Solid rear subframe mounts

[astondg]

Background:
- I have a 323i that is my road car and also used for motorsport and track days. I do about 7 Autocross (dirt circuit) events a year and probably the same number of track days / sprints, all up about 15 to 20 events a year. It has Koni adjustable dampers, about 1" lower King springs, M3 offset poly control arm bushes, front adjustable strut top mounts, rear adjustable poly trailing arm bushes, semi-race Toyo Trampio R1R tyres. I think that is about all for the suspension.
- I know a mechanic here in Brisbane that I sometimes take my 323i to. he seems to know what he is talking about and he raced an E30 with an S14 and M3 drivetrain and suspension (it was probably at least part way to Group A type spec but in a normal E30 bodyshell) and now he is building up an E36 M3 (but using a 4 door factory Motorsport shell) for racing. He has also built, or at least worked on, other E30s and BMWs for racing.

Anyway I took my car there today for a service and asked him to check out the rear subframe bushes for me. He said they were completely worn out. I asked him if he could raplace them for me and he said he normally replaces the bushes with solid mounts (he said steel and aluminium) which makes a big difference. He knows and I reminded him that this is still my road car (I mentioned it twice) but he still said that he thought they would be the best way to go and they wouldn't be too bad on the road. I tend to trust him because he has always been honest with me and the two major parts he fitted to my car, a motorsport 4.1 LSD and full 2.5" straight through exhaust with headers, have been very good and are probably the 2 best bits of my car. The exhaust can drone only very slightly at constant higher speeds and the diff chatters slightly but I am happy with that and I wouldn't swap them. They actually make it sound a bit race car like which is kind of nice . BUT I know a few of you recommend against even poly bushes for the subframe because of the harshness. I'm not exactly sure what to do? He said the solid mounts make the handling a lot better which is nice (at the moment it is feeling a little wayward at the back on the dirt which doesn't give me confidence) but I also don't want it really harsh, I still have to drive it around everyday.

What are peoples opinions?

Thankyou,
Aston

[kam-325i]

It can be done, but all of the noise and stress's that are normally absorbed by the bush's will be transmitted to the chassis, They mush also allow some flex, or something will snap.

I would not personally recommend them for a car that will be used on the road.

[astondg]

So you think something might break? Is that the general opinion? I guess he would have had his race E30 like that but then it would have had a lot less stuff to break (it was a stripped out race car) and maybe he had to fix it often.

I would be interested to hear from M5Pilot, I think he is normally against even the poly bushes but my situation might be a little different because I do a bit of motorsport?

Aston

[Ziggy]

I'm not sure about those bushes specifically, but in general, solid bushes are a bad idea on anything but a pure race car. Like Kam said - if they can't move, something that wasn't supposed to, will!

[kam-325i]

My thinking is this, bushes are design to allow some flexing and to absorb stress'es. Its easier and cheaper to replace a bush that has worn. If you use a solid bush over a flexi one, 90% of the stress that would normaly be absorbed by the bush will be tranmitted to the chasis, this is fine if the chasis has been modified to accept this extra stress, Most motor sport cars, and the M3 have a bonded windscreen and rearscreen, this adds to the stiffness of the chasis, a normal E30 front and rear screen is not.

You can run with solid bushes, but you must think about where the stress that would of been absorbed is going......

[astondg]

I think his E30 race car had plexi glass (or whatever it's called) type windows, I could be wrong though. If they were then they wouldn't give much strength would they? I have seen an onboard camera in the Carrera Cup out here (GT3s with the same type rear windows) and their windows wobbled and flexed the whole time.

BUT ok, I will stay away from the solid mounts. What about the poly bushes for my situation? I really would like to improve the handling for the track if I could, and that mechanic says that the subframe bushes make a big difference, but I do still have to use it for my road car.

Thanks,
Aston

[kam-325i]

The Stiffer bush's will make the car handle better on the track, but for this better handling, your sacrifice will be more road noise tranmitted thru to the chassis and cabin, I know of loads of peolpe how have fitted the stiffer poly bush's (powerflex), they say they are fine if you are willing to put up with the louder noise....

I just think that the solid ones would be great for a track/race only car, but to much for a car that will be used on the roads.