GTD Front Control Arms - Which bushes ?

Mark Charlton

GT40s Supporter
Lifetime Supporter
How timely... I am doing exactly the same, however I prefer to replace with rubber bushes not poly. Does anyone know if these were a stock item that was used on other cars? They seem to be very close to 1"Ø, 2" Long and 1/2" bolt.
 
Mark, why prefer rubber? lots of reasons not to, I have boxes of old rubber bushes thrown out when changing to polybushes, unless of course like most development engineers who use rod end ( heim ) joints.
 

Mark Charlton

GT40s Supporter
Lifetime Supporter
Hello Frank, I have replaced all of the rod ends with Heim joints, however the upper and lower A-Arms as they secure to the chassis are still rubber bushes. As I am refurbishing the arms to remove the silly chrome, I would like to replace the 35 year-old bushings. There are only 8. I do not like the poly bushings others are so fond of because I've had them on other cars and I find them harsh and extremely noisy over time. I didn't expect that finding 1" diameter x 2" long (2.25" inner) bushings would be so complicated. I'm sure many people know what they were drawn from, and where to obtain them, but knowledge sharing appears to be like gold these days.
 
Mark, with the car on jacks and the front suspension off the ground, remove the spring / shock unit totally from the car. Now move the hub up and down on the A Arms, just as it would move on the road in use. Not only will you find that the rubber bushes will strongly resist movement, but the greater the distance moved will increase the resistance by the rubber bushes winding up in their sleeves. Now imagine the same operation useing Rod ends ( helms) , which would freely allow the suspension operate freely and for the geometry changes designed into the system to work. Try it, and you will dump the rubber bushes ! Frank
 

Mark Charlton

GT40s Supporter
Lifetime Supporter
Thanks Frank. Any pictures of what you propose? Do I not need to modify all the A-arms and chassis mounts?
 
Mark, it's a long and complex thing, depends on how far you want to go. The original GTD front suspension is typical kit car build, useing production car parts , and the geometry is not at all good, I am sure you have experienced that, and suffers from many problems including a lot of understeer ( push ) at high speed. There are 5 critical geometric angles, each which must work together, in the front suspension, and we all have our own opinions on those. I like much greater KPI and caster angles, others will argue that, but unless all of these work together it's a compromise. I will see if I have any pictures on this iPad , it's 3.30 am and I am replying here in bed to you guys - sad ?
 

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Terry Oxandale

Skinny Man
I agree with Frank, mainly due to such an improvement to a previous project (strut suspension), where I significantly modified the towers to increase caster to be more appropriate with the high kingpin that the OEM strut mandated. For my application, it reduced the inherently poor camber gain in turns when the outside suspension compresses, but must be taken in coordination with the steering angle of the upright.
 
Thanks Terry, at last someone is understanding my ramblings , To prove my point on the resistance caused by the rubber bushes, I demonstrated this by removing the spring / shock unit and then driving the car down the road purely on the rubber bushes ! Frank
 

Neil

Supporter
Frank, the rubber bushings should not provide that much resistance. Maybe you have over-tightened their mounting bolts.
 

Terry Oxandale

Skinny Man
I think most rubber bushing applications are based on the torsional strength of the bushing (because they don't rotate in the housing, but rather deform, or shear), and not as much on compressive strength, hence the need to tighten the installation of the rubber bushings at the normal or expected ride height, instead of at full droop. I didn't comment earlier, but my thoughts were to use a thin poly sleeve bearing on either the ID or OD of the bushing, allowing it to rotate in the housings, yet provide the more compliant attribute (???) of the rubber, or now hybrid, bushing. With the housings used in the initial post, I can't imagine the use of a rubber bushing due to insufficient wall thickness to perform as standard rubber bushings do, and if one chose to allow the rubber to rotate in the housing, I can't imagine it lasting very long. Poly bushings, even without the specialized lubricant supplied with them, will rotate within the housing with much less degradation.
 
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Neil, the origin of this post askes about bushes in the GTDs, and these are assembled as press fit bushes, hence little or no rotation.
 

Malcolm

Supporter
then they would be acting like a crude heim joint. Metal rotating on metal like that would be bad. If you don't want poly bushes which come in different hardness of material, because of increased noise I would be amazed if you could hear that difference in a gt40 replica of any make. The engine is inches from your ear!
 

Glenn M

Supporter
then they would be acting like a crude heim joint. Metal rotating on metal like that would be bad. If you don't want poly bushes which come in different hardness of material, because of increased noise I would be amazed if you could hear that difference in a gt40 replica of any make. The engine is inches from your ear!

PARDON?
 

Mark Charlton

GT40s Supporter
Lifetime Supporter
Gents, many thanks for the discussion and photos (Frank). Mark Sibley has been very helpful and responsive, and does have some of the later-style GTD bushings on-hand which will work for my needs. I am not unhappy with how my car performs or handles/rides after having almost every connection EXCEPT those at the chassis ends of the front a-arms replaced with Heim joints. I don't have the engineering expertise to argue minutia, but I agree that what Frank is proposing would be an improvement — I'm just not sure that for my needs the extra effort and cost of making new A-Arms is worth the benefit for how I use the car. As always, great discussions, and I very much appreciate all the input!
 
Thanks to advice and help from both Andy at Tornado and Frank on his trick with the steering control arms, I now have a front suspension that is starting to come together.

I really appreciate all the help I get on this forum

I’ve now got the new Pro Alloy radiator in, replaced all the water pipes front to back, new aircon condenser and pipes in, new starter motor and oil cooler in.....just in time for spring and fingers crossed Le Mans.
 

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