Polybushes transmitting axle noise!?

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Datsundrew
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Polybushes transmitting axle noise!?

Post by Datsundrew » Mon Jan 01, 2018 11:43 pm

I took the scimitar to its first car show gathering today and did just over 100 miles. Apart from a fuel line blockage on the way all is well, nothing blew up and nothing fell off, but there is one problem......the noise!!!
As some will have read, I've fitted a Diesel engine on original mounts, gearbox too (all new) these work well with very little noise or vibration.........and then you drive it and it all changes dramatically! On acceleration or overrun the noise transmitted through the axle is awful, between the two is the sweet spot where all is calm and quiet. When driving, you can almost feel the noise coming from the seat belt area, which has made me think that this could be down to the polybushes I fitted on the trailing arms? It's such a shame, as otherwise it's really smooth, coasting in neutral (and cornering) it's great.



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Polybushes transmitting axle noise!?

Post by scimjim » Mon Jan 01, 2018 11:45 pm

General consensus has always been not to polybush the back end except for full on compie cars.


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Polybushes transmitting axle noise!?

Post by Datsundrew » Tue Jan 02, 2018 2:54 pm

I though that that might be the case Jim! Thinking logically, apart from the suspension struts at the rear, the axle is held in very rigidly compared to the drivetrain which are on fluid filled mounts, and therefore very well dampened.
I'm wondering if the diff noise that's generally talked about is not so much it being excessively noisy, but just transmitted to the chassis easily?
Thinking about other cars I've owned that have been very smooth, they all have bigger, softer mounts. Rather than buying OE bushes, I'm wondering if I could go a step further and make up new trailing arms to accept wider diameter bushes?
I'm in no great hurry, but it does ruin the ride :roll:



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Roger Pennington
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Polybushes transmitting axle noise!?

Post by Roger Pennington » Tue Jan 02, 2018 3:25 pm

Datsundrew wrote:
Tue Jan 02, 2018 2:54 pm
I'm wondering if the diff noise that's generally talked about is not so much it being excessively noisy, but just transmitted to the chassis easily?
Yes, I've always believed that to be the case. After all, basically the same axle was used in the Jaguar MK2, not a car noted for ride harshness or mechanical noise :wink: Rear axles *can* be quiet, but if there is any noise, then it will be transmitted into the body more easily (the estate layout may not help either). The trade-off is better handling (Ford owners pay good money to have their suspension converted to "our type" :lol: ). I'm still on OE rubber on mine.


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Polybushes transmitting axle noise!?

Post by philhoward » Tue Jan 02, 2018 3:48 pm

It might not be comparing apples with apples - the "modern" way is to have the moving suspension bits (with pivot bushes) onto a subframe that is then mounted to the body with nice, compliant (read: soft) bushes. The bushes on the Scimitar act as both the isolation and bearing surfaces and it's a trade-off between the two. Stiffer mounting is certainly provided by Poly bushes, but then you get a much higher transmission of any noise to the chassis which then acts as a huge tuning fork..

Rubber behaves a lot differently to poly with its rising rate deflection characteristics - as I'm sure you know. I don't know if there's any other way of mounting the axle to reduce the noise transmission aside from going back to rubber? From memory, there's not much scope to go for a bigger diameter bush as there's not much spare room there as standard unless you start moving the mounting points to allow for it?

Rubber bushes at one end may reduce it sufficiently though?

As for the noise, some do, some don't but are you sure it's not the prop complaining (angles and UJ phasing spring to mind)?


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Polybushes transmitting axle noise!?

Post by efi_sprintgte » Tue Jan 02, 2018 4:18 pm

Yes my immediate suspicion reading this was the custom prop shaft..

Agree with all that has been said about the bushes however.


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Polybushes transmitting axle noise!?

Post by Coupe Racing » Tue Jan 02, 2018 6:55 pm

There is a mention of a quiet spot IE on and off the throttle. Sounds like a noisy diff to me

Mind you take the rest seats and sound insulation out of a RWD modern and they are pretty noisy ,
Try the same modern with the door cards removed if you want noise


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Polybushes transmitting axle noise!?

Post by Nigel Clark » Tue Jan 02, 2018 8:04 pm

Different set up entirely but on IRS Triumphs e.g. TR6, there's been quite some discussion about polybush diff mountings on the TR Register forum in the past. The general consensus was to stick with OE rubber bushes for the diff as poly was noisy.

In contrast, I have fitted polybushes throughout on my GT6, including the diff and don't have a problem with diff noise. I chose the blue 'comfort' bushes rather than red 'competition' bushes. It could be that the softer polybushes are fine, while harder ones are responsible for transmitting too much noise from the diff. If/when I need to rebush the back of my SE6A, I will look for OE rubber to be on the safe side.

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Polybushes transmitting axle noise!?

Post by weaver » Tue Jan 02, 2018 8:06 pm

I see that you are comparing polly to rubber. If the old metalastic bushes are still available could someone please point me in the right direction. Cheers Dave



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Polybushes transmitting axle noise!?

Post by philhoward » Tue Jan 02, 2018 8:12 pm

I have a feeling Mark (gtcse8) has some?


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Polybushes transmitting axle noise!?

Post by Roger Pennington » Tue Jan 02, 2018 8:58 pm

Nigel Clark wrote:
Tue Jan 02, 2018 8:04 pm
In contrast, I have fitted polybushes throughout on my GT6, including the diff and don't have a problem with diff noise.
Presumably you have a quiet axle then? If you've got a quiet axle, it probably won't matter what kind of bushes you use, it's when you've got a noisy one that some types of bushes increase the chance of noise transmission :)


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Polybushes transmitting axle noise!?

Post by Datsundrew » Tue Jan 02, 2018 11:12 pm

I'm sure you're right, and it's got a noisy diff. The prop was custom made, balanced and cost just shy of £400 by a very reputable company, so I'm sure it's not that. I've had cars with bad props/ centre bearings etc, and it's definitely not that sort of noise or vibration. I'm going to put up with it for now and fit an amp to the radio lol. I've yet to add sound deadening in the back which will obviously help, but the good stuff is expensive, so it'll have to wait a bit.
Considering that the chassis is galvanised etc, it would make sense that any rubber type mods should be done on the axle end.



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