Rear end slide

[BCMikeMR2]

Took the MR2 out to the autocross on the weekend..first time for me in a mid engine car, I usually drive front wheel drive classic Mini, so this was a bit different. My times were good considering it is a stock 91 NA with street tires..but I had one recurring problem. The course ended with a RH turn and then into a short stop box. I was consistantly loosing the rear end as I was coming around the RH turn and slowing down..no problems everywhere else as I could accelerate coming out of the other turns..any thoughts on how to deal with this..driving style?? tire pressure? Not wanting to modify the car as I want to keep it stock..

[Gairloch]

Do you know what your alignment settings are?

I take it you were trail braking through the turn?

What tire and tire pressures were you running? I have found that even small changes in tire pressures can make a tremendous difference.

[NONACK]

I have found, in AutoXing my 2, that the rear pressure can be very picky. Too low=slide, as well as too high, but if you're braking really hard in a corner THE ASS END IS COMING OUT. Just be ready for it with the right amount of countersteer. Anyway, if it only happens through the finish it probably looks intentional and shweet.

[MR2 Dad]

Mike,

It is the curse of the swing axle. You are unweighting the rear, the axles tuck under, the contact patchs shrink and the rear steps out. Done right you can do terrific cloverleafs, 180's and 360's. For your problem course you will need to brake early, change to the 93 and later positrack rear end with the revised links, or modify (look at 1961 Porsche Super 90 - leaf spring accross diferential limits swing arm drop).

Enjoy !

MR2 Dad

[Mr2crazybyong]

like MR2 Dad said, you have to modify especially the stock suspension, IMO this is the weakest handling part of our cars.. there have been many incidents and accidents due to this, happened to me too.. especially when you are new to MR2s.. so upgrade your suspension, it would be a great investment on your car.. stock sucks! peace out! hehe...

[David]

brake before you hit the corner, then when you exit bring it to a stop in the box.

[BCMikeMR2]

I was runing 40lbs in the rears and 38 in the fronts. Tire temps seemed fairly even across the tread. What pressures are other people running? Tires are Bridgestones, 195 front 205 rear. I realize I could upgrade suspension and get different tires, but I want to see what I can do with the car as it is. It sounds like I can play with the rear tire pressure, and then brake earlier entering the last corner..one of those 'slow down to go faster' moments on the course! I was entering the corner very fast as it was after a straight line, and then heavy braking coming out, which seems to be the worst thing for creating the slide.

[PearlJammzz]

If you are worried about points (not sure if yer racing in a class or what) a 93+ cross member, sway bar, sway bar end links, as well as struts/springs are much better than 91-92. Would be a decent upgrade and still keep you 'stock' I believe.

[David]

just upgrade the suspension to a '93+

[Gairloch]

Originally Posted by BCMikeMR2

I was runing 40lbs in the rears and 38 in the fronts. Tire temps seemed fairly even across the tread. What pressures are other people running? Tires are Bridgestones, 195 front 205 rear. I realize I could upgrade suspension and get different tires, but I want to see what I can do with the car as it is. It sounds like I can play with the rear tire pressure, and then brake earlier entering the last corner..one of those 'slow down to go faster' moments on the course! I was entering the corner very fast as it was after a straight line, and then heavy braking coming out, which seems to be the worst thing for creating the slide.

I'm with you on trying to figure things out before you start changing parts. I've talked to many autocrossers that prefer the '91 suspension over the '93. I think ultimately that ends up just being a matter of what fits your driving style better.

So you weren't even braking before you entered the corner? Was the slide starting when you hit the brakes or as you were crossing the apex of the corner?

If it was starting when you hit the brakes you were likely applying them while the chassis was still unsettled. If you apply hard braking while the chassis is still in movement you're demanding even more of an already compromised traction circumstance which is likely to create a slide. If you don't have room to let the car come to neutral before needing to brake for the stop box then you'd have to break your braking zone into 2 pieces. Brake hard once before the turn then again as you're coming out of it. If you do have room then you need to learn how to feel when the weight of the car has returned to neutral before applying brakes.

If the slide was starting anywhere around the apex of the corner than you're simply overdriving your available traction. The solution to this problem is to reduce your corner entry speed.

You hit the nail on the head, the mid-rear platform is deadly in enforcing "slow in, fast out". There are two ways to go about learning the traction limits of your car. One is to approach it from over the top, the second is to approach it from underneath. Approaching it from over the top will entail a lot of spins and loss of control. Approaching it from underneath *feels* slower but ultimately because you're not losing as much time overall it is faster. With both approaches you have to tune your "butt dyno". You have to learn how to feel exactly what the car is doing at any given moment. Consciousness of this is one of the biggest differences between an experienced driver and a newbie. As your comfort level increases and your butt dyno is more finely tuned your use of the ultimate traction available in your car is better and better creating faster and faster lap times. Most autocrosses aren't about how big your motor is and how hard you tromp on the throttle, it's about how much speed you can carry through the corners.

The suspension was changed in '93 due to the fact that under heavy corner loading the rear wheels were prone to shifting to a toe out arrangement causing the rear to come around. The point of the newer suspension geometry was to reduce the car's propensity for oversteering during heavy cornering. I think that if you do some research you will find that most serious MR2 autocrossers actually prefer the '91 suspension because they are aware of the car's behavior and are able to use it's propensity to rotate to get around corners more quickly.

I referred to 'ultimate traction' a couple of times. I don't know if you're aware of this but there is a finite amount of traction available at any given moment. Every input you give consumes some of this traction and if you demand more than it has to give you start sliding.

Here is a more detailed explanation of that concept. Chapter 2

Here is a pretty good illustration of the concept.

[David]

damn, i was just about to post that ^

This may sound stupid to go off a video game, but gran turismo has a pretty good explanation on wait transfers for every layout including MR. the main thing is your tires can only do 100% work. it sounds like your pushing (+100%), which means you can do less braking while turning (brake on the straight) and you'll have more traction when you turn.

[BCMikeMR2]

Thanks for all the info, I think I need to come in to the last corner slower as I was coming in fast and then the rear end was losing it as I braked. I think seat time and learning to read the car over time will help..next time out in a few weeks so will try again..although the course is different each time, so may not have the same issue to deal with, but this is good info to apply to any of the courses..

[Gairloch]

Your instinct of addressing the loose nut behind the wheel first is a good one.

[mopwer]

Try more toe in on the rear.

Greg

[Gairloch]

F150? Silverado? It does already have hooks in place.

[Spektrewing386]

so how did these guys do those 180 degree hairpins?

[Gairloch]

If you watch the tail end of the second one you can see it. The red MR2 going into the tight left hand around the grass.

What he's doing is load up the suspension on the left side of the car then kicks all the weight back over to the right be beginning his left hand turn and gets hard into the throttle. If you unsettle the chassis you can get throttle on oversteer with even an under powered rwd car.

Another thing you can do is toss the weight as above then tap the brake to start the rear sliding before you get on the gas.

What they are doing is really impressive. Takes some talent and or a lot of practice to make a car move like that.

Let me know if that doesn't make sense

[Mr2crazybyong]

don't they use the handbrake too?

[Spektrewing386]

i dont think so, i still see the wheels turning in the back

[Gairloch]

e-brake is another way to go about it but it's really not necessary.