Brass Bushings
 

At Kurt's lift day Chris and I were discussing suspension bushings. Chris told me that his were made of brass as opposed to the stock rubber. The brass bushings supposedly make for a stiffer ride.

However I never followed up to ask why brass was used and thought we could capture that knowledge here.

My understanding is that bushings are supposed to flex or allow for some give (hence the use of rubber or plastic (in pedal/shifter apps). How does brass flex or give? I always thought it was really strong, thus giving a really unforgiving bumpy ride?

So, my question is, why is brass the material of choice? And then: why not a different metal?

In this case I suspect these are the bushings offered by Elephant Racing. Chuck's products are wonderfully thought out. I the case of these bushings the do not need to flex. Suspension a-arms and spring plates work by swinging an arc around a fixed point. This fixed point is the front or rear torsion bar. the arc created is the suspension travel. Suspension engineers are constantly in battle with having a compliant suspension that does not transmit road noise (think rough roads and tire hum/vibration) that also is precise in its movement.

These brass bushings are to correct a few problems with the alternatives. When new the stock OEM rubber bushings work well. They reduce noise transmission into the tub and function to move the a-arm or spring plate in its intended arc of travel. As time moves on, the rubber flows, cracks and compresses causing changes in suspension geometry and ability to perform.

The rubber bushings are hard to replace up front. They need to be vulcanized to the a-arm to work properly. this is beyond the typical repair facility. The polymer bushings are a cheap alternative to stock rubber. These urethane composites have problems with fit (wrong diameters) and require a lot of trial and error work that is best done using a lathe. In the end, they still squeak, pop, bind but still function to limit geometry changes.

Enter the Poly-Bronze bushing. These are hard bushings. Its more like a bearing than a bushing. These can provide nearly friction free performance while maintaining strict suspension geometry. Some people claim a bit more noise transmission into the chassis but I suspect there would be a smoother ride as the suspension is now free to move as it was originally intended.

I think these are a great upgrade, especially if you are tracking the car.

The current issue of Grassroots Motorsports has an article describing this exact issue and the relationship of toe-in (out) to the elasticity of OEM, aftermarket bushings on either F or R drive cars designed for street / track in the context of both static and dynamic effect / situ.

If you don't subscribe to this magazine I highly recommend it. Non of the extraneous fluff of other mags. It has good tuning (read: DIY mfg. and low $) comparisons on "old school" tricks and tuning and current stuff as well as good general technical info. They recently have done multi issue series (restoration, improve suspension, better power, grip, etc..) on a 914, a 951 and an early 911. There is always some P-car project underway and the information on "other" cars often crosses makes / models.

Jase

Almost every suspension joint in my car is either Delrin [hard polymer material] or monoball. Funniest story I've heard in a while was from a guy who sold his killer 951 to a guy in NY. He had over $50K in this car, which was very nicely prepared. Monoballs everywhere, and Delrin in the other places. The NY guy called to complain that the car's suspension rattled, and insisted that he get $10K to replace the noisy bushings. :roll:

Jase is right about Grassroots Motorsport. Its perfect Dorki reading. If you are thinking of doing a NASA event next season, just join NASA now - you get a subscription to Grassroots Motorsport as part of the deal. Schweet!

and their recent spin off mag for historics :!:

another vote for Grassroots and their content
 

I've been subscribing to Grassroots since they were a bi-monthly magazine held together w/ staples and the front/back, inside/out covers accounted for about HALF the color pictures in the entire magazine :D

Back then (before there was internet in my life) it was the only place that I could find out where, or for how much, I could buy a 5 pt harness etc. Sport Compact/European Car could tell you where to get the latest faux carbon shift knob :roll:

Their "Classic" offshoot is worth mentioning too.

That's true. I subscribe to that as well but, it has somewhat less p-car content :oops:

Here is the ER site and page for the bushings. To add to Jamie's description, there is a pretty good explanation for why this vendor has made these on the page. Interesting to note that he now offers the PB spring plate bushings for those of us who upgraded MY (MY = middle year ;)) or earlier cars to the sway-away adj. springplates. (have on my '76).

Jase

Jamie pretty much nailed the explanation. According to Chuck, the ploy/bronze bushings should ride almost as smooth as stock rubber. The truth is that this is almost impossible to verify, since I know of no one - myself included - that has ever taken a stock, well tuned suspension (i.e. no worn out bushings and shocks) and ONLY replaced the bushings to see what the effect was. In almost every case, LeShandleritis comes on strong and you go from a stock suspension with Boges and 18mm torsion bars to 21/22mm t bars, Bilstiens and the polybronze busings. So yeah, the new suspension is stiffer - but why? You've changed too many variables to be able to say the polybronze do or do not contribute to harshness.

On my recent suspension rebulld, I went with the polybronze in the front and the spring plates. It was not cheap. The reason I'll tell you when we're all BSing at the track is that I really liked the way the help keep the geometry in line and controls the tendency for the rear wheels to toe out when cornering at 9/10s. The reality is I have no idea what that means - I used them because I'm lazy. Functionally, they operate in the same manner as the replacement polycarbonate bushings - i.e. the a-arms twist inside the bushing rather than deform it like with rubber - then I didn't think there'd be much of a performance difference between the two. However, talking to Charlie, I was told to expect about an hour or so of work to fit each bushing. 4 hours total for the front. Screw that - for $240, they were installed in about 15 minutes. The rears were similar.

Oh, and I hope they do some of the cool keep the suspension in line stuff on the track. I'll let you know next year. :D

Looks like everybody is headed East. I went West and replaced my bushings with Netrix standard replacements and new Bilstiens. My reasons may leave the competetion set agast, so here they are. Pick em apart as you will.

Porsche designed a very good suspension for driving fast. Who am I to reengineer that. While I admit some improvements are possible the general risk of success depends on the quality of the replacement parts and the skill of the installer. No matter how good, the parts WILL not be superior to, but MAY be equal to, Porsche parts. If you have the sure knowledge and direction than the risk may be small to you, but not to me.

My general mind set is not to modify. I deviated from these principals by lowering the SC to European specs and ditching the CAT. I do intend to drive fast, I don't intend to race. Couldn't compete with the over physcied, deep pocketed, achieve driven, pagid owning younger set anyway. :P

The thing is, John, you did reengineer the suspension. The workings of the plastic Weltmeister bushings in the front of the car are fundamentally different than the OEM rubber bushings. As far as I know, with ALL aftermarket bushings, the a-arm rotates inside the bushing. Only in the factory rubber bushings do the a-arms deform the bushing, not rotate within it.

So the first choice is to decide: aftermarket or OEM rubber. OEM rubber will cold flow and wear out again in a few years, deform more under load, and is the most expensive solution - about $500 for the front 4 bushings.

If you choose to go aftermarket, your choices are the poly-bronze or the polycarbonite. With both, the a-arms rotate, not deform the bushing. The advantage of the poly-bronze is that the rotation surface has less friction so the suspension travel is smoother, being metal they hold the suspension geometery better under extreme loads, and they are easier (quicker) to install. The disadvantage is that they are much more (10x) expensive. In either case, however, you have, in essence, reengineered the suspension.