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Canter lever/ inboard suspension.

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Hello everyone,

I’m new here.

I find myself in the position where I’m faced with what direction to take with the suspension on my e30. I’ve already concluded the factory rear setup won’t work for my needs.

So do I go to a levered inboard setup or stick with the common coilover conversion. I’m leaning to the canter lever set up but is there really any significant gains to be had?

thanks,

Leigh.

What are your needs that can't be satisfied with proper springs & dampers on the OEM suspension design? More suspension travel, fitting wider tires, reducing unsprung weight, support for huge aero loads (3rd spring)? Hard to know what to suggest without understanding the problem, or desired goal.

hi,

Firstly I found at my desired ride height the factory style suspension would coil bound the inboard spring set up sitting on top of the arms. Or the factory spring perches would smash into each other. The spring rate was also massive just to support the car under normal track driving. That’s why I got rid of that.

coilover conversion will require some chassis reinforcement but so will the canter lever setup, running the stock set up would require more to get the car as low as it is currently. The subframe is already 25mm higher then stock to correct arm alignment at the new ride height which also reduces this stock spring clearance. Also partly why the e30 dtm cars didn’t use it either.

what I’d like to achieve from one over the other is mechanical grip, and reduced unsprung weight if possible.

aero loads will be low as is the designed function of the car. Tyre size has been chosen and will fit without any issues.

bit of a waffle there. But hope that makes sense?

thanks, Leigh.

I would suggest the coilover conversion is the most straight-forward solution. While there are theoretical advantages to a push-rod (Cantilever) suspension, the magnitude of the effects are pretty small, unless you are already a tubeframe car, or just want to take on the project I don't think you need it to reach your goals.

Proper damper valving, spring / ARB selection, and alignment along with the all important tires will be what gets you the mechanical grip you are looking for. All of that can be done with a coilover conversion.

The best way to reduce your unsprung weight is by lightweight wheel and rotor selection. I think you can save more there than half the weight of a damper.

Good luck with your project and suspension tuning.

You didn't mention if you are required to use the basic OEM rear suspension design and are working around it's limitations, or if you are free to modify it as desired?

IIRC, the non-M3 E30 used a semi-trailing arm rear suspension which causes toe variation during it's travel, which together with the camber change, caused some 'interesting' handling characteristics - as you've found out. The usual 'cure' would be to use a high spring rate to minimise the travel - which you've found unacceptable.

There may be some confusion in the suspension options being discussed - a true cantilever suspension normally uses a longer top arm/wishbone extension with the pivot point being somewhere in the middle and the inboard extension compresses a 'coil-over' spring-damper assembly. Unless you wish to convert to a "double wishbone", or multi-link design, that isn't really an option. An intermediate option may be to fabricate a triangulated "cage" with a braced, vertical extension to which a coil-over can be fitted, probably in a horizontal orientation, and acting against suitably re-inforced parts of the roll cage structure.

There is also the potential for a push, or pull, rod linkage where the coil-over is located apart from the actual suspension and operated via linkages - while popular in some series, some thought has to be put into the loadings of the components, especially the pushrods as they will tend to flex and even buckle under high loads.

There are some options, if you're on a tight budget, you may be able to sink the spring seats in the wishbones to gain a little clearance, or look at some of the commercial conversions that use fabricated replacement wishbones and either buy one of them or fabricate your own. If you have deeper pockets, and the right contacts, apparently the Z3 rear axle can be fitted with some work.

An intermediate option may be to convert it to a beam axle - this will add unsprung weight but, if the track is reasonably smooth, it has the advantage of simplified installation, strength, simplified geometry, and zero camber change in roll so the tyres will be more evenly loaded.

As David said, a surprising reduction in unsprung mass can be made by careful choice of wheel and brake components, as can the selection of coil-over type, and spring diameter and installed length. The advantage extends beyond simple compliance to the track surface, but it's not only less overall mass to accelerate and decellerate, but the rotating mass also contributes to the "flywheel" affect, so any reduction there will have even greater affect in overall performance.

Thanks for the advice David,

hi Gord,

there’s no limits I have with suspension in any series I’m aware of that I may wish to join. Also In saying that I’m not building it for any series in mind. More to be the car I wish it to be. If that makes sense?

I can fit z3 rear suspension easy enough but really it puts me back at the same problem. And given I’ve already reinforced the rear arms then there’s little to no gain to be had going down that road.

and you are right about the camber and toe gain issues. That’s why I’ve raised the subframe to move the arms range of motion back into acceptable tolerances.

the last piece of the puzzle now is what to do with rear dampers. coil overs are commonly done. Will let me run softer springs being behind the axle. And are simple enough to do.

however. I like to do what’s the best possible option. Hence my curiosity about like you suggested, building a pivot point acted on by a vertical rod to actuate horizontal dampers inside the car. I’ve been calling them cantilever. My apologies if it’s not the correct technical term.

also given that the roll cage is still to be fabricated and there is some chassis reinforcement to be done regardless I figured either two options aren’t much more work then the other.

I’ve been doing some reading and can’t tell through the contradictions online if one system is significantly better then the other. Other then one looking 200x cooler. But what’s that if it’s not better.

Being a simple soul, I'd probably go for a 4 link, Watts linkage, vertical coilover setup with a suitable width diff'. But that's me.

You didn't mention the engine and any potential power increases - you may need to uprate the differential assembly and axles, anyway?

NZ has a thriving E30 race series - http://www.bmwraceseries.co.nz/ - when the lockdown ends and racing resumes, you may like to check out when they have meetings [edit - for some reason I thought you were in Auckland, but they have meetings nationally] at Puke' and Hampton', and have a chat to the guys in the pits. It seems they have some classes for highly modified vehicles, and you may get some very useful information, tips and sources.

Thanks gord.

engine and drive line are remaining original, but modified slightly for power. Only looking to run around 300whp. Mainly cos I’m a bit of a purest but also cos it lets me run in the most amount of classes. Such as bmw open class or historical classes. But I’ll cross that bridge when I come to it.

thanks for the advice and input.

Hi Leigh,

A little late to the party on this one! From my perspective, using an inboard actuated spring & damper on a suspension design like this doesn't really make sense, I think this would be overcomplicating things. Inboard actuation makes sense when it's designed into the concept of the car and suspension as a whole from the start. Sure if you're completely re-engineering that end of the car with a new geometry and suspension layout then looking going inboard could be a legitimate option.

However, comparing this to a simple direct acting coilover, I'm quite sure the inboard option would end up being heavier and end up with a higher CoG with all things considered (as well as being a lot more work and expense). Unfortunately, we seem to be in the middle of a period of having inboard dampers being fashionable. An inboard suspension is a great option for some applications, but not all. For the type of car, racing and suspension layout you're talking about, a simple direct acting coilover will meet all your needs and more 🙂

Thanks for the insight Tim.

Tbh I hadn’t considered the effects of CoG much on the inboard set up. It does make a whole lot more sense now you mention that.

I think I will take the advice and stick with the coil overs.

thanks to everyone else as well for the advice and insight.

No problem, Leigh 🙂. This is what our forum is for!

Keep us in the loop with how you get on with it, our Build Threads section would be a great place for that too.