fatbillybob

Hi,

Anyone using full delrin suspension bushings? I wonder how long they last. These are a race only product since they are considered non-compressible solid link second only to Heim joints.

Steve Weiner-Rennsport Systems

Hi:

I would not use delrin in anything. Its a member of the nylon fmaily and absorbs water over time causing some swelling and then, squeaking.

The polyurethanes are really better and if you need custom bushings, a variety of shore hardnesses can be used.

Petevb

I'll call you on that generalization, Steve...

You might ask what type of Delrin they are using. Delrin is a trade name for acetal, and there are literally hundreds of "flavors" of this plastic with different fillers. Some of these make outstanding bearing materials, with low cf and moisture absorption (below .1%- good dimensional stability). Others, including the "regular" types used most often, can have 10x that moisture absorption. Polyurethanes, also available in many flavors, are often in the .15% moisture absorption range.

Of course I have no idea if anyone is making bushings out of the right grades, so Steve could still be right given what's out on the market. I've recently put a couple delrin bushings in my 951, but the 911 is all heim joints.

fatbillybob

Pete,

Did the Delrin last? How did it compare with the feel of Heims? I did not know there was many types of Acetyl out there. Asuming most racing is in the dry water absorption is not an issue anyway? I believe the compressive strength etc of all acetyls are similar? While poeple think of Delrin as nylon I believe there is significant differences between Nylon and Delrin. Nylon melts under frictional load much faster than Delrin and also does not have the abrasion resistence a bushing needs...yes? I would think that the Delrin family all perform similarly and have been quoted in many sources that I have read as bushing material of choice between Heims and Poly U.

Steve,

I think that racers who are not using heims or rubber are using Delrin for a near solid suspension. PolyU is used by the track/street crowd as a compromise and they always squeak right? Nobody in a racecar cares if the Delrin makes noise only if it performs. I am not sure there is much difference between new rubber and used PU. I have seen old PU grind away under loads and look all chewed up causing the a-arms to rock in their bores. Under those conditions rubber is probably better.

Steve Weiner-Rennsport Systems

Hi Pete:

I'm only addressing the delrin bushings that people make from what is commonly available. I've had FAR better results with urethanes.

These are made in hardnesses from 60 to 100 and work very well. Of course spherical bearings are much much better if one has the budget.

Like everything else, the "devil is in the details" and for most folks, they will have better luck with urethanes of the proper hardness,....
They will not squeak if they are properly fitted and lubricated. Some folks install zerk fittings and grease channels for easy long-term maintenance.

I agree, a good rubber bushing is better than some of the urethanes and other plastics I have seen used on 911's in the past 30 years.

Petevb

Fatbillybob-

For practical experience I’ll have to defer to Steve and others. I’ve barely used the options, I have only had the delrin on my 951 (trailing arm bushings) for a short time, and can't be sure how it's going to hold up. One issue with Delrin is that the bushings/ metal requires filing and preparation to fit and work.

A little more about the plastics, FYI:

Most delrin does have poor dimensional stability, and parts will absorb water from humidity in the air and warp to some degree. I don’t know if this or just regular wear (and resultant plastic on plastic friction increase) would cause the squeaking. Acetals can vary in most properties significantly and this includes modulus of elasticity/ hardness, but the things they are likely to use for this application should all be similar to one another. If you want to know one property, it’s coefficient of friction. I would hope you could find someone that uses a good low cf teflon filled grade, though it would be somewhat more expensive. Significantly cleaning up the metal side carefully (the part that sits on the ID of the bushing) is going to be required. I believe Steve’s experience, so you probably need to do something different if you want better results. The surface preparation and materials will be key- the range of properties you could expect to see could easily vary from >.5 cf, (which would grind itself up pretty quickly) to about .1 cf (close to as low as you can get- equivalent to smooth hard stainless on Teflon, which is what they often use in Heim joints, BTW).

By the way, nylon has the same variability- some types (Nylatron, Nylon GS, etc) have very low cfs and excellent bearing properties. It also has a significantly higher melting temp than Acetal- I wouldn’t expect “melting” issues as much as simple wear due to the (on average higher) cf. Finally, I suspect Delrin also squeaks precisely for the reason you want to use it- it’s hard. I don’t know if there is enough load/ deflection of something softer that a harder material would be a performance advantage.

M758

I am using delrin for the bushings on the steel front control arms on my 944-spec car. These are the bushings the attach the control arms to the front crossmember. On these cars the stock rubbers do not work well since the arm slides back over the rubber and eventually contacts the crossmember. I tried poly bushings and found the these same braking forces cut the outer ring if the bushing causing the same arm to crossmember contact. I installed delrin bushings and these have held-up much better. No problems or issues at all. They also provice a nice sharp turn in response. Based on class rules I may run any material except metal. In my opinion these are the best alternatives.

Matt Romanowski

With any bushing/ bearing one of the most important things to remember is the fit. So many people just "jam" everything together, not noticing that there is so much stiction that the suspension won't ever move. It's amazing how much stiction there is even in a car with complete spherical bearings. It's easy to see why F1 is getting away from them.

Matt

MJR911

Matt- what is FI moving to then?

Matt Romanowski

F1 is moving to fissures (sp?). They are carbon fibers joints of sorts. If you look closely at the bottom front control arm mounts, you can see the small band holding everything together. They truly trusts in composites.

Matt

fatbillybob

Romoman brings up an interesteing point about "stiction". A bearing joint would provide the most free movement to sub F-1 cars. Even rubber bushings have this unwanted property. The A-arms twist the rubber in the bushing bore or the outside of bushings metal washers ride as a bearing surface on the chassis attachment points. Both make lots of friction. Also, what may seem like too much stiction by feel of you rocking the arms may be insignificant to a car under load changes of several hundred pounds per corner. If you made the suspension rock freely by small pressure of your hand you may find this to be too much "play" under driving conditions. Then any hard suspension PU or delrin would be perhaps worse than rubber...you think?

Matt Romanowski

They key to any bushing is that there is no play. Installing suspension bushings actually is very time consuming in that they need to be fitted to the other parts. You hear that many people just press everything together, but this is what creates the stiction.

To truly assemble a suspension, the parts must have a very slight clearance, just outside of a press fit, to allow the lubrication to have an intermediary layer. This creates the a fit tight enough to remove play, but loose enough to avoid stiction.

Anyone who has cornerbalanced a car can attest to the stiction. Why do you have to bounce the car and roll it after making adjustments?

Matt