, so if you want to tweak your grip it should always be done by raising pressures (as you lower tire pressure, you get more grip, more grip, then suddenly hit a point where the tires start rolling on the sidewall, which you don't want to get to). So set the front & rear to the ideal pressures, then raise the rear pressure a little to reduce grip in the rear. (for example E46 M3 is supposed to be good at 35F and 42R)

4. Sway bars or spring rates (stiffer in rear and softer in front). You get more oversteer from a car with a stiff rear. Basically stiffer = less grip, it means the whole end of the car will slide rather than the wheels acting independently and sticking (this is why Lotus and McLaren don't use sway bars at all). You don't want to overdo this as a way to get oversteer, because it makes the ride harder (a stiffer sway is just a form of stiffer spring), and it also just reduces overall grip (unless your sways were so severe that you were leaning and getting out of camber, in which case stiffer can mean more grip). But many OEM cars are shipped much stiffer in the front than the rear - they are dialed to have more grip in back and not enough in front, so you can undo that. Note that lots of "tuners" just mindlessly put bigger sways on front and back, when in fact the OEM front bar might be just fine, and putting on a stiffer front just makes things worse. BTW a lot of people make the mistake of just going stiff, thinking stiffer = better handling; in fact you want some weight transfer because weight transfer is what gives you control over the grip. The ideal car will either oversteer or understeer depending on what you do to it, and weight transfer lets you do that.

5. Narrower rear tires. Basically undoing the staggered setup a bit. This obviously decreases max grip in the rear. Many cars now are shipped on rear tires that are really too big. See below for more notes.

6. Wider front tires. Going wider in the front as part of a package of going narrower in the rear may make sense; for example a lot of cars now are sold on a 235/265 stagger, and they may do well on a 245/245 square setup. However, many people mistakenly think wider = faster, and will do something like change the 235/265 to 265/265. Wider is not always better, particularly in the front. It makes turning feel heavier and makes turning response not as sharp. It produces more tire scrubbing in low speed full lock turn. It takes the tires longer to heat up, so it can actually make you slower in autocross scenarios. It makes the tires heavier which makes you slower.


Beware copying racers' setups.

Racers run way more negative camber, like -3 to -5 degrees. That's partly because they are taking hard turns all the time, with few straights, but it's also because they are running true competition slicks, which are a very different tire compound and need/want the greater camber.

Racers run much wider tires. This is good for them for a few reasons. One is they never actually make very sharp turns (most race cars have horrible turning radii), they only ever turn the wheels slightly, so they don't need the nimbleness of narrow tires. The other is that they are going so fast that they can warm up the wide tires - under street or autocross type use very wide tires never come up to temp and thus can actually have less grip than narrow tires.

Also more generally, race cars are usually set up because of the weird specs and rules of their class, not because it's the best way for them to be set up.


A collection of quotes that I found informative :


"The way you take a corner in a 911 is brake in a straight line before
entering the corner and get your right foot on the throttle before
turning into the corner. Use light changes on the throttle to keep the
rear end stable and use weight transfer to control understeer/oversteer.
The front end will bite and turn in. At or before Apex, start rolling in
throttle. Most corners you can be at full throttle before corner exit.
There is not another car out there that can come off a corner as fast as
these cars, but a lot of cars that can enter faster. You do not want to
drive it just trying to push through a corner like a front engine
understeering car."

"I race a FWD 1999 Honda Civic in the SSC class in SCCA
races. The trick to get FWD cars to rotate is to pump up the rear tires.
The rules won't let us modify suspensions and, in the Civic, there's
very little camber available, so we run with cold air pressures of 33F
and 37R. In a Neon I used to race, I would run with over 40 psi in the
rear."

"When I got my 2004 M3 I played with air pressures and ended up setting
them at 35F and 42R. This was on a car with the factory alignment and
Michelin Pilot Sports. At the Summit Point track in WV, with these air
pressures, I got NO understeer in the M3. Mr. B"

"To use oversteer to rotate your car prior to the apex you turn in early
and trail brake hard. The heavy braking while turning shifts your weight
forward reducing traction in the rear which induces oversteer. You
better not plan on lifting to catch the car though or you will be in the
weeds. The right thing to do is to transition from trail braking to
fairly heavy throttle (depending on the corner) to shift the weight back
to the rear taking the car from oversteer to neutral at the apex.

Because you come into the corner early and fast and brake very late this
can be very fast but it's not for beginners. One false move and you are
in a world of hurt. 

If you really feel the need to change your car to help with this the
best way to help the weight transfer. If you have 2 way adjustable
shocks increase rebound in back allowing the rear to lift more under
braking or decrease compression in front allowing the it to dip more or
both. 

If you don't have adjustable shocks do something to increase turn in
like softening the front bar or stiffening the rear. The downside is
that this will also increase oversteer under acceleration at and after
the apex. "

"Same principles as the M3 game, Ron. For less understeer: more camber
in front, less camber in rear, higher pressure in rear, less pressure in
front. Anything to increase grip in front and reduce grip in the rear
will result in more neutral handling. Of course, you can go too far in
one direction and create an oversteering monster a la 930s, et al."

"My car is my daily driver, and I care how quiet and comfortable it is.
I think that all camber plates have monoball mountings, so there is NO
rubber bushing at the top of the strut."

"There is almorst no increase in noise & rattle with the Tarett camber
plates. removing the rubber makes a big difference in turn in and
getting the car to take a set."

"If I remember right when we built the Spec Boxster we were
after 3.5 degrees in the front. You will pick up lots of camber when you
lower the car. I thick we were at 1.9 degrees without any shims. After
that the rule of thumb was .1 degrees for every mm of shim. I thick we
went with 16mm of shim to get our 3.5 degrees."

"I'm an instructor with LCAs and only use 1.8 in front. It's enough to
really help the tire life. I'd be faster with more camber but I drive
the car a lot on regular roads and don't want to muck up the everyday
handling."

"Track driving is a different story with different settings. Don't fall
into the "Negative-camber-mania" accompanied by excessive lowering"

"Worst case is a lowered car on stock bars, which will have lots of body
roll and the camber will go positive (or less negative) with only a bit
of compression. Add that to the positive camber from body roll and your
outside front tire could go several degrees positive in a hard turn. "

"On my car--light weight with fairly stiff suspension and not overly
lowered--I couldn't even use -1.5� of static front camber at street
speeds. It was cornering so flat that it wasn't "using up" the camber it
had. Front end bite was actually much better with -.8� front camber. At
the track, I'd probably want that -1.5� or even more."

"collectively- we spend a lot of time tuning our cars for the "ultimate
set up" with high amounts of camber- stiffness in the sway bars etc.
this is a DRY set up and will in fact get you in trouble on a wet
course."

"You'll find that you lose a lot of feel with the 26/24mm sways because
the car isn't rolling at all. That probably makes it faster, but IMHO
it's not as fun. And without body roll the car can seem a bit more
unpredictable /unstable."

"My understanding, and I'll confirm once I get lsd, is that you should
run without a rear sway if you don't have lsd, run with rear sway if you
do."

" Take out that front sway and put the stock one back in. Then put the
rear sway on full stiff. I don't understand why so many people continue
to put stiffer front sways on the front of the Turbo's. Even on full
soft, it's got to be well stiffer than the stock unit. You just
increased your front spring rate (as springs and sways work together)
and added understeer. Especially H&R's which are pretty darn stiff.


"FWIW I ran the GT3 rear bar (full stiff) with an OEM M030 front on my
car when I was on the stock M030 dampers and H&R springs and the car
drove great compared to when the stock rear bar was on there. Helped the
car rotate, reduces the understeer some and was a cheap solution to
making the car handle better with what was on the car at the time.
Totally worth the time and money if you ask me"

" Steve and others, I've not had good results running less negative
camber up front. My car is AWD still, and I played around with a
different setting earlier this year with horrible "Put it on the
trailer" results... Setting the car up with as little as 1/4 less
negative camber up front than the rear made the car a handful to drive,
to the point that I was off course twice on highspeed corners... I am
having my best results running about .4-.5 degrees MORE negative camber
up front than in the rear, and tend to float around at -3.0 to -2.8
upfront and -2.5 to -2.2 in the rear. This is on my 18X9 and 18X12 CCWs
with NT01s on. I also run the front swaybar in the softest setting and
the rear in the middle of three settings... If I want more rear
rotation, I tend to go stiffer to the inside hole. 

I've also raised my rear ride height by about 3/8th inch and that has
helped with high speed braking a little bit... I think that additional
height may have put the rear wing in the air a little more to help with
some downforce out back... Can't wait to get the GT2 nose on the car to
see how it balances things out... " 

"My observation is that the rear sway setting affects your ride quality
quite a bit, more than the front. And the front setting affects steering
feel."

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