Bertrand Daoust

I have a set of Delrin bushing waiting to be install on my car. Thanks Roger.

My car as 120K km and as the original ones.

First question,

Do the ones I have are done?
I think so as the car tramline a bit. With the new (to me) Carrera III.

Is this doable quite easily?
I search and it looks like it's not a big job.
I would like to do this before Frenzy but don't want to f.... things up and mist the event.

What do you think?

Thanks.

ammonman

The factory bushings are likely perished if original simply due to their age. If you've had any oil or PS fluid leaks then they are almost certain to be soft. The job is pretty straight forward and made MUCH easier if you have an air compressor and an air hammer with chisel. This makes short work of getting the old bushes out. If no air hammer you will have to use the old fashioned hammer method to fold the metal portions of the old bushes inward to release them from the mount holes in the rack. Not technical, just sweat and time. Be sure to get some new sealing rings for the Banjo bolts that attach the fluid supply and return lines to the rack. You will need to drop it down a bit to get the old bushes out. Removing the lines gives the necessary room to work. IIRC when I did mine I did not need to uncouple the steering shaft from the rack.

Mike

Adamant1971

Hey Bertrand,

Don't be so sure they are done. Our cooler climate and only 120k they may be ok.

I went back to 17" et 65 on my car because of the tramlining I was getting from my Carerra III's.

If you do decide to do them just use a hole saw to drill between the steel sleeve and rack and you will have the bushings out in a flash. The biggest pain will just be the mess of ATF.

928Myles

Bertrand,
I noticed a big difference when I replaced my original ones at a similar distance (130k km).
Was a realtively easy job to do (once you knew what to do).
Read the guides and it should only take a couple of hours or so.

Myles

ALKada

It will go much quicker if you have access to a rack to put the 928 up on. Also, if you have some help to get the rack back up to start your bolts back, it would be much easier. Clean, clean, clean before you start.

You're going to love the Delrin bushings. It made a huge difference with mine.

Bertrand Daoust

Thanks guys.

I just found this:
https://rennlist.com/forums/attachme...rininstall.pdf

A nice step by step procedure for this job.

Thanks Stratford!

FredR

Bertrand,

The Delrin bushes are a no brainer at this stage in your 928's life. Whether or not the rack can/does float to any significant amount remains to be seen but sooner or later it will degrade and eventually affect the steering. I let the main dealers do mine and it did not cost me much to have them fitted so it must have taken them little time and effort but that was before they jacked their manhour pricing enormously to be in line with main dealer rates.

I doubt the tramlining has anything to do with the rack and everything to do with your wheels and the front offset. The Carerra 3 wheel sets appear to have an ET of 55mm which if my memory serves me correctly means they end up with a scrub radius of "zero". When I researched this concept an article I read at the time advised that having a scrub radius of zero is worse than having psotive scrub radius in terms of the phenomena that happen. All I can say is that I have run several sets of wheels and the take offs I used with ET52 tramlined and did nothing for the stop watch so I got rid of them selling them to a friend with a 911 C4 whose car they suited very well. I currently run ET68 and have never had any such issues.

Plenty of folks run ET55 and report no issues so I can only assume that either car behaviour varies example to example or driver sensitivity to such is the variable. I have always been vey sensitive to small vibrations, twitchiness and the like so that is where I would put my money. That being said I suspect tramlining is more something that one "feels" uneasy about as opposed to being a real danger or threat but I sure hate the sensation.

Best thing you can do is fit the delrin and perhaps experiment with hot air pressure a bit given I suspect that many folks run with too much pressure. On my late 90 S4 the warm weather seemed to play havoc with the RDK system. In fact it was so successful I have a feeling they dropped it from later GTS's after fitting a sensor with a lower trigger pressure for the 17 inch wheels fitted. On my late S4 I jumpered the alarm and abandoned the system, on my GTS I think the PO [the owner of the main dealers] did this just after the car was delivered. Bottom line folks are/were using air pressure to avoid RDK alarms. With the wheels you have fitted I suspect you have done this mod yourself but...?

If you have not done so already target hot front air pressure in the 30 to 33 psig range and see how it feels and whether or not it modulates your perception of tramlining. 18 inch rubber with much stiffer side walls does not need as much pressure to support the tyre itself and if you study the load ratings you wll invariably find that at the tyre's rated conditions of pressure and load pro rated to the corner weight of 400kg you can reduce the tire pressure to what at face value seem like rediculously low pressures with numbers in the region of the low 20's and who told me this? The technical manager of a major tyre company for the Middle east Region during a timed event we had- and the stop watch suggested he was right.

Rgds

Fred

85euro928

Bertrand,
When I first installed my Carerra IIIs the tramlining was awful. This spring I had new tires installed going from a 295 to a 285 on the rear and the tramlining went away.

Bertrand Daoust

Thanks guys.

Fred, I'M already driving the car at lower tire pressure than stock.
Righ now I'm at 32lb cold. I'll try at lower pressure.

Thom, I'm running 265/40-18 in the back (with 15mm spacers) and 235/40-18 in front. Michelin Super Sport.
What size and brand are you running? Front and back.

I'm starting to think that maybe the front 235mm compare to the 225 stock and the 57mm offset compare to the 65 stock is what is causing the light tramlining.

Thanks again.

FredR

Tramlining is influenced by a number of parameters but basically the more grip you have the more tramlining is felt. Thus going to a narrower section will help reduce the perceived impact. Tyres that are well worn and thus closer to a slick will generate more grip and thus more tramlining potential- racers shave their tyres to more approximate slicks to get more grip. Too much tyre pressure will enhance tramlining as will worn suspension bushes that permit the wheels to move around more than the original design ever intended and this probably applies to just about every 928 still on original bushes. More grip, more lateral force more movement- a vicious circle type of thing. More camber, more lateral grip can aslo cause more tramlining thus when tuning the suspension one has to understand all these interactions to arrive at something that is optimal for the intended use.

Reduce the offset by a significant amount same thing happens- I am sure that Porsche will tell you that if you reduce the front offset on a 928 by 10mm that would be considered a major change to the suspension design that corrupts the design intent.

That one can add quite a bit to the 928's lateral adhesion and use it without too much of a negative reaction says a lot about the car's inherent abilities but as I am experiencing recently, the harder you push something the quicker it wears out!

Rgds

Fred

Adamant1971

I second that. When I had my 295's on the C3's it was horrible. I went down to a 265 and it was worlds better. Basically going with more than an 1" difference in tread width from front to back causes issues. Along with the extra grip also affecting it.

Also these cars were designed and engineered to operate with much thinner tires and less gripy rubber that what is available today. Any time you deviate from the factory geometry it's going to have an effect.

If you still have the stock wheels I would do a same day comparison with both sets and see how it handles.

Bertrand Daoust

I could try this.

I would start with the front. And see...

Thanks.

Imo000

Remove the spacer and see if it improves.

Bertrand Daoust

I can try this also.

Thanks Imo.

jcorenman

Alignment is an issue also. I am not sure about camber, but certainly toe-in has a big effect. And if there is any looseness in the steering-- rack bushings, tie-rods or the rack itself-- then toe alignment becomes a big crap-shoot. Front wheel offset is part of the equation also.

The factory toe spec is 15' +/- 5' total toe-in. What works best for me is setting it towards the toed-in end of that spec (i.e. 15-20').

Where this all goes astray is worn parts. If the rack mounting bushings are loose then the rack will move around as the loads change, changing the steering angle but also changing toe-in as the rack flops around-- because the tie-rods are not aligned with the rack axis.

Loose tie-rods also allow the toe-in to change as loads change.

But the big wildcard in my experience is the rack itself-- if the internal bushings are worn then the rack ends move around, which also changes toe. This is often not diagnosed, particularly if a centering bolt is used-- which pushes the rack rod towards the rear at the driver's end and increases toe-in if the bushings are worn. So it gets aligned properly, the centering bolt is pulled out and it winds up toed-out.

Try this experiment: Grab both front tires and pull the front sides firmly together, pulling any looseness in the toed-in direction. Measure the total toe-in. Now push the tires apart at the front, shifting any looseness towards toe-out. Measure again-- same amount of toe-in? Great! Set it to 15-20' and go driving.

What usually happens is that you get one reading after pushing the front of the tires together, and a different reading after pushing them apart. The rack is moving on its mounts, or the rod is moving within the housing (worn internal bushings), or a tie-rod is loose. Wiggle things and find it, and fix it. Use your fingers to sense relative movement between rack rod and housing, or tie-rods, or rack housing and the cross-member. Fingers are very good at feeling small motions.

On the road this contributes to tramlining. As one side of the tire rides up a shallow rut in the pavement, the wheel wants to turn in that direction. And if things are loose then that's exactly what it does.