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What size poly bush for engine mounts?
DIY Si - 13/8/12 at 05:20 PM

Evening all. Having finally gotten to the stage of having my engine and box sat in the shell of my Sprite, I now need to make the engine mounts. However, there isn't really a lot of room, so the current thinking is to go with something similar to Furyrebuild's:

Description
Description

Only it would be to fit in here somewhere (sorry I haven't got a more side on pic, been too busy):



The only thing I want to alter is to use a bigger diameter bush, as I intend this car to be as usable as possible and want good vibration damping through the mounts as well as them limiting engine roll. However, I have no idea what might be suitable but was thinking of something in the 2-2.5" range might do. So any suggestions from you guys before I go ahead and get something?

mark chandler - 13/8/12 at 06:31 PM

I would look to use a pair of landrover 90 pandard rod poly bushes (nice and cheap) , if you get a complete panard rod off Ebay you will get some nice solid bits of steel to hold the bushes then chop and weld

LAND ROVER POLYURETHENE BUSH PANARD ROD SET DEFENDER / DISCOVERY | eBay

Regards Mark

DIY Si - 13/8/12 at 10:03 PM

Would they be big enough to use as engine mounts? For some reason I think engine mounts should be bigger, but I suppose if they'll deal with suspension on a landy, they must be up to it.

FuryRebuild - 13/8/12 at 10:50 PM

I feel like I've come of age ... I've been quoted

Mine were 1/2" ID, if that helps. If you need OD as well as a comparison, then shout.

It didn't take much machining to make it all fit.

Thanks
Mark

DIY Si - 13/8/12 at 11:17 PM

Mark,

If you could let me know the OD that would be good. I'm hoping the since I'm making it all from scratch, there shouldn't be much machining to do!

FuryRebuild - 13/8/12 at 11:24 PM

No Problem. 26.94mm OD, and 37mm length of inner shaft, with inner diameter of 25.4mm

We machined a piece of mild steel tube to be the bush carrier, and gave it a couple of thou to allow for contraction of the steel after welding. your mileage may vary.

cliftyhanger - 14/8/12 at 06:09 AM

I will stick my oar in here....

You did say you wanted something suited to a road car, plus I suspect the shell would benefit from more vibration-absorbtion than a small bush will give.

MK1 golf's use a larger engine-mounting bush, over 2" diameter from memory, and should be available as polybush. Saying that, decent quality rubber bushes last very well, as long as they are oe and not repro.

edit here is a pic I just found on the bay
VW GOLF CADDY MK1 20V TURBO ENGINE MOUNTING | eBay

[Edited on 14/8/12 by cliftyhanger]

loggyboy - 14/8/12 at 07:30 AM

quote:
Originally posted by DIY Si
as I intend this car to be as usable as possible and want good vibration damping through the mounts as well as them limiting engine roll.

Doesn't sound like PU mounts are the right material for your setup and use IMO. Ignoring the size of your engine, you want good vib damping AND less movement, well its always a trade off, but PU mounts are only 1 step down from solid mounts so possiblly too extreme? I would think good quality rubber mounts may suit better? even consider 2 mounts per side to restrict movement but retain the absorption of rubber.

FuryRebuild - 14/8/12 at 07:50 AM

Hi Loggyboy

I think you've hit the nail on the head there, and I'm not sure if I've gone too far. I'm all for removing most road compromises from my car (in fact it's been a continuous journey since I started hill-climbing), and I'm used to the first 20 miles on any journey giving my lymphatic system a thorough workout. Most unions and cable connectors have been and the remainder will be uprated to cope with the vibration.

HOWEVER, if it proves too much I will investigate your ideas, and possibly build a mount that still fits my constraints but possibly has two bushes, one rotated at 90 degrees to the other to absorb vibration and inertial load as the beans are applied.

Have you seen the system Porsche have applied to their latest 911? They have active damping on the engine mounts to manage the twitching it makes when changing gear. In essence, during the change the damper stiffens, and then relaxes again. The advantage is good isolation during the cruise, and when cornering and changing gear, there isn't a 100kg mass twitching and moving up to an inch on its mounts. I think it makes sense for those that break when enteringing the corner, and even worse, those that change mid corner as well as improving hard changes at full-chat accelleration.

After all, part of this fun is all the management we apply using wetware. Saying that, the Porsche idea is a cracking one.

[Edited on 14/8/12 by FuryRebuild]

owelly - 14/8/12 at 12:24 PM

I used locost wishbone bushes from Rally Design. Thus...


(Don't worry, it looked better blathered in paint...)

And the front mount which was built into the strut brace..:



I used the same bushes for the gearbox mount:




This set-up stops too many vibrations from shaking the car to bits whilst keeping the engine and box pretty rigid in the chassis.

DIY Si - 14/8/12 at 03:58 PM

Thanks all for your ideas and comments. Part of the problem is that I have little to compare any options to. I've had a mini with a high revving engine, but it was all solid mounted. The same was true in my BEC Indy, the engine was just bolted in place. The mini was a bit much, but the Indy I found OK and not a problem. So I'm not sure if rubber or PU will be the right choice. The Mazda engine is quite smooth, so there shouldn't be too much harsh vibration anyway.