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MG MGB GT V8 Factory Originals Technical - Traction Bars

I am planning on fabricating my own traction bars(anti-tramp) for my conversion(190 bhp engine) using chromemoly tubing,any suggestions on diameter and wall thickness?

Thanks in advance
Trevor Burnett
t burnett

Trevor, on my midget I used a thick wall mild steel tubing about 3/4" in diameter with about a .125" wall thickness. . I used larger thin wall tubing on the ends which would take polyurethane rear spring shackle bushings so that the bars would have a little compliance yet be stiff enough to keep the spring under control.
Bill Young

Bill's dimensions sound about right.

I would add that chrome-moly steel is simply the wrong material for the job. The main reason mild steel (e.g. "1020") is more appropriate is that chrome-moly welds are brittle unless properly stress relieved. Stress relieving isn't a job for home workshops. Furthermore, if you use the wrong filler rod or wire, stress relieving won't work properly. You'll save a lot of money by using mild steel, and your traction bars will actually be much less likely to fail catastrophically.
Curtis Jacobson

Trevor,

On my V8 MGB I just used 1/2" solid bar welded onto tube at 90deg with a V8 spec front lower arm bush pressed in at the front end and a Rose joint at the other.

The picture shows my design (still on the old wheels in picture now on new 15" minilites)

cheers
Steve



Steve Danson

whoops missed the picture - here it is
Steve

Steve Danson

Steve, the only problem with that sort of arrangement is that the trapezoid you have created, all with solid joints, tries to deform as the diff goes through the suspension travel and the part that gives is the leaf spring, which will assume the shape of a shallow 'S'.

This can be avoided in two ways. Either hard mount one end of the arm and have a rubber buffer on the other end so that it limits spring wind up when it contacts the bottom of the spring (it need be no more than a fcation of an inch off the spring at normal ride height), or rigid mount as you have but arrange for the spring to float between the two spring plates. The latter requires tubular spacers over the U bolts so they can move back and forth over the spring enough to accomodate the geometry change, but no more.

I use the latter on my race car, but would opt for the simpler former method on a street car.
Bill Spohn

Bill,

correct me if I have miss understood you.
You say my traction control arms need a rubber bush at one end?
I have a solid rose joint at the axle end and a rubber bush at the other. the rubber bush I used is a front lower arm bush V8 spec
Are you saying I need to redesign?
thanks
Steve
Steve Danson

Bill how about the buffer and two rose joints,that is a rose joint at either end using 3/4" sch.80 piping suitably connected to the rose joints,await your comments.
rgds Trevor Burnett
t burnett

I'm certainly no expert at suspension design, but from what I've learned over the years the best design for a simple anti tramp bar for a street car needs to form as nearly a perfect parallelogram as possible when installed on the car. The length between the front and rear pivot points should be the same distance as that of the leaf spring measured along the top of the main leaf from the front spring eye center to the center of the spring beneath the axle. The distance the bar is mounted below or above the spring main leaf should also be equal in front and at the rear. In addition to prevent binding during the suspension movement you should incorporate some sort of compliance either in the form of a urethane or rubber bushing or a spring loaded bar. I designed the rear bars on my Midget along those lines and they do an effective job at eliminating wheel hop while not impacting the ride quality. Steve, from your photo it appears that your bar isn't parallel to the spring so a lot of compliance needs to be taken up by the rear bushing. It should work, but to get a soft enough bushing you will loose some tramp control. It's a trade off.
As a leaf spring compresses the distance between the front spring mount and the axle center gets longer. The traction bar must either accomodate this change or bind causing poor ride quality. Mounting the bar as close to a parallel as possible and measuring the bars length against the spring main leaf length means the least movement in relation to each part and less binding. You can then use stiffer bushings and those will allow less spring flex to control axle tramp.



Bill Young

Bill,

thanks for the information. Very interesting. Never thought about the parallelagram when I designed it.
I will measure tomorrow and let you know the distances. Having looked at the picture again I think it looks worse than it actually is because of the camera angle (fingers crossed)
cheers
Steve
Steve Danson

Not a great photo, but shows one of my bars installed. As close to parallel with the spring as I could make it.

Bill Young

Something I've noticed that will help.
The traction bar mount point below the middle of the spring is below a curving surface. As the spring decompresses and become more curved, the axle moves a little closer to the spring hanger. However as the spring curves, the mount point below it (which is at 90 degrees to the immediate surface)moves backwards. Flex a ruler or something and you'll see.
However, on mine there is an over all tightening up of about half a mm despite 90degree surface etc so some rubber bushing is a good idea.
Peter Sherman

I would have thought the bulk of the fore and aft movement of the axle as the spring compresses and rebounds would have been accommodated by the swinging rear shackle. Then, the distance of the axle from the front spring pivot remains essentially unchanged in bump and rebound as the front portion of the spring - which is fairly flat already - will remain virtually unchanged in length.
If this wasn't the case, there would be pretty noticeable rear-steer as in when the car rolls in a sharp turn and one spring compresses while the other rebounds. It also seems that manufacturers cant the spring forwards by putting the front pivot several inches lower than the rear one so that the front portion of the spring is more parallel to the ground. This should also offset rear-steer in roll.
I used a design of anti-tramp bar where the front portion of the bar pivots on the same point as the front spring mount and operates a bellcrank which presses on the top of the spring when the axle tries to pivot upwards at the front. Seems to work quite well without interfering with suspension travel. Cheers, Bob
G Elwin

Bob,
Your thinking is correct.

The front location point is the pivot about which the rest of the spring and axle ass'y moves.

Splines on the driveshaft allow the live axle to travel rearward a little as the spring compresses and flattens and the rear shackle ass'y lets the spring move back and forth as required.

If this were not the case then it certainly would "steer from the rear"...not a pleasant prospect!

Some compliance is needed for those times when the rear axle hits a bump on one wheel only and a bushing normally is sufficient.

In earlier cars this was sometimes accomplished with a sliding block arrangement but the result was essentially the same.

Pete.
Peter Thomas

This sort of thing Bob?
http://www.mgbmga.com/tech/mgb4A.htm
The best/easiest approach, I think, unless you are using composite springs.
Peter Sherman

Hi Peter, Yes essentially the same thing - you can see a picture of the bars in the MGC section of the British V8 "How it was done" photo library. Cheers, Bob
G Elwin

This thread was discussed between 06/05/2009 and 24/05/2009

MG MGB GT V8 Factory Originals Technical index

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