DIY uprated Dogbone Guide

[Basil]

Been running this mod for several months now - very happy with the performance of it. Decided to post up a guide to help any other Brisky members to see what's involved.
Been a lot of interest in firming up the dogbone mount, with the resultant improvement in handling, cleaner gear changes and reducing wheel hop, etc. Jabbasport offers their (by all accounts) superbly engineered complete mount as a direct fit-in.
Downsides of a firmer dogbone mount include added vibration while waiting in traffic, with resultant unhappy SWMBO. Apparently this does settle slightly with time though (I mean the vibration, not SWMBO)…

After an Internet search, I came across several references to DIY filling of the existing mounts with polyurethane, a flexible but reasonably hard rubber.
Basically it is similar in prep to epoxy glue. You mix two liquids together that react. Looks and pours like motor oil. You can pour it into the mount and after it cures you are left with a firm rubber.

Devcon Flexane is one that has been mentioned favorably several times so I went with this. It’s available from Lawson in the UK for about 33 quid for 500g – which is enough for about five filled mounts (GB anyone?) link: Devcon Flexane Liquid Urethanes - Lawson HIS - UK

It is available in three hardness levels (the units are referred to as “Shore A Durometer” numbers). Higher is harder. For an idea, an eraser is about 42 , regular silicone sealant is about 30-45 Shore and a bowling ball is pretty much off the scale….

The common mistake is to make the mount too hard – some people have used 90 Shore polyurethane and have regretted this and been forced to drill holes in the filled mount to drop the stiffness.

The Devcon Flexane stuff is available in three levels of hardness, Shore 60, 80 and 94. I wanted to go with the 60, as I run a SMF and don’t want too much vibration, but all I could find locally was the 80 (which actually cures to a shore A hardness of 87).

Steps:

• Remove the OEM dogbone – just two bolts:
• Clean it carefully – I used hot water and washing up liquid while scrubbing with SWMBO’s toothbrush (!!). Scraped it afterwards with a sharp blade to reveal 'new-looking' rubber. Not only to remove any debris but also to make sure that any mould release agent used in initial manufacture is removed.
• Tape up one side of the Dogbone and place it upside down.
• Mix up a measured quantity of the poly (100cc was the perfect amount = 1/5 of the total can). If you have an electronic balance you can mix it pretty easily, otherwise you have to dump all the hardener into the resin, which wastes a lot of stuff.
• Pour into the upside down dogbone and allow to fully cure for a full 3-7 days (!!)before re-fitting to the car.
• Fit it back in place – I intend to coat the bolts with blue loctite and torque them to spec:

[EDIT] has stood up to over 6 months of use and abuse nicely! My console was out at the time as I had a spot of bother with a bolt breaking off while removing the OEM bushes. It's was removed and re-threaded and re-installed.
Not too stiff but I’m almost always alone in the car and SWMBO is pretty tolerant (NOT!)…(Also I'm not worried about having to put in a few holes with the drill away from the OEM bits....) [EDIT] settled very nicely - minimal vibes at idle after 500km running-in, SWMBO (chief critic-hasn't even noticed any difference!)

Not look likely to ever come out, as the poly fully surrounds the OEM center and the rubber has been well abraded allowing it to stick. Also, it’s been left to fully cure to make sure that it doesn’t deform and come out. The properties sheet indicates a lot of ability to stretch and return to normal….

If I had a choice, I would have gone with the Shore A 60 stuff – deffo be softer/ less hard-core (no pun intended)…Oh well...[EDIT] but the one I used works well.

I wonder if a similar approach can be used to stiffen OEM console bushes while they are still in place without disturbing anything. Would need a clever use of tape/ silicone grease/plasticine and a dab hand at pouring, but it may be an idea….

The poly is really neat stuff - with a little imagination it can be used for a lot of DIY things - my 'helper' mate who has years of mechanical experience in the aircraft industry was quite wowed by it.....

Most posts I’ve seen about recommend against any stiffening of the transmission mounts btw…


Cheers

Bas

[Lummox]

Great write up Bas, I can see a little production line forming now...lol

[Basil]

Thank you for your blessing O Great one!

[skudmissile]

I can confirm the bush does work and its all stayed together after 5k miles and a track day and still offers very little movement.

I would be willing to make and distribute them for you bas? with you getting a % cut for the orginal idea?

Pm if interested.

This is a very good mod indeed! better than the one jabba have had made

[beedie]

I'm currently planningg to take off my Jabba one as in traffic it is just terrible. now wondering if this is the way to go.
Thnanks Bas.
Any one tried out both methods and notices if it's any softer on the vibration front?
Scott