What's the point of poly bushes?

[johnsimister]

I have just carried out a bit of reverse engineering. My car came with polyurethane wishbone bushes and the rear suspension has always felt a bit harsh and noisy over bumps and sharp ridges. The new TTR rear Konis have sorted the damping out but the harshness remained. So I've swapped the poly bushes, both inner and outer, for regular rubber ones from Miles Wilkins.

The difference is remarkable. Much better compliance over sharp disturbances, a better feeling of flow, no more vibration transmitted through the structure from the back under hard cornering or acceleration. I can see why poly bushes make sense on a track, where cornering forces are higher and ultimate precision matters more than refinement, but I can't see the point on the road. That said, I've left the front ones installed because there's no harshness problem there. Maybe it's to do with the front wishbones' pivot points being less spread out so there's more lateral leverage on the bushes over bumps.

Going off the topic slightly, I also carried out a tweak to a new, too-stiff trackrod end passed on to me by Lotus engineer and Elan owner Nick Adams. He drove my car and could feel the stiffness in the steering. With TRE disconnected from the steering arm but still attached to the rack, clamp the threaded pin in the chuck of a reversible electric drill (firmly to avoid thread-ruining slippage) and rotate the pin back and forth for a few sustained bursts, ensuring there's still plenty of grease in there. Soon the stiffness is replaced by easier movement, but don't overdo it or the ball will get too hot and the nylon cup inside might melt. Once it has all cooled down the TRE works beautifully, and my steering is no longer full of stiction.

Apologies for two topics in one posting but I thought the second one might be useful.

I'm sure Mr Markintheforest will agree on the poly bushes...

John

[Elanintheforest]

Of Course! But they do sharpen the car up for the track, no doubt.

One of the great pleasures of the Elan (and Jaguar) is the compliant suspension, and to change that for a road car just seems plain daft.

I dread to think what a Morgan or Healey would feel like with polybushes!

Mark

[gerrym]

John, one of the "points" is that the poly bushes are almost immune to softening by lube oil, eg those used on the torque reaction arms on the diff. You could argue that a badly softened rubber bush would give a worse ride than a good condition and lubed poly bush. But overall agree they are not a perfect match to a road car suspension.

Regards

Gerry

[1964 S1]

Hi John, when you used the words, "a better feeling of flow" I realized you and I have a bit more than a bit in common.
I'm thinking about Jim Clark and Colin Chapman, in that order, and it's road cars, not race track cars.
For all the talk and modification evaluations here, and I love them all, I still wonder, can anyone, adjust a stock Elan to a "better feeling of flow"?
1963 - 2009. Drive on a back country undulating, sometimes bumpy, sometimes gravel, then two lane blacktop with flat out straights followed by a sharp left, the rural type thrashing an Elan's designed and built for, the driver's enjoyment. Why change what it was, and mostly, what it is, still the best. Name a funner car...
I appreciate my stock Elans for what they are, not what they "couldaben" to quote Brando or Stallone because it already still is.
Eric
I know, half you guys out there have vastly improved upon your stock Lotuses. This is about poly bushes.
ps; I won't bring up the baby Elan thing right now, but I've heard their poly bushes are very very tiny.

[paddy]

I recently replaced all suspension bushes front and back, and decided to use rubber instead of poly. When I ordered them from Steve at SJ he said that he highly recommended poly bushes for the rear inboard bushes but rubber everywhere else, which is what I did. I found that in fact the bushes he supplied were more compliant than the original rubber metalastik ones so they would be less harsh than the original.

Now, after the latest round of replacing dampers, rack, adjusting for bump steer, etc, I find the Elan is much more comfortable to drive over a bumpy road, with fewer rattles, than either of our daily drivers. The Volvo (which has also just had every suspension joint, bush and damper replaced in an effort to remove an annoying knocking) still can't keep it's back wheels stuck to the floor over bumpy sections. I guess wide, low profile tyres and tyre pressures must have a lot to do with it, because the sprung to unsprung weight ratio must be a lot higher than the Elan's.

Paddy

[rocket]

Do polybushes not come in softer versions?

Ian

[RotoFlexible]

I think that the poly bushes (presumably inboard rear) have reduced or eliminated the "trailing throttle oversteer" that is sometimes associated with Elans. I used to notice this (primarily on the track) but it doesn't seem to be present now in my limited opportunities to exercise the handling characteristics.

My observations are not particularly reliable since so much has changed in the car since it was last on the track almost 30 years ago. I am pleased with the car's ride and handling now, even with poly bushes, slightly stiffer springs, and CV driveshafts. That said, it would be fun to revert to the original components (if I could do so instantaneously and without effort!) for purposes of comparison.

[elansprint]

One would have thought the rear bushes would have made less difference than the front as the std metalastic rear bush has 2 steel tubes with very little rubber in between
Ian

[johnsimister]

One would have thought the rear bushes would have made less difference than the front as the std metalastic rear bush has 2 steel tubes with very little rubber in between
Ian

Little rubber in the outer ones, yes, but a lot in the inner ones.

I've been thinking more about this. The 'issue' here is longitudinal compliance. The rubber bushes have their crush tubes bonded to the rubber, and the tubes are the only contact with the chassis (inner) or upright (outer). So the wishbone is free to move longitudinally against the rubber's elasticity by the amount permitted by the extra length of the crush tube relative to the bush's outer shell. The poly bushes are not bonded to anything, so the longitudinal location and compliance are achieved by the top-hat part of the bush, squeezed between wishbone and chassis/upright. This will resist longitudinal movement, by progressive compression of the top-hat section, more than a rubber bush's shearing and stretching motion, so will feel harsher for a given hardness of rubber or polyurethane. So it's not so much that the PU is harder, more the way it is used.

John