Mechanical Grip

[Jean]

Has anybody here researched or done any testing on increasing e30 mechanical grip?

In my example specifically, 2500lb car on coilovers and 15x8 tires with 225 width Dot R tires.

I am wondering if going with wider tires (possibly will require bodywork) would be the next logical step VS aero ?

How much difference or improvement can a better diff provide ... 25% vs 40% vs gear diff ?

[frankenbeemer]

Good question. I don't know the answer, but I'm interested.

[CorvallisBMW]

How much extra grip is dependent on several dozen factors. Spring rates, roll bar rates, damping/compression rates, roll center height, tire size, sidewall flex, tire compound, wheelbase, track width, aerodynamics influences, speed, tire slip angle, weight transfer for/aft and side/side, turn radius and rate, etc...

Even F1 teams can't calculate everything that goes in to grip. The best thing you can do is follow "best practices" when modifying your car. Lower cars have a lower center of gravity, meaning less weight transfer. Stiffer springs and shocks can help keep wheels on the ground, more tire usually equals more grip, wider track usually equals more grip, etc.

Diff lock-up ratio will only matter if you're in a turn and actively putting down enough power to cause wheel slip.

Are you racing on a track or autoX? Aero for those two different driving styles is very different.

[Jean]

Autocross... I am just trying to learn more on what will make the biggest difference basically.

I met with a friend of mine today for lunch and we were talking about this... so I figured maybe some of you have gone through this already.

I see cars even at the local autocross events use Aero, and since it's not free (weight) it must work even at these speeds....

[L8 apexr]

There isn't really a right answer,if your car is optimised(just having coilovers means jack)and using every bit of the 225 to their maximum then yes going to wider tires will improve mechancial grip.Of course that increase will only come if the compound used allows the tire to get to optimum temp.
Basically you need to know for sure that your alignment is perfect and air pressure(proper tire temp spread),the correct amount of roll resistance AND the correct balance between spring and bar rate,the compound used is correct and that you able to drive it consistantly to the limit.
From my experience sway bars should be used cautiously,they are definately not a bigger=better deal.

[CorvallisBMW]

Very aggressive aero can work. Large front splitter, dive planes, large rear wings, etc. You just won't get any benefit from under body trays, rear diffusers, trunk lip spoilers, things like that. They all require much higher speeds to generate downforce.

For autoX the best thing you can do is stiffen up your suspension, get adjustable swaybars you you can tune the rotation of your car (under/oversteer) and wide, sticky tires.

Also, don't forget tire pressure. It may be one of the most-overlooked things you can do and has a huge benefit. Also remember that r-comp tires are designed to work at hi temperatures, they won't get warm enough on a 60-second run to reach optimum grip levels.

[z31maniac]

Custom SLA arrangment up front?

[Jean]

We'll see tomorrow how the car behaves and go from there.... I have a feeling this car will get yet another change.

DRS? i kid i kid.

[L8 apexr]

Ahh autox,I assumed track day stuff.My earlier posts still applies but autox is a different animal,response is key.
Unhook one end of the rear sway,you need to be able to use both rear wheels to their max for corner exit drive-the sway takes weight of the inside rear and adds(most)of it to the outside rear.In effect you have reduced grip from the inside due to lack of weight on it and reduced grip at the outside rear due to being overloaded.
I'd suggest an adjustable front bar to tune with differeing suface temps/grip levels.At least -2.5 front camber and front toe out,if its driven to events than about 1/8 to a 1/4".
What tires are you using?,if your looking to go fast you need to run R-comps-streets suck once you start using R's.For autox go to the shortest tire you can run,I assume your in a class you can run wider wheels?.If so the widest wheels with the widest Hoosier's you can run will be fastest,the 275 15's will be the right tire for that weight.They work fine on a 9" wheel(10 would be better),you only get a few good runs from hoosiers before they start to fall off.If those aren't in the budget the next best for grip is likely the khumo 710,great grip and long life so they are what I tend to run(my open wheel car uses non dot slicks,too light for R's to get up to temp).710's don't have a decent 15" size though,205-50 and 225-50.
Stay away from toyo R888,RA1,nitto nt01 etc as they are decent for a track day but too hard to use for autox.I tried the R888's on the corolla(225 R888 to a 215 710) back to back with khumo's,the 710's were over 3 seconds faster on a 50 second course.Car felt terrible on them as well.
Good luck at the event today,I'm still a month away from the 1st event up here in the frozen north.

[Jean]

Ahh autox,I assumed track day stuff.My earlier posts still applies but autox is a different animal,response is key.
Unhook one end of the rear sway,you need to be able to use both rear wheels to their max for corner exit drive-the sway takes weight of the inside rear and adds(most)of it to the outside rear.In effect you have reduced grip from the inside due to lack of weight on it and reduced grip at the outside rear due to being overloaded.
I'd suggest an adjustable front bar to tune with differeing suface temps/grip levels.At least -2.5 front camber and front toe out,if its driven to events than about 1/8 to a 1/4".
What tires are you using?,if your looking to go fast you need to run R-comps-streets suck once you start using R's.For autox go to the shortest tire you can run,I assume your in a class you can run wider wheels?.If so the widest wheels with the widest Hoosier's you can run will be fastest,the 275 15's will be the right tire for that weight.They work fine on a 9" wheel(10 would be better),you only get a few good runs from hoosiers before they start to fall off.If those aren't in the budget the next best for grip is likely the khumo 710,great grip and long life so they are what I tend to run(my open wheel car uses non dot slicks,too light for R's to get up to temp).710's don't have a decent 15" size though,205-50 and 225-50.
Stay away from toyo R888,RA1,nitto nt01 etc as they are decent for a track day but too hard to use for autox.I tried the R888's on the corolla(225 R888 to a 215 710) back to back with khumo's,the 710's were over 3 seconds faster on a 50 second course.Car felt terrible on them as well.
Good luck at the event today,I'm still a month away from the 1st event up here in the frozen north.

I have some of the stuff you listed already, no rear sway, cabrio front sway and getting m3 end links for next event. Wheels are TRM 15x8, tires are V710 225/50 size, heat cycled. My spring rates are 550front/675rear, gc coils and camber plates, about -2.5 or 3.0 degrees front camber, shocks for now are just re-valved bilstein sports.

We did good, top time of the day was about 47 second in the highly modified AH v8 car. Me and my co-driver got into low 55 and high 54 second passes. I think the only car that beat us in SM was a new v8 mustang with suspension and WIDE tires :(

There were a few bmws but not e30s today. M6, e46m3 supercharged, z3m roadster, z4m coupe, z3, e36 m3 etc etc. ... I believe we were the fastest bmw, at least in the first 2 run groups.

Two issues need to be addressed though.

1. there is almost no feed back in brakes.... hard to tell where the limit is before it locks up.

2. can really feel the diff unload, spin the inside rear, and then hook up .... really need to see about getting a different diff... maybe just a 40% lock will do?

Handling is fine, power is more than enough for now... at least with only 225 width tires.

[L8 apexr]

When you say heat cycled I assume you meant by the tirerack prior to use?.One thing with r-comps is they are a little harder to tell whats going on,no noise feedback so they can lock quite easily.Part of it is the added - camber reducing front contact patch,especially the inside front while trail braking.I've always been a little too aggressive with the trail braking,slowly learning to brake very hard slightly earlier(and mostly going straight) and then less trail brake pressure as you bend the car in.
What kind of diff are you currently using?

[joshh]

The guys winning nations are using the widest tire they can fit on the car. With the softest compound (Hoosiers without a doubt right now). They started fitting 275 and bigger tires on 6.5" wheels because the grip out weighs any other problems the tires create. Specially since the Hoosiers heat up so fast. Ridiculous I know but the rules allow it. So much for "stock" class.
If you're in a class that has a maximum width then you have to focus on all the other items you can adjust.
But tires and the compound are always the first mod that makes the largest difference. If you can afford it of course.
Aero is the last thing I'd worry about unless you're driving a Mod car.

[Fusion]

Awesome info in here once again.
L8: Since you just brought up the diff - I run a big GP 5 mile track (no official events) and on the longest straight, which is a 600 meter start/finish straight, with a quick left-right "S" turn just before it and a looong right after it. On the last turn I'm in 3rd, full throttle max rev out, into fourth, full throttle up to about 5000 or so rpm and over 100mph, then I have the let go and start braking for the long right.
Since there's nowhere on the track I could currently use fifth gear with the 3.73 open and the top speed it offers, should I be looking for a S3.91/4.10? Will the torquiness be beneficial on the track, specially the pretty steep hill climb portion?

[nrubenstein]

Awesome info in here once again.
L8: Since you just brought up the diff - I run a big GP 5 mile track (no official events) and on the longest straight, which is a 600 meter start/finish straight, with a quick left-right "S" turn just before it and a looong right after it. On the last turn I'm in 3rd, full throttle max rev out, into fourth, full throttle up to about 5000 or so rpm and over 100mph, then I have the let go and start braking for the long right.
Since there's nowhere on the track I could currently use fifth gear with the 3.73 open and the top speed it offers, should I be looking for a S3.91/4.10? Will the torquiness be beneficial on the track, specially the pretty steep hill climb portion?

Maybe. You really have to work out your speeds across the track and figure out whether you'll be doing a bunch of extra shifting. Just because you're better off in that specific spot doesn't mean that it will work out well in another part of the track.

Basically, the question in this case is "Will you have to use 4th instead of 3rd in a bunch of places?"

[Fusion]

I need to get that damn track day app working and then I'll be able to make precise judgements like that, or atleast the camera. But guessing based on the run two weeks ago, I'd say there might be two spots between turns where I'm currently only in third, the diff change may require 3-4-3. I'll study it, I'll be doing susp. changes now anyways so I'm just thinking ahead with the differential. Also because they're not easy to find once you finally make a decision :/.