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Mini Cooper S (4g63 swapped AWD) Setup for Rally Racing has too much understeer

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I am looking for some help and ideas on how I can improve the suspension and balance of my car. I have finally started racing my project car in SCCA rallycross (amature rally with no jumps and no roll cages so basically dirt autocross lol). During the first points race of the season (mid feb), I learned that the car has horrendous understeering and it is nearly impossible to get any real rotation in turns while maintaining speed/momentum. Ideally, I don't want to lower rear grip to balance the car's handling, I would like to improve the front grip as much as possible. I won't be able to fix everything before the next race, but with 1 points race per month, I could tackle several small modifications and repairs between race events and leave the biggest ones for between the seasons. My car is in the highest AWD class (modified AWD) so not much is considered against the rules. It is a road legal car so I won't be removing airbags.

Setup: The car is engine swapped and converted to AWD from a 1991 eclipse GSX. So cast iron engine with very heavy gearbox and transfer case. I am estimating the total car weight around 3000 lbs (probably a little less) with front to rear weight distribution at ~70/30. I am running 16 inch wheels with blizzacs in the front and fairly worn all seasons in the rear (4 blizzacs had even worse understeer). Front suspension (LCA, bushings, knuckles, hubs, but not springs and struts) and brakes are from the eclipse GSX, while the rear is from the Mini Countryman SUV to allow for AWD "bolt in" trailing arms and hubs. The front differential is open, but the center diff is a viscous coupler as is the rear. Currently, the car does not have any sway bars hooked up due to packaging. There is a way to possibly connect the rear with some fabrication but the front doesn't have enough space to rotate. My previous attempt on the rear swaybar did not have enough room and collided with the trailing arm under high compression (picture attached). The car also has a 3 inch lift via spacers on top of the strut towers and is running springs and struts for the SUV version of the car (countryman). They are stiffer than the mini cooper's and have more stroke on the strut. This is where I am stuck now.

What are my possible corrective actions or improvements? Here are somethings I have been thinking about. Is there something I am not considering or should avoid doing?

1) The front of the car is HEAVY. If I can move ~300 lbs to the rear, I would be about 60/40 depending where that weight actually sits. The issue is that by shifting what I can to the rear (battery, radiator) and lightening forward components (bucket seats, tubular front subframe, aluminum front brake calipers, remove ABS), I realistically could only move maybe 100 lbs and a bit over half of that being on the simple side (battery, bucket seats, brakes).

2) Shift brake bias more to the rear. The brakes are likely not matched as I went with OE options for the exclipse and countryman so the rears I suspect are undersized. I could improve the brake pad material and/or install a bias knob to lessen pressure to the front brakes.

3) Make a visous coupler eliminator to lock the center differential. This would shift the fwd bias transmission to be full 50/50 4wd. This could improve acceleration and lighten the force going to the front wheels and possibly promote a little more oversteer, but it would limit driveability on the street so I would need to swap it in and out (less than an hour probably to swap the viscous coupler for the eliminator shaft).

4) Add sway bars. Right now, this would probably be limited to the rear sway bar and it would need to be installed inside the boot with long tie rods through the floor to allow for the full range of movement. In order to fit one in the front, I would likely need to fabrication a fully tubular subframe to allow space for a sway bar to function.

Attached Files

When exactly does it understeer - on entry, mid-point/apex, on exit? Is it worse or better under power?

I would suggest first checking the suspension geometry front and rear - a quick look suggests you have probably raised the front roll centre which will increase roll resistance and load the outer tyre more, and the rear looks like it has a lot of camber camber gain in compression/roll which may be increasing rear grip.

1/ yes, that would certainly help - and don't forget moving the driver back even a small maount gan help - especially if a bigger chap/lass.

2/ that should help if trail braking into the corner(s). It will also give options for trimming it for different conditions. A cheaper option may be selecting higher co-efficient of friction rear pads?

3/ that may aid straight line acc'n, but will usually increase understeer as the front tyres will be forced to slip more - however, if they have more weight they may force the rears to slip a little more. If you have an eliminator shaft may be worth trying - and if it doesn't work you can keep it as an emergency replacement if you have a coupler fail on an event.

4/ that should help as increasing rear roll resistance will bias the lateral grip forward. Using springs would compromise the suspension travel and conformity over yumps and dips. A front one shouldn't be needed.

Do you really need the vehicle lifted? While it allows softer, longer travel springs for better traction over undulating ground and helps absorb impact after jumps, it will also increase load transfer under braking, cornering, and acc'n - only the last will be a benefit.

If you can, run it as low as practical while avoiding hitting the ground - if running a sump/underbody protection plate, some light contact is going to be an acceptable compromise.

It's expensive, but you should find a useful benefit if you fit a torque biasing (Torsen/gear type) front differential. They limit wheel slip under drive without locking the two sdes together. Alternatively a single acting plate type may work as it will only lock under power, not braking.

I would suggest fitting at least a minimum cage - it's easy to "trip" in ruts and can save you a head-ache.

Hey Gord, thanks so much for the detailed reply. Those are great points so let me try to address each one individually.

I didn't take a huge note of this before, but I would say the understeer happens the worst in the mid corner and I need to slow significantly to get any real turn in by the exit of the corner. Otherwise it just carries through. I experimented a lot with harder left foot braking and pendulum turns in most corners on a different track. This track had more ruts about 30cm deep so I was also "hooking" the front inside tire on the rut to help turn. Now this is the first time the car has ever drifted instead of plowing through the dirt. Because of the 3 different major changes (track with ruts, heavier left foot braking, and pendulum turns), it's hard to say where the biggest change came in. I didn't have a memory card during that race 🤦‍♂️ but, I have one this weekend for a 2 day race at the same track. I can hopefully post up a video or two next week of the runs (interior and exterior videos) and get your impressions.

The other thing for this race is that I connected the rear sway bar. I didn't do any measurements or calcs because it's the only bar I have available, but I'm sure it's stiffer than stock even at the softest setting.

Yes, I'll definitely need to sketch or model out the suspension geometry to get a more precise understanding of the movement and relative movement front versus rear. The rear does gain a lot of camber under load. This was the stock Mini Cooper design and I only changed it to adjustable bars. That being said, if I have room, I have considered lowering the center lower arm connection which would decrease camber gain under load, but I don't know how far I should go to change camber gain which can be useful in corners. When I measured years ago I think it was around 2-3 degrees of camber gain through the travel with maybe 2 of those happening from ride height to bump stop engagement. Ideally, I'm of the philosophy that decreasing rear grip should be the last resort when balancing the car, but I am limited on front corrections.

1/ I'm pretty short, but I'll try to sit as far back as I can while having good posture. Nore sure if I'll be done with the battery relocation before Saturday morning, but I should.

2/ well it wasn't cheaper to get the rear pads, however, it was loads easier and faster to install. Once I get the bias valve hooked up later, I'll balance out the compounds and then use the bias to balance it out. I still have to bed them in before the race as well.

3/ that's very true. I don't have a coupler eliminator, but I do have a spare viscous coupler that I planned to cut up on the lathe and weld into an elimination coupler. But right now that seems like very high effort for likely not the effect im looking for. Plus it takes handbrake turns completely off the table.

4/ I'll have to calculate out a bar stiffness and see what I can come up with. This street sway bar is likely too stiff, but it'll be helpful to see the extremes and note the effects to narrow down the direction I should probably go. If it's too loose, I could put the same tire compound I have up front to the rear which should limit oversteer a little and then give more straight line performance too. I'll have my 4 off-road tires and street tires to swap around during the event.

The lift from my perspective is pretty critical with the AWD swap as the transfer case, driveshaft carrier bearings, and exhaust are the lowest hanging components. I actually ripped every the exhaust hanger and dragged my exhaust back to the staging area with safety wire lol. A skid plate may help to protect the bottom much better while allowing me to lower the car slightly. Now I did fix all the bump steer when I originally built the front so I may need to change mounting locations and whatnot to support lowering the car without reintroducing that aspect.

I agree, since the front is the only remaining open diff on the car, some sort of biasing would be extremely helpful. I'll need to think more about whether I should do 1, 1.5, or 2 way locking and adjustability. Since the 4g63 and transmission aftermarket is have is much more drag racing oriented, finding the right parts combination may be a touch difficult/expensive.

I don't think I have seen anything flip in these races and they even have trucks competing, but it's likely something I'll look at more seriously as this goes further beyond daily and into dedicated race car territory. It'll also never be competitive at the top levels anyway so losing the "dailyness" would be a bummer.

- Ryan

This is the current trunk layout with the electric AC compressor, intercooler tank, and battery box. This is without the false floor in place

Attached Files

Ah, yes, a skip plate may be a wise move, and with 30cm ruts I can see the need for the high ride height.

I would hope you've some substantial plating underneath the components in the boot (trunk), as even a mild head-on could otherwise send them through the back seat and potentially injure or kill you.

While the Mitsi' may be more commonly used for drag and sprints over there, don't forget they've had a very long career in the World Rally Championship, so there will be solutions for the problems you're having, the trick is finding them at a reasonable cost.

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