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MG MGB Technical - Lowering a vehicle with rubber bumper cross member

My crome bumper V8 has the rubber cross member fitted as standard. I have already got what is considered as a lowered (rubber bumper) ride height. This measures 14.5 inches from the front wheel centre to the underside of the crome strip.

I'd idealy like to take the front down another 3/4 inch, but have noticed the lower a-arms are already canted upwards (from body to axle). However further lowering will increase this angle, which I understand will lead to worsened bump steer characteristics, not to mention the roll centre.

Does any one have advice on how I could acheive a further 3/4 lowering without further inclination of the a-arms, (cost sensitive solutions prefered). Or any advice on the handling characteristics if I proceded to lower it without regard to the a-arm angle.

Thanks
James
James

James...

I believe that you are now at the point that you would need to convert over to a chrome bumpered MGB front crossmember, then use the appropriate aftermarket parts to lower accordingly. I believe that you will need a chrome bumpered MGB steering column to keep the geometry if you go this route.

Hope that helps.

rick
rick ingram

Rick is right.

Luigi
Loogie

I put the chrome bumper front cross member under the late model B. You can use the stock modern steering by bolting 5/8" square tubing on top of the steering rack mounts and extending the mounting for the rack and pinion system out, forward and down. The stock steering rack etc. will fit and the ackerman will be right on.
Barry
Barry Parkinson

Hi James

I went through the exercise of putting in a CB cross member on my RB car.

I gleaned a bit of info from the V8 board, and seem to remember that there may be a crankshaft pulley clearance problem when using a CB cross member with a V8. (mine is a B series).

Is this an issue with yours?

Re the steering box mountings, although I used the RB mounts, and welded them to the CB cross member and shortened the pinion shaft, I can see that Barrys method would work. I would however suggest that a solid section is used. It won't crush or deform, and you will be able to drill and tap threads into it for the steering box mounting bolts.

Cheers
Ian F

Ian Fraser

Isn't it possible to get vertically offset swivel axles to lower the front end?
Simon Jansen

Can the crank pully interference with the front crossmember be addressed by relieving the crossmember locally? The Ford guys gut a BIG notch in it for pan clearanve, certainly a small notch for front pulley clearance shouldn't be a big issue?

Simon-
offset swivels exacerbate the bump steer issue if used without additional correction, the relationship of the travel arcs of the steering arm and tie rod is changed. So yes, a good solution, but not without it's own difficulties
greg fast

Greg, is that because the tie rod end is moved up too? If just the stub axle part is moved won't the geometry be the same? My suspension geometry knowledge is limited so sorry if this is a silly question! I keep thinking I should buy some of that Lego Mindstorms to build models of suspensions so I can learn how it all works.
Simon Jansen

At rest the steering tie rods and the rack should all be in nearly a straight line from side to side. If you start having the tie rods point up or down at rest, you are going to get bump steer.
Barry
Barry Parkinson

Gents,

You have provided some pointers here for further investigation such as the engine and rack clearance to the cross member, thank you.

I did find a company in the US which sells the 'lowered' stub axles. My understanding of these products is that the relationship between the upper and lower pivot and the steering arm remains, with only the axle being an inch higher, thus all geometry is preserved.

The lowered axles are available from mgbracing.com, or email mgltd@yahoo.com. They cost $340 plus shipping or $480 with new king pins. Whilst this is good value considering the casting and machining required, I am still looking for a more cost effective solution.

Regards, James
James

Having the track rods in a straight line when a car is at rest may or may not prevent bump steer. What is important is the relationship of the track rod to the upper and lower control arms as the suspension goes into bump and rebound. See the following for bump steer theory and how it is corrected.

http://www.ffcobra.com/FAQ/bumpsteer.html

Clifton
Clifton Gordon

Simon-

Yes. The steering arm is bolted to the stubs axle.

I won't tell you the kind of ruckus raised when I suggested heating and bending the arms back into position the last time I suggested it.

Other options include repositioning the steering rack or fabricating another set of arms with revised relationship between the swivel and tie rod end.
greg fast

Easy, Just cut off the raised bits on your rubber bumper cross member. Where the Rubber bumper crossmember bolts onto the car it is raised up about an inch or so by a hollow spacer. Cut off this raised bit so the car sits flat on the crossmember for the front bolt of each side. For the rear bolt you need to weld on or bolt in an piece of steel as you will have cut off the top of the bracket. Then as per Barry or Ian for the steering.
If only MG had used solid spacers in '75 !
Peter

If the bottom A arm and the tie rod are pretty close to being parallel and the tie rods and rack are pretty close to a straight line at rest, then you shouldn't have a significant bump steer problem.
Barry
Barry Parkinson

The tie rods and A arms on chrome bumper MGB's with stock components aren't parallel. On my 74 the rack and tie rods are in a straight line, but the A arms point down, 3/4" to be exact. On the 68 GT I used to own the A arms were flat and the tie rods pointed up. The attaching points for the A arms, tie rods and steering rack are all fixed by design and if you look at any stock chrome bumper car the A arms will not be parallel to the tie rods. That said, my 74 has straight rack and tie rods so it shouldn't have bump steer right, Right? If I change springs to lower the car one inch my tie rods are now pointing up. Do I now have a bump steer problem? No, I wouldn't unless I had a bump problem previously. Same thing if the suspension is raised, the tie rods will be pointing down and still no bump steer problems.

Bump steer is about what happens to toe in/out during suspension bump/rebound. The upper arm/shock lever on MGB, lower A arm and tie rods are all working together and if designed correctly there will be very little change in the toe in regardless of the suspension position. Change the position or dimensions of any of these parts and you can introduce problems that may need correcting if you want the car to handle as it should.

I have a home built bump tool so I did some measurements on my MGB yesterday. I removed the left spring and measured bump and found very little change for 2" of bump but at 3" it was around .035". Rebound to 1" was also good, rebound hit the bump stop rubber at less than 2".

FWIW, MGB A arms are 11 1/2" center of povit pin to center of lower trunion bolt. Tie rods are 9 1/2" from center of inner ball to center of outer tie rod end. Shock arm is 8 1/8" from center of shock pin/crank to center of upper trunion bolt. All mearsuements are approximate.

Last year I spent lots of time trying to sort out bump steer problems on a Legends race car my son owned. The cars are a poor design and you are limited on what can be changed. I never could get the bump as close as I measured on my MGB. The only changes they allowed was adding spacers in the tie rod ends. As a result of that exercise I have a rather extensive collection of suspension books. Very interesting subject but not always easy to understand.

Clifton


Clifton Gordon

Clifton
your measurements of bump steer at the extreme range of travel, point out the problems with lowering a suspension. As you measured, the radius from the pivot point to the tie rod end are an average of the upper shock arm length and the lower A arm length. This average changes as the wheel moves up and down. The shorter length of the upper arm results in the top of the wheel moving in (and the wheel steering out) when the travel is at high or low limits.
Theoretically, you could compensate for this by having the tie rod run at an angle from the rack to the suspension. However what would work well on an up movement would be worse for a down movement.

Besides reading and looking at the thing trying to figure this out as we have both done, I messed around with mounting the late model steering rack different ways on an early cross member and by bouncing the suspension up and down and seeing and measuring the results. (With the fenders off, you can see the wheels change their direction as you bounce the car. It's also easier to do with a set of bad shocks that don't damp the bounce.)I found that with the A arm being parallel to the ground (approx) at rest and the tie rod being close to parallel also, that bump steer went away.
Barry
Barry Parkinson

I always wondered if bump steer is really bad wouldn't you end up bending your steering arms? If I understand properly bump steer is when the wheel goes up or down it is as if the steering arm length is changing (because of the non parallel geometery) making the wheel toe in or out. If both wheels go up at the same time and both steering arms now need to be effectively shorter won't something have to give?

Simon
Simon Jansen

Hi Simon

Wih regard to both wheels being in bump, both wheels toe in together (or out, depending on the errors in the setup), there is nothing physical that constrains this except the inertia of the wheel(due both to its mass and gyroscopic effect).

However this inertia effect can be quite large.

I have seen a TF that had a highly modified suspension setup (lengthened kingpin, repositioned dampers, neg camber lower wishbone) that did not include the needed changes to rack position and tie rod lengths. Bump steer was way too much, and the car was difficult to control on bumpy roads. It was finally converted back to standard after two occasions where the eye at the end of the steering arm split.

Hi Clifton
I did much the same thing after transferring my RB steering box to a CB cross member. I wanted to check that things were in the right place.

I assembled the suspension, no spring, no bump rubbers, empty dampers, and the rubber bush connections a bit loose so that they could rotate. I clamped a straightedge to the machined face of the stub axle, and used the light from a single headlight bulb mounted on the ceiling to project a shadow of the straightedge on to the floor. Unfortunatly I did not take any measurements, but I was able to satisfy myself that that there was only marginal toe change from full bump to full droop even with the bump rubbers removed.

I would expect that with an undamaged standard setup, fitting shortened springs would not materially induce excessive bump steer, but there would be a minor change to toe in/out. Once toe in/out is adjusted for the new ride height, their should not be any problems apart from the downside of shorter bump travel.

Cheers
Ian F





Ian Fraser

This thread was discussed between 07/02/2005 and 13/02/2005

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