BimmaruJim
BimmaruJim
9/24/20 10:43 p.m.

Hey all,

Have jumped in to check out GRM in the past and really appreciate the info in here a lot. Was hoping to ask a question about suspension and wide body race cars that's been on my mind for some time.

Background - Amongst many other projects, I have one 1988 BMW E30 325is that I've completely restored mechanically and it is an absolute blast to drive. Now I have an 87' 325is that I'm starting to dig in to and, since I already have one nice E30 the way BMW intended it, and since this one is in somewhat rough shape, I'd like to throw all caution to the wind and try something crazy with the 87.

I've always LOVED the way the McLaren BMW E21 turbo race car looked and from what I can gather, when it wasn't broken, it was quite a performer as well. http://www.speedhunters.com/2009/09/car_feature_gt_gt_mclaren_bmw_e21_turbo/ In my mind's eye, I see building my own E30 variant. Yes it's probably crazy, will probably be an expensive custom nightmare, I should probably have an aerospace degree to even consider it etc etc etc. Oh well. It would be a full race car. Not a "I'd like it to break track records and be comfortable on my daily commute to Best Buy" build...

One of the hallmarks of the race cars of this era that made them look so awesome was the wide bodies and wild aero integrated into those wide bodies. The aero in and of itself I'm not even ready to tackle on the drawing board yet. What I've always wondered, though, is how suspension is working in these cases.

When cars go wide like the McLaren BMW E21 Turbo (I.e. for performance and not just looks) - is it simply for wider tires, is it to accommodate some kind of wider track from a different suspension system, or both? It doesn't take long to look at a race car like that one and see its tires and realize they were certainly trying to pack in some big meaty pieces of rubber, but is all the suspension behind those wheels still the same geometry as a stock E21 in that case but with higher end components? Or is the geometry totally changed as well? I feel like the idea of just hanging some super wide wheels and tires off a suspension that's normally designed to have a more compact wheel and tire much closer in to the body would just create a giant stressful lever arm way out there that would wear components out mega quick. And maybe that's just it too - stop being poor and be ready to replace stuff all the time b'cuz race car. 

In the E30's case - there is the obvious E30 M3 with a slightly wider stance than my regular ol' 325is. From what I can tell, aside from different front control arms (Which I actually have on my 88 325is), it doesn't seem like there was a different suspension system design from the 325is, but maybe just nicer components for more racing applications? Was it just then the wheels and tires that filled out that wider body? In my case, I'd probably be looking to go even wider than the M3 to accommodate all my crazy aero I haven't figured out yet... So perhaps something more dramatic would be needed. The front suspension setup seems like it's been played with a fair amount over history on these cars. The rear being a semi trailing arm might be a challenge.... If only there were nice images online of the underneath of that E21... I believe it, too, in stock form, was a semi trailing arm like the E30? 

z31maniac
z31maniac MegaDork
9/25/20 6:51 a.m.

I searched for "bmw e21 rear suspension" and found this in about 30 seconds. It's very similar to the E30. 

http://the320i.blogspot.com/2010/07/before-rear-differential-subframe-and.html

The difference being on a normal E30 the springs are separate from the shocks. Other than that, without plans of what you want to do, where you want to race, etc, it's going to be hard for people to give you any advice.

BimmaruJim
BimmaruJim New Reader
9/25/20 7:09 a.m.

I did the same thing. Should have been more clear. I was referencing the McLaren BMW E21 turbo when I said I wished I could find good pictures of the underneath for an idea of the suspension setup. This is the closest I got midway down the page - http://www.bmwe21.net/?page_id=496

As for people giving advice, I'm not really looking for how to set up the car for a particular track yet. Chances are that based on my area, it will be somewhere in the upper Midwest more often than not like Road America or Blackhawk, etc. Admittedly, I'm not as concerned with making a car to go out and beat everyone with or compete in a particular class/series/division/whatever. I just want to make a car that emulates the race cars of that E21 turbo era and has some level of actual real thought for performance put into it rather than just slapping parts on to look cool. 

So at this point I'm still just wondering - are wide body race cars just using the same stock suspension geometry with big fat wheels and tires and a body wrapped around it all? Or are they going completely custom with suspension geometry, mounting points, subframes, hardware, etc in order to accommodate the wider track?

Just learning at this point. The specifics and details will come later.

adam525i (Forum Supporter)
adam525i (Forum Supporter) GRM+ Memberand HalfDork
9/25/20 7:26 a.m.
BimmaruJim said:

So at this point I'm still just wondering - are wide body race cars just using the same stock suspension geometry with big fat wheels and tires and a body wrapped around it all? Or are they going completely custom with suspension geometry, mounting points, subframes, hardware, etc in order to accommodate the wider track?

I think the answer to this is both depending on the series the car is competing in and the rules involved.

This thread might be interesting to you, the style of flares you are talking about are a recent addition - https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/1987-bmw-325e/75671/page1/

Start reading!

ojannen
ojannen Reader
9/25/20 8:00 a.m.

If you start with 15x9s and 245 section Miata tires, you are going to be required to run a wide body.  I don't think you need any reinforcements or gemoetry changes.  Your scrub radius will get weird and kill your steering feel though.  I would also look into a camber and toe kit for the rear so you can lower it and get the stance right.

Unfortunately, you can't go any wider than that on the street due to tire availability.  You would have to add spacers for a specific look at that point.

Vorshlag won the GRM 2K challenge a while back using an E30 with box flares.  I could see an alternate build with the DTM style flares and roughly the same fitment.

Slippery (Forum Supporter)
Slippery (Forum Supporter) GRM+ Memberand UltraDork
9/25/20 8:24 a.m.

I am sure the car you are referencing has completely custom suspension pickup points as well as suspension. 

Watch this movie, you will like it:

z31maniac
z31maniac MegaDork
9/25/20 9:31 a.m.
ojannen said:

If you start with 15x9s and 245 section Miata tires, you are going to be required to run a wide body.  I don't think you need any reinforcements or gemoetry changes.  Your scrub radius will get weird and kill your steering feel though.  I would also look into a camber and toe kit for the rear so you can lower it and get the stance right.

Unfortunately, you can't go any wider than that on the street due to tire availability.  You would have to add spacers for a specific look at that point.

Vorshlag won the GRM 2K challenge a while back using an E30 with box flares.  I could see an alternate build with the DTM style flares and roughly the same fitment.

Being that 245/40 are about 1" diameter less than stock, I'd bet a heavy pull on the stock fenders would work unless you're shooting for DTM-style ride height. But then you get into another can of worms with roll centers, etc. 

BimmaruJim
BimmaruJim New Reader
9/25/20 12:02 p.m.

In reply to Slippery (Forum Supporter) :

That's pretty much what I figured, and I'm assuming the custom suspension would then be designed with a wider track, rather than a stock geometry with a wider track simply achieved by big ol' meaty rubber and wide wheels stuck on the end. 

I already like the looks of that movie based on the E9 on the front - going to check it out! Much appreciated.

BimmaruJim
BimmaruJim New Reader
9/25/20 12:23 p.m.
ojannen said:

If you start with 15x9s and 245 section Miata tires, you are going to be required to run a wide body.  I don't think you need any reinforcements or gemoetry changes.  Your scrub radius will get weird and kill your steering feel though.  I would also look into a camber and toe kit for the rear so you can lower it and get the stance right.

Unfortunately, you can't go any wider than that on the street due to tire availability.  You would have to add spacers for a specific look at that point.

Vorshlag won the GRM 2K challenge a while back using an E30 with box flares.  I could see an alternate build with the DTM style flares and roughly the same fitment.

Just now checked out that Vorshlag car and it looks cool! Can't believe I hadn't seen that yet. This thread on their build is extraordinarily well documented and extensive - pretty cool stuff I'm going to continue to dig in to - https://www.vorshlag.com/forums/forum/vorshlag-v8-swap-talk/engine-swap-discussions/6108-vorshlag-2010-grm-challenge-car-bmw-e30-v8

Looks like they were obviously going with the budget/parts bin approach and made something work, so that's encouraging to see considering I wouldn't be trying to limit myself quite that much. 

ojannen
ojannen Reader
9/25/20 12:47 p.m.

In reply to BimmaruJim :

My vote is to decide ride height and wheel/tire package first, then figure out what works with it.  If you are going for the classic DTM look, it is going to be tough to reproduce the stance of fat bias ply tires on 16" wheels if you want street legal tires.  Do you go with shorter overall tires on 15" wheels or low profile sidewalls on 17" wheels?  You can always use different offsets and widths for a Cars & Coffee setup and a separate race track setup.

BimmaruJim
BimmaruJim New Reader
9/25/20 1:25 p.m.
ojannen said:

In reply to BimmaruJim :

My vote is to decide ride height and wheel/tire package first, then figure out what works with it.  If you are going for the classic DTM look, it is going to be tough to reproduce the stance of fat bias ply tires on 16" wheels if you want street legal tires.  Do you go with shorter overall tires on 15" wheels or low profile sidewalls on 17" wheels?  You can always use different offsets and widths for a Cars & Coffee setup and a separate race track setup.

Cool I'm following what you're getting at and that makes sense. 

Wheels and tires are quite the pandora's box with the huge variety out there and the effects they can have on the performance/feel of the car. Obviously staying more close to stock makes things easy, there's more documentation of what to do and not to do, but when you go full custom, it gets pretty wild. 

The books I've gotten into thus far have yet to crack into wheels and tires very specifically at this point (At least in terms of sizes, and offset and changes to these things can affect the car). I'd love for anyone on here to school me further on wide wheels/tires if they're feeling particularly scholarly.

Obviously my motivation here, as with many others I'm sure, comes from a historic and really badass look. I want the car to look a certain way. That said, I want it to perform well, too. Conveniently, the cars I'm trying to get it to look like, also seemed to perform quite well, too. What is it that the super wide wheels and tires are doing for these DTM cars? Did they, like you say, start out knowing what wheels/tires/ride height they wanted and then just wrapping everything else in and around it? Usually it appears they have a staggered setup with more meat in the back than in the front. I can only assume that more meat in the back means more grip to put power down, and less meat up front means less effort needed to turn the tires and less wear on the components up front related to steering? But then just slapping big tires in back and slightly smaller ones up front without doing anything else gets you to the point where I would imagine understeer comes in to play... Unless you start playing with bars and alignment? 

If anyone is aware of any great writeups, videos, book recommendations, OR alternate realities I can look into where I was born into a family of race car engineers, aerospace scientists and machinists to learn more on this, please shout them out!

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