935 Style a-arms: pitfalls and benefits

[Kroggers]

I am in discussions with Cary at ERP about his nice 935 style front suspension a-arms.

I am converting to coilover suspension front and rear and removing the torsion bars.

I have the Rebel Racing rear coil over kit to go in and will probably get the Rebel Racing RSR front kit.

But I am considering if I stay with the factory a-arms (with the torsion bars removed), or pull them off and replace with the ERP set-up.

The car is used predominantly on track, but with some limited road use - I would love to hear from those that have the ERP 935 set-up installed and get some real life feedback - do you drive at all on the street?

[MikeD930]

My 930 has full ERP 935 suspension setup. I drive it on the street 99% of the time. I like it a lot. A month ago I drove 800 mi (each way) to meet with crazy 930 guys for track day at VIR and mountain driving in NC/VA hills.

If you can put up with lots of road feedback then you'll like this setup.

[Flieger]

The 935 crossmember, being milled out of billet Aluminum is the very expensive part. Clint reports that it does not weigh any less than the factory Aluminum crossmember. The Steel A-arms are where the weight savings are. If you could have some Aluminum arms fabbed up to go in the stock location and use a stock ball joint, that would be a good way to go. The 935 arms are longer for less camber change with vertical wheel motion but if you are not using 935 slicks with a square shoulder, that may be of little benefit.

[RWebb]

they likely need to be forged Al, hence spendy

I'd love to have some tho - so if anyone does a production run...

[Kroggers]

I just find the design on the 935 a-arms to look weak, and that it would brake or bend if you were to hit a pothole.

The weight gain is low down and at the front, so not the focus point for removing weight. So, what is the advantages to having the 935 a-arms over say a good fresh factory set of a-arms with the Rebel Racing bushings and a good set of coilover struts? What (apart from weight) are you really gaining with the 935 a-arms?

[Flieger]

They are longer and lighter, so less camber change during vertical wheel movement and less weight.

Yes, they are much weaker than the steel A-arms. That is only possible because they do not need to take huge bending loads like the steel arms do with torsion bars. They take compression and tension which means thin hollow sections like Aluminum tubes are efficient and the extra material and thickness of the Steel beam is superfluous. The coil-over spring is taking the springing load so that load is now being borne by the top strut mount, so reinforcement like with a brace is recommended. Before it only took the damper loads so thin sheet metal was sufficient.

If you do not need the weight savings, then just use your current arms and the Rebel Racing A-arm bushings.

[RWebb]

Originally Posted by Kroggers

I just find the design on the 935 a-arms to look weak, and that it would brake or bend if you were to hit a pothole.

The weight gain is low down and at the front, so not the focus point for removing weight. So, what is the advantages to having the 935 a-arms over say a good fresh factory set of a-arms with the Rebel Racing bushings and a good set of coilover struts? What (apart from weight) are you really gaining with the 935 a-arms?

I spoke to one of the original guys to manf. these for race cars about your first point, as I was interested in putting them on my car. I don't want to use his name w/o permission, but he is famous in our community.

He indicated that they might be fine, but did not want to go out on a limb re street use of his product. I still have my steel arms.

You can lose a fair amt. of wt. with the 935 arms, but you are right re the location of the wt.

[Kroggers]

I hear you Flieger & RWebb, so for someone like me that does not competitively race the car the factory a-arms are a great option and the ERP 935 arms are overkill...

Hmmm - how about aluminum a-arms that fit in the stock fittings but are for coilover use only and therefore do not need to support the torsion bars. Any one fancy making these

[Jeff Alton]

Lots of intersting info here....

The crossmember, although billet aluminum, would not be expensive to make at all. Once programed, I would bet it can be machined in less than 30 mins in the appropriate machine. The rod ends, and ball joints would be costly I would think though.

We have run both the ERP and the TRG versions. The arm that supports the ball joint and pivots off the aluminum cross member is STEEL. The track rod portion is aluminum, and it does not see anywhere near as much stress as the lateral link. Lots of race cars running this set up and I have not heard of one failing (doesnt mean it hasn't happened) and one could argue the track is a more harsh environment than the street.

The lateral link can be length adjusted so that you can get maximum camber with the use of a factory upper strut tower (without camber boxes welded in).

Cheers

[Kroggers]

Jeff, how about designing an aluminum copy of the factory a-arm for use with coilover struts - that way you could reduce the weight and not worry about the torsion bar - the a-arm could be reduced in size in the middle maybe, and maintain the factory configuration...

This might be a stupid idea, so sorry if it is :s

[Kroggers]

Did Porsche not make aluminum a-arms for the 944 Turbo?

[RWebb]

Originally Posted by Jeff Alton

...one could argue the track is a more harsh environment than the street.

...

Jeff, that is true in some ways, BUT I rarely find large bumps, boulders, or stumps on or adjacent to the track. Those are the dangers on roadways re F. suspension failure.

[Flieger]

Unfortunately, the Aluminum arms in the stock location would be a custom-fabricated thing. I have not seen any cars with them.

I recommend using your existing A-arms and the Rebel Racing A-arm bearings.

I am also sending you a PM regarding Clint.

[Jeff Alton]

I agree, until you go off the track! The curbs can be big though! I wasn't saying it was more harsh, but could be. Likely the person you spoke to, (I think I know who it is) does not want to endorse them for street us simply as a liabilty issue....

Pal,

That would work, but I am not sure how much you would be gaining. Plus once you manufacture them, and then buy some new ball joints and Elephant or Rebel mounts the price starts going up...

Cheers

[dad911]

IMHO the main advantage to this setup is to make camber changes easier. You can do coilovers with the stock arms, however I went down this slippery slope about 10 years ago, and I wouldn't do it again unless I brought the fully welded cage out to the upper strut mounts for reinforcement, which you wouldn't want to do in a street car.

[Kroggers]

Flieger, I already have the RR bushings in the garage ready to go onto my current a-arms. Clint supplied them to me earlier to help with marketing and for me to test - they will be going on shortly.

I was interested to know the benefit of the ERP 935 a-arms, and to see if they would change my car radically. And the answer is "no"...

I got your PM and will reply shortly.

Jeff, I was just thinking with the keyboard and sharing my thought. I have no idea if it would be a good idea or even if you would benefit in any way - just thinking

[Flieger]

One thing I would worry about is thread engagement with the Heim joint. I have seen some people use a standard steel A-arm and cut the ball joint bit off and thread in a Heim joint to get a bit longer arm and some more camber adjustment. You would need to have some way to tell how much thread engagement was occurring, otherwise you could adjust too far and... snap! You are now sideways, heading for the dirt, and possibly rolling over. There is also a reason why Clint does not like the idea of offset ball joints on very high-grip racecars with sticky slicks.

[Kroggers]

I like the idea of removing weight wherever possible on the car. I also like to have components on the car that takes lead from modern technology and engineering - where the benefit is higher than the cost.

The Rebel Racing bushings and coilover kit is a great example. I am just not sure if the 935 a-arms are?

I can understand the need for the strong steel a-arms when the torsion bar is running threw them. But the factory a-arms are getting old, and new replacements from porsche are not cheap. I would have thought there would be improvements to be made here, if one was to run with a coilover set-up. A lighter aluminum a-arm that sits in the factory position and uses factory hardware (or if you want Rebel Racing or Elephant Racing bushings).

I would have thought that an aluminum solution could be made that would cost the same as new replacement a-arms if not less - or am I wrong?

[Flieger]

I am not sure how big the market is. Those with coil-overs are often racers who just go for the 935 setup. Or they have wide-body cars and just drive on the street so the normal arms are good enough.

I agree with you, Paul (sorry, no Norwegian accent marks). I would like an Aluminum arm that fits in the factory location- if I had coil-overs. You would need to do some bonding of Stainless Steel to Aluminum so I think JB weld or other epoxy would be necessary for the bushing inner races, just like normal. It would be nice to use a factory ball joint but it would be a bit of work to design an elegant solution to join a thick flange for the ball joint to some hollow Aluminum tubes.

Might almost be easier (not cheaper) to make out of carbon fiber, like an F1 car.

The problem, too is manufacturing. A weldment would need to be hand-done each time so very time and money intensive. A good jig would help. CNC would be easily repeatable but could waste a lot of material and machine time is not always cheap if you need a many-axis machine or special tooling. Design is one thing, turning it into reality is another. I'd be willing to help design if someone had money or skills to fabricate.

[winders]

Pål,

I think you would be best served using suspension components for the applications for which they are designed.

The clones of the 935 front suspension were not designed for use on the street. The weight savings you would see is quite minimal anyway and not worth the risk. The real advantages of the 935 front suspension would not be realized on a car subject the compromises required for street use. In other words, the benefit is not even close to matching the cost.

That means the only reasonable choice is using the stock A-arm on a car that is driven on the street on a regular basis.

Scott