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MG MGB Technical - Quick Question - Subframe Query

Hi,

Just a quick one, will a chrome bumper front subframe fit onto rubber bumper shell?

Whats the difference between a chrome bumper and rubber bumper subframe? are the body mounting points the same? steering rack mounting points etc?

Cheers
Gavin
Gavin Jewkes

The subframemounting holes are the same but the platform height is different, (you will lower the front end) the steering rack will fit the mounts but the column alignment will be incorrect. The hole in the bulkhead is different also. Its a lot easier to fit shorter springs and bumpstops if you just want to lower the car.
jim soutar

The rubber bumper one has the body sitting about an inch higher due to extensions on top of the front crossmember. Also the steering mounting brackets are different. You can fit a CB front onto a RB car. The four body mounting bolts are exactly the same so it just bolts on. The front end of the car will sit lower (good!) but your steering column now won't line up correctly. Check the archives to see how people deal with that issue.

If you have the crossmember off the car check the steering mounting brackets for cracks where they are welded onto the frame. They often seem to crack there.

Simon
Simon Jansen

Gavin-
The use of shortened coil springs in the front suspension should be avoided as they will leave the front suspension travel with less upward travel, result in an extreme “Toe out” front wheel misalignment that will result in accelerated tire tread wear, as well as create a bad tendency toward bump steer as a result of the change of angle of the tie rods. Braking distances will also increase as a result of the deformation of the contact patch of the tires. In addition, these shorter springs will need to have a much stiffer rate to partially compensate for the resultant decrease in upward suspension travel and to prevent the suspension from crashing into its bump stops, thus creating very harsh ride qualities. The most comprehensive approach to lowering the front end is to install a Chrome Bumper front crossmember, complete with steering rack and steering column, and then use an engine front plate from an engine intended for installation in a Chrome Bumper model car.

Be advised that the steering rack mounting brackets of the two different front crossmembers are set at different angles, and that the steering racks have different length pinion shafts and use different length steering columns, and thus cannot be interchanged except as a complete steering system, provided that the appropriate corresponding steering rack mounts are welded onto the front crossmember. For the rear suspension, you need only cut off the front spring hangers and weld in new Chrome Bumper leaf spring mount assemblies. These are available from the British Motor Heritage Trust (BMIHT Part #’s BMH9002 for the right side, BMH9003 for the left side). These can be seen on the British Motor Heritage Trust webpage at: http://www.bmh-ltd.com/p61r.asp . You will also need to change the rear bump stops so that you will still have adequate suspension travel, and the limiting straps. Fortunately, the pedestals (BMC Part # AHH 7355) are the same on all four-cylinder models.
Stephen Strange

I have shortened springs fitted to mine and have not had any problems with it. I am not an expert driver nor a racer but to me the car seems to handle fine. My mechanic who does the 6 month WOF inspections said it was one of the best MGs he's driven (granted it is basically rebuilt so 'new'). No sign of bump steer or tyre wear issues. I fitted shorter but stiffer springs, uprated front damper valves to stiffen them up, shorter bump stops, a fatter front ARB and of course made sure all the bushes and mechanicals are in top condition and have good tires. To me it has been a perfectly acceptable way to lower the front end. The ride is firmer but not harsh. The only downside I have found so far is the front crossmember sits too low for me to get my trolley jack under!

Simon
Simon Jansen

This thread was discussed between 11/08/2011 and 12/08/2011

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