72 911 targa too stiff after new bushings

[sbrown.nw]

Hi guys,

It's been about 3 months after I had new Elephant Racing rubber bushings and Elephant Racing adjustable spring plates installed on my 1972 911 targa plus a corner balance and alignment. I've had three sets of tires on the car since the work has been done: Bridgestone RE-11a (replaced after puncture just after your shop did the work and were 4 years old), Dunlop Direzza ZII Star Spec and finally some Bridgestone RE-71. I like the Bridgestone RE-71 tires the best out of the 3 and remind me of the Kumho Escta XS autocross DOT tires (which had very stiff sidewalls) I had on before the RE-11a ones.

The car seems to be ridding much stiffer (too stiff for mostly road driving). I wonder if it is the new Elephant Racing rubber bushings as not much else has changed? Here is what I know about the car over the 10 years I've had it:

- car has a 1985 euro (3.2 and 915) drive train, plus MA Shaw fiberglass front and rear bumpers plus deck lid / duck tail, lightweight Corbeau seats and 17" 7.5" and 9" Rota Foxes rims
- should have factory torsion bars
- should have green Bilsteins (these should be the HD, which are not as stiff as the sport if I recall correctly), done about 8 years ago
- has Weltmeister adjustable sway bars, done about 8 years ago and the bars, especially the rear one, have been setup stiffer / less body roll before
- had front and rear polyurathane bushings (noisy ones) installed replacing the factory ones and Weltmesiter adjustable spring plates put on about 8 years ago and then the recent Elephant Racing bushings and spring plates installation a few months ago
- 1976 alloy trailing arms installed 6 years ago with Elephant Racing poly bronze bushings
- there is a bump steer kit installed in the front, this was done before I bought the car and not sure which make it is
- not sure what has been done to the bump stops but the car has been noticeably lower (about 6 years ago) than it was setup just a couple months ago from your shop

The car has never ridden this stiff as I feel every road imperfection and is causing me back lower back pain. I am quite concerned about this and I am hoping there is something that can be done. Could there have been an Elephant Racing rubber bushing order mix up with Pelican Parts?

I would like to hear your thoughts and I hope something can be done to reduce the ride stiffness.

TIA,

Scott

[eimkeith]

Were the rubber bits tightened at loaded ride height, or on the lift at full droop?


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[Tony V]

Recently changed the tires on my '74 Targa from Michelin Pilot Sport AS to The RE71's. I thought the tire shop had inflated the tires to 60 PSI they were so hard riding. Guess it's the price you pay for the precision feel and handling of the Bridgstones.

[Knockdown]

I have a set of 15" wheels with higher and softer sidewalls for the rough streets around here, and change to a lower profile, harder sidewall when I want to get the most out of the car. Big difference in ride and easy to swap wheels.

[sbrown.nw]

Hi Keith and Tony, thanks for getting back to me.

@Keith, I am not sure what position the car was in when the rubber bits were tightened. What is recommended and what is the result if it is not done the recommended way?

@Tony, yes the RE-71 are stiff, but I've had tires with stiff sidewalls on before (the RE-11a and Kumhos, all the same size on the 17" Rota Foxes) before the new bushings.

[eimkeith]

Spotty coverage where I am, sorry.

The rubber behaves as a spring and twists a certain range before it limits travel itself (or tears)
Typically you want to tighten up rubber bushings at ride height to maximize 'travel' before they affect twisting movement.

Not sure if I'm explaining it well, but probably worth loosening the spring plate bushing covers at ride height and re-tightening them to see if it improves anything...


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[sbrown.nw]

Thanks Keith for getting back to me. That is what I expected by having all the bushing points tightened at ride height. If I have the shop do that, will alignment or corner balance specs have to be changed or reset?

The best way to describe the stiffness change is when I went from factory torsion bars to 21/27 on my 78 911 SC targa, definitely a spring rate change. Bridgestone describes the sidewall of the RE-71 to be a little stiffer than the RE-11a (again, previous tire) and I think the Kumho Ecsta XS was right around there in stiffness and the Dunlop tires being any stiffer. I will get in touch with the shop next week and also follow up with Elephant Racing to see if they would expect the spring rate / ride quality to change going from the polyeurathane bushings to their rubber ones...

[sbrown.nw]

@eimkeith, I had the shop that did the bushings replacement, alignment and corner balance do what you suggested and it may have made a very minor change, if any. The RE-71r tires will be replaced with Bridgestone S-04 Pole Positions next week due to the 71's extremely stiff sidewalls.

[eimkeith]

Definitely keep posting your findings! Good data here.


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[chris_seven]

Surely the stiffer bushes only impact on NHV and in helping to maintain wheel alignment.

If you calculate the wheel rates the rear bush contributes about 1% of this value so even a bush which is 20% stiffer than stock only has a 0.2% increase in wheel rate.

I haven't looked at control arm bushes but again their impact on wheel rate will be minimal.

The change in bush stiffness with ambient temperature change is likely to be more significant.

[Driven97]

Just to be clear, were the *only* changes the bushings and tires? And the car was fine before but now it's not? Did the ride height change? Is the car lower after the bushing install? If the car was fine before but now it's not we can narrow this mystery down very quickly. I highly doubt the bushings themselves are the answer.

Other notes:

• Am I reading it correctly that poly bushings came out and Elephant rubber ones went in? Like, as in softer bushings?
  
• Factory torsion bars are crazy soft - like softer than Cadillac soft. It should ride fairly comfy even with the minor weight reduction your car has. If you're lower than stock there is a decent chance you are getting to the bumpstops which would cause discomfort and ill handling.
  
• Setting the swaybars to stiffer settings would definitely reduce one wheel bump performance. It shouldn't be a night and day thing unless it was a HUGE adjustment, but also something to look at.
  
• The Corbeau A4 seats have a suspension base, right? As in they're not shelled race seats?
  
• The RE-71R are probably the single most aggressive 200tw street tire available. The sidewalls are brutally stiff, waaaay stiffer than the RE11, stiffer than the ZIIss, and stiffer than that old Kumho XS (which was also a street tire, BTW.) They're made with significant concessions to comfort, road noise, and standing water performance to simply be the stickiest thing possible in the dry. If comfort is a concern, that's too much tire.
  
• Do you know how many miles are on the shocks? Time isn't always the best measure. Actually mileage isn't either, but it's better than age. Again, the shocks didn't "blow" during the bushing install of course, but it could be something you could look at to improve your ride quality.

You can't have your cake and eat it too. These cars are old. The suspension and chassis was designed in the early 60s. The bodies aren't very stiff, especially non-coupes. The whole chassis acts like an undamped spring in both torsion and bending. If you want comfort, you'll have to ride high and soft, and use correct damping, put some soft sidewall in there, and dial down the roll control. If you want to run low and stiff and have the sticky tires AND have comfort too I'd recommend swapping to a more modern 911.

[sbrown.nw]

@driven97, I've been told the shocks have been good. I have already tried stiffer torsion bars on my car with the older poly bushings (not ER poly bronze) before the ER rubber ones went it, and with this config, the ride was too stiff, so I went back to factory torsion bars (had similar experience on my first 911 which was a SC, both targas).

Car is a touch lower and sway bars were not stiffened. The Cobrea A4 seats do not have a suspension base that I know of. I will be going to the S-04 tires in a week.

I think @tonyv's comment sums everything up. The RE-71r sidewalls are just too stiff for my preference, most likely his plus other posts that I've researched.

[sbrown.nw]

Anyone want to buy some 17" RE-71r tires with about 250 miles on them for track or autocross in the Seattle area?

[RSTarga]

Sounds like you found your problem, 17" wheels with low profile tires + heavy wheels make for a stiff ride.

[sbrown.nw]

@rstarga, I've had 17" wheels on my targa for almost 9 years with many aggressive tire sets with no problem like the RE-71r tires, which is why I'm going to the S-04 PP. So I would not necessarily agree with your general 17" wheel statement.

[manbridge 74]

Yeah the ER bushings will give a harsher feel as I just did them front and rear myself.

I like:
-15 inch wheels with a taller sidewall and lower pressures if you have a real light weight car. I run as low as 26/30 and no sidewall roll over. Separate wheel set for track duty
- Near stock torsion bar sizes. Removing 10% of total weight already gives a stiffer feel with 19/23. Currently 19/24 and 2400lbs with me on board. Plenty stiff for street use and IMO the 911 is not really analagous to a fighter plane but more a fighter-bomber. They were critized for being too heavy in 1965. Re-valving shocks to have less high speed compression or possibly a digressive curve would help even more.
- Not to big on sways. Some guys like no sways and stiff tbars but I haven't tried it and roads around here aren't suitable for gonzo spring rates IMO.
- Not to low on ride height. Once the front control arms are angled up from road surface or center of rear wheel hub is above torsion bar center the suspension loses too much of its range.

Sort of a monster post but I've been enjoying the firmer feel and the steadiness that the ER bushings provide.

[sbrown.nw]

@eimkeith and others, I may have spoken too soon. I did not know the shop inflated the tires back up to 30/34 after they did the bushing connection points loosen and re-tighten. I dropped the pressure back to 26/30 (was 23/27) and at the new 26/30 the ride quality seems better / less stiff. I still plan on swapping the RE-71r tires for S-04 Pole Positions.

[sbrown.nw]

@manbride74, interesting you had the same experience as I did will the ride quality stiffening up going with the ER rubber "sport" bushings while suspension experts, including ER themselves say the ride quality should not change. I've had the Bridgestone Pole Position tires on the car for about a month now (replaced the stiffer RE-71r) and the ride quality is better but not like what I had before the bushing swap and still seems on the too stiff side. I am going in for an oil change tomorrow and will have my shop double check the shocks / struts to make sure everything is ok. If they are sport, I might go to HD shocks / struts. The car corners nicely but can feel more twitchy (most likely to the stiffer ride) and pavement quality / bumps / etc. are that much more noticeable with the change.

To sum things up, I've had 17" tires with extreme performance tires on (Bridgestone RE-11a, Bridgestone and Kumho Escta XS) with a nice sporty and perfect for me ride and cornering before the bushing change. The suspension experts, including ER, say the ride quality should not change but it did for two people who actually did the install. What is going on here?

[Driven97]

Kind of a longshot, but the bushings could be installed "set" in a way that has an effect on spring rate. Rubber bushings aren't designed to rotate / slide at all, they're dependent on a twisting action. They then act as a spring the more you rotate them from the "rest" position.

On another car of mine I was able to lower it noticeably by removing the shock / spring, loosening all the suspension bolts, compressing it fully, then tightening all the bolts with bushings before reinstalling the shock. It was a multilink car with a lot of bushings, but it gained me a legal advantage in autocross.

I still think something else is going on. The difference between poly to rubber bushings, even the sport ones, should be an improvement in ride, not a regression. I'd look elsewhere.

[chris_seven]

Originally Posted by Driven97

Kind of a longshot, but the bushings could be installed "set" in a way that has an effect on spring rate. Rubber bushings aren't designed to rotate / slide at all, they're dependent on a twisting action. They then act as a spring the more you rotate them from the "rest" position.

On another car of mine I was able to lower it noticeably by removing the shock / spring, loosening all the suspension bolts, compressing it fully, then tightening all the bolts with bushings before reinstalling the shock. It was a multilink car with a lot of bushings, but it gained me a legal advantage in autocross.

I still think something else is going on. The difference between poly to rubber bushings, even the sport ones, should be an improvement in ride, not a regression. I'd look elsewhere.

Just calcuate the wheel rate due to a rubber bush that twists and a bush that slides is at the most 1%.

The calculation is very straightforward.

If you compare the modulus of a 60 ShoreA and an 80 Shore A NBR you will find the difference negligible.

As the front bushes in a 911 are NOT a metallastic design they are not tightened in the same manner as the rear spring plate bush the fitting will have no influence on stored energy.

The rear bush can store some energy if the cover is tightened at full droop compared to being tightened at the ride position but again the energy stored is not really significant compared to the energy stored in the spring.

The difference in Modulus between standard NBR and sport NBR is about 20%.

The high frequncy vibrations transmitted may differ but ride stiffness just should't be affected by these bushes.