Fail-Safe Rear Exhaust Mount

[revilla]

My exhaust is an Rx00 VHPD SV system, now with an Edwards Motorsport custom 4-2-1 centre section. The rear silencer doesn't come far enough forward to pick up on the centre mounting bracket neatly (the bracket aligns with the clamp that secures the silencer to the collector). I know I could get a bracket welded to the silencer extending forward but I think it would look mess and I prefer the clean lines of the system as it is. I'm not worried about the lack of centre support - with the cat fitted I've actually stood in it the centre (and I'm quite large!) and it is rock solid. What has always concerned me though was the fact that the while system was supported by a Mini exhaust bobbin which, if it failed, could allow the whole system to drop.

I did think about adding some fail-safe strapping across the two studs of the bobbin to add some security but it wasn't easy to find suitable components, it would have looked messy and probably wouldn't have worked so well.

So instead I thought about this:

 

The idea being to replace the rubber bobbin with a set of rubber bushes above, below and between with a solid M8 8.8 bolt through middle so that even if the rubber failed, the exhaust could not come off.

I checked to make sure that the holes in the exhaust and the chassis bracket are large enough to allow the bolt to pivot through a large angle, ensuring that the load is always on the rubber spacers rather than being a shear load on the bolt. I planned to sandwich each of the 26mm wide rubber bushes between 25mm M8 stainless penny washers to distribute the load evenly.

This is the mount I actually made up:

I've fitted it to the car and it all aligns neatly. I pivoted the mounting bracket a little and carefully aligned the exhaust to ensure that the two brackets were exactly parallel, which is alwaya a good idea using the original bobbin too.

It seems to have pretty much the same stiffness and range of movement as the original bobbin when pushing and pulling the system around. I'm sure it will provide sufficient vibration isolation and flexibility to allow the exhaust to move as required in use, but I will keep an eye on it to make sure it is holding up. You can actually tune the stiffness of it all my tightening the nyloc nut to adjust the compression of the bushes.

I've been out for a good drive and all seems fine so far.

These are the rubber spacers bushes I used: http://www.hawkeng.co.uk/store.php?crn=552&rn=653&action=show_detail. The bolt is an M8 8.8 high tensile; I didn't go any higher as 10.9 and 12.9 bolts can be prone to brittle fractures and if the exhaust took a severe knock on the ground it is possible that the bolt could experience a bending load if was forced to move beyond the range of free movement afforded by the bracket holes (but there is so.muxh free movement available that this would mean moving beyond the point where the original bobbin would have been badly damaged anyway). I used a nyloc nut and inserted the bolt from above so it wouldn't tend to fall out even if the nut came off.

The only concern I have is the possibility of the nyloc nut walking off as you can't clamp it down hard, but think this is extremely unlikely. I will be keeping an eye on it and if it does show any sign of movement I'll do something about it - maybe using a 5mm longer bolt and a lock nut under the nyloc to allow me to tighten it positively.

So far I'm quite happy with the way this has worked out and a lot less worried than I was about using a single mounting point for the silencer.

[Jonathan Kay]

Very neat.

It seems to have pretty much the same stiffness and range of movement as the original bobbin when pushing and pulling the system around. I'm sure it will provide sufficient vibration isolation and flexibility to allow the exhaust to move as required in use, but I will keep an eye on it to make sure it is holding up. You can actually tune the stiffness of it all my tightening the nyloc nut to adjust the compression of the bushes.

Quite a lot of modes to control there... and presumably bigger forces in rocking in pitch because there's only one bracket? 

Which hardness of bush did you choose?

Jonathan

PS: I was told off at my initial inspection because the steering rack bolts were upside down and failed dangerous.

PPS: The Mini bobbin is the only component common to my Seven and my Windcheetah.

[Bricol]

Had one bobbin fail in 18 years . . . or rather, the usual once in a while glance over the whole car showed it to be perishing, so I swopped it.

I simply remade the centre mounting on the car to pick-up on the bracket on my silencer.

[Wrightpayne]

I think it would be better without the metal washers either side of the chassis bracket and exhaust bracket - I think it would give a little more flex and transmit less noise.

whilst a neat execution, the question in my mind is why are the majority of car exhausts suspended on rubber rather than having through bolts? 

My solution would be a loop of bicycle brake cable looped between the top fixing and bottom fixing or chassis bracket

[Stridey]

I used a bolt and skateboard truck bushings and washers which are more urethaney and reboundey than rubber ones.

And a tip for adjust height of the bracket is to use it as a jacking point.

[7 wonders of the world]

I use the same principle but without the washers and using less rubber....

works perfectly - much better than bobbins 

[Ian B]

Looks like a neat solution - I've had a couple of bobbins fail over the years (but a lot of miles!).

My only concern might be heat transfer down the bolt to the nylon in the nyloc??

 

[7 wonders of the world]

Mines fine right on the arse end, I've checked it a few times including after a dyno session, if your concerned though you could use a Philidas (Aerolock) nut

[TomB]

I think the only thing you forgot to tell us was the length of the bolt. Looks really neat.

[DJ.]

That does look neat

I take it you didn't notice any obvious increase in noise/vibration after the change.

Duncan

[7 wonders of the world]

Glad to see Mike sorted you out with a bypass section.... 

[revilla]

@Jonathan - Not so many modes in practice I think. The engine mounts are very rigid and there's enough length of pipe and available flex between the engine and rear mount that most modes can be discounted I would suggest. I don't think there's much need to allow for linear vertical movement other than to provide some cushioning of shock loads causes by the inertia of the silencer over bumps etc. There might be some lateral and longitudinal movement if the engine moves a bit on the mounts. I should imagine the only significant movement in the bush is in the roll axis when inertial forces on the mass of the silencer, which is outboard of the mount, cause the whole exhaust system to flex in torsion.

Shore hardness ratings for the rubber meant very little to me in practical terms, I had no "feel" for how stiff different rated rubbers would be, so I ordered the ones I linked to in their default harness and thought I'd see if they were about right; they were!

@Bricol - I've had a couple of near-failures as you describe, but never actually lost one. Maybe I was worrying unnecessarily but I feel better now. My silencer doesn't have any kind of front bracket to pick up with the centre mount. I could have it modified to weld one on I guess but it would have been a little messy as that bolt hole in the chassis for the centre mount is right where my silencer clamp and lambda boss are.

@Wrightpayne - It might give more flex without the washers but I think from the feel of it that the flex is about right. My initial concern about not using the washers was that the load would not be even across the rubber and the edges of the brackets might just bite in. In reality I don't think it would make much difference as if you look at the pictures both the exhaust and chassis brackets are as wide as the washers.

I haven't noticed any noise or vibration from it at all. There doesn't seem to be anything transmitted to the chassis through the mount. It doesn't sound or feel any different to the way it did with a bobbin.

And as you point out, most exhaust systems are suspended on rubber, but the original Mini bobbin I believe was intended to suspend the exhaust in tension rather than to be used under the exhaust in compression as on our cars, and with nasty shear and torsional loads if the exhaust and chassis brackets are even slightly misaligned or out of parallel. I think there's a lot more movement in the engines and exhausts of most tintops than on our cars, and there would normally be multiple points of suspension too.

@Ian B - Yes I share your concern there and I will keep a close eye on it to see how it holds up. As suggested by 7 Wonders an Aerolock nut or even two nuts locked against each other may be an alternative solution.

@TomB - The measurements of all of the parts are marked on the first diagram. The bolt was an M8 X 50mm hex headed BZP grade 8.8. I avoided capheads as they are usually stainless or 12.9 and I wanted to avoid the risk of brittle failures. The washers were M8 X 25mm, the rubber spacers are 26mm diameter and 5mm and 15mm thickness.

@DJ - No nothing at all to report.

@7 Wonders - Yes he made me up a nice collector, thanks for the recommendation. I can really feel a lift in torque around 5000rpm and the sound is just incredible!

[DJ.]

I think I will copy your design, and give it a go

 

Duncan

[Paul 160]

One possible advantage of the bobbin is if the mount takes a hit. I don't know how but I took a knock to my rear exhaust mount pushing it almost parallel with the ground and pretty much folding the bobbin at right angles. At first glance you didn't notice it because the exhaust was still in same place. I'm not sure what would have happened if it weren't a bobbin, maybe torn the whole thing out the body? It might be something to think about though I still can't quite work out how it happened in the first place..!

Paul.

[Bizzielizzie13]

A 'Binx' nut could replace the 'Nyloc' if there is any heat transfer worries? Maybe an oversize hole in the either or both of the brackets to provide more bobbin like movement? Looks a lovely bit of thoughtful engineering though.

[revilla]

The holes in both of the brackets were oversized as supplied by enough to allow something like 35-40° (I estimate) movement of the bolt. I checked this before assembling as I wanted to ensure that over the likely range of movement in use the load would be borne by the rubber and not by the bolt "locking" in the holes, which would have imparted large bending loads. I've not come across "Binx" nuts ... I'll look them up thank you.

[revilla]

This little video from my mobile shows the mount flexing as I pulled and pushed on the tailpipe, rolling the exhaust side to side: