Use of Poly-bushes in a Tornado, GT-Forte, GTD etc.

[Andy Green]

I’ve currently got Rally Design Poly-bushes fitted to my chassis and after recommending them to John Christian, an issue has arisen, specifically around the use of the bushes in the rear suspension. See previous post: http://www.gt40s.com/forum/gt40-tech-chassis-brakes-tires-wheels/35870-poly-bushes.html#post354779
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<st1:City w:st="on">Westfield</st1:City>, however when used in a ’40 there seems to be excessive play after a while. As supplied the inner sleeve is a light push fit into the bush, but when I’ve fitted these before from other manufacturers, they are really tight fit. I’ve measured the Shore hardness of the bushes, which is about 80, so no issues there. <o></o>
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On the front suspension, both the inner joints are used in pairs therefore the forces in the bush is halved. However on the rear inner the same bush is used singularly, effectively doubling the load.<o></o>
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Also to function as designed, the Poly bushes has to be fully supported (i.e. clamped between 2 plates to prevent the bush deforming) whereas as on the rear top inner mount on a Tornado this is not the case, with the bolt tube coming through the chassis and then welded. <o></o>
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So a few recommendations:<o></o>

• Replace the inner bushes with a rod ends. Adding one to the top link and using a L/H thread makes changing the camber much easier<o></o>
• Fit a stiffer bush, it may be easiest to replace the current bush with a hard nylon etc.<o></o>
• Fit a larger inner tube, which would effectively put more pre-load on the current bush, causing it to stiffen <o></o>
• Use the original specification metalastic bush, however I’d still be concerned about the long term durability of this.<o></o>
• For the top inner mount on a Tornado, grind back the chassis support tube sufficient to add a larger washer too effectively prevent the bush from deforming. There is sufficient material to grind 3mm off the tube for a washer and not touch the weld.<o></o>

I’ve replaced the top inner link bush with a L/H rod end and replaced the lower inner bush with Nylatron<o></o>
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Hope this all makes sense<o></o>
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Regards,<o></o>
Andy<o></o>

 

[Mick Ridley]

Hi Andy,

Very intresting what you say about the bushes, and sleeves. As I have those bushes for my GT40, I looked to see how tight the sleeves were in the bushes I have

The sleeves supplied are a light push fit in the bushes, But I have had to make my own sleeves, because I need slightly longer ones. So these longer ones were made from 16mm Stainless steel bar, and they are a much tighter fit, when I measured them, the supplied ones were 0.12mm smaller, so it would appear the supplied ones are made from 5/8" (15.88mm)

So a easy way to make the sleeves a tighter fit, is just to replace the 5/8"sleeves with 16 mm sleeves.

Thanks for pointing out the problem

mick

 

[Phil]

Andy:
I can tell you that part of the problem with the rear suspension bushes is the fact that the radius rods, upper transverse link, and innner-lower wishbone mounts all work in two axes.
Your uprights don't travel straight up and down but in an arc described by the two radius rods, this puts not only rotation in the bushes, but a twist and consequently excessive wear.
The radius rods also twist as well as rotate so there is a bit of binding there also.
You would also be surprised how much bind there is in the rear.
High quality heim joints do a much better job and have zero bind if setup correctly such as the way that you did the inner pickup point on the transverse link. Give them room to work and they are very smooth.
You probably wont encounter the same condition in the front because the wishbones are a fixed structure using only rotation at the mount without twisting, so the only bind will be friction from the bushes.
Installing the bushes with silicone grease also smoothes them out a bit.
Hope some of this helps
Cheers
Phil

 

[Andy Green]

Hi Phil,
Thanks for the comments. I must admit I’d forgotten about the rotational aspect of the bush loading and this was probably the more significant element of the failure mode that John Christian noted when he used them as supplied.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com<img src=" /><o></o>
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Whilst my proposed solution will not give any rotational freedom on the bottom inner joint (a tight fitting nylatron bush) there should still be sufficient compliant in the outer pair of lower bushes to compensate.<o></o>
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Hi Mick,<o></o>
No problems, that’s what this forum is all about<o></o>
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Regards,<o></o>
Andy<o></o>

 

[Geoff Deakin]

Hi Andy,

An interesting thread.

On the front wishbones, I've got a rubber bush which appears to be bonded to the inner steel tube.

Should the wishbone pivot on the bush, or should the bush and tube pivot on the mounting bolt?

I've got to make up steel spacers, to locate the wishbone fore and aft, and am unsure whether the spacers should bear on the steel tube or the rubber bush.

By the way, clutch slave cylinder mods a great success!

Cheers,

Geoff.

 

[Andy Green]

Hi Geoff,

Sounds like you’ve got Metalastic bushes. The idea is that the wishbones pivots on the rubber, therefore the inner pivot tube needs to be clamped hard to the chassis mount. Therefore your steel spacers need to bear onto the steel inner tube<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com<img src=" /><o></o>
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Really pleased to hear that the modified clutch slave cylinder did the job for you<o></o>

Regards,<o></o>
Andy

 

[Geoff Deakin]

Thanks, Andy.

Another problem solved.

When I bought the car, there was no fore and aft location on the bushes for the wishbones.

I have trimmed the bushes to length to suit the mounting eye width, and temporarily spaced everything with washers, prior to alignment checks.

Thanks.

Cheers,

Geoff.

 

[Jon Briggs]

Andy,
Thanks for the info. Its always good to solve problems before they become problems. Although looking at the various bits of suspension scattered about my garage a complete change to rod ends may be in order.

 

[Thomas Rouw]

Hi Andy,

it might prove easier (no welding) to replace the problem bushes with spherical bearings instead of rod ends, positive effect on altering camber angles is not there, but you can pressfit the into the original place and make up for a stronger total than the rodend conversion.

common conversion in racecars that bind the bushes under braking.

use minebea sphericals and rod ends they are incredible quality.

regards Thomas