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MG MGB Technical - RV8 anti-tramp bars... anyone have data?

Hi all,

Has anyone got a picture or drawings of RV8 rear shock brackets and front spring hangers? I'd like to fit RV8 anti-tramp bars (traction bars for the US guys...) to my '67 GT and was wondering what the dimensions are for the spring hangers and spring clamp plates.

I can't afford composite rear springs so I know I can't just fit RV8 clamp plates as the shock mounting is in the wrong place. I'm not troubled by the idea of having some made though, as long as I have a pattern to work to.

The main reason I'd like to use RV8 parts is to save myself making the bars themselves - I figure I can just go and buy a set of RV8 bars and bushes and then fabricate the mountings.

Any thoughts? Thanks in advance for any data or opinions!
--
Olly
Oliver Stephenson

Olly
You would be well advised to start this thread on the V8 Conversions bit as these guys are probably more likely to have considered this due to having more power than the standard 'B'
Bob

Oliver-
If you're serious about this conversion be advised that that solid-rod traction bars will cause the rear multileaf springs to bind as they flex, resulting in a very stiff ride on bad roads. The RV8 used a tapered single-leaf spring that could tolerate this while the multileaf springs of the MGB really don't. For an isometric drawing that will enable you to fabricate the necessary antitramp bar anchoring bracket, you'll need to purchase a copy of Roger William's "How To Improve MGB, MGC, And MGB V8". It's on page 57.
Steve S.

I thought they only caused the springs to bind if you got the arc wrong... if the bar doesn't try to tilt the axle forward or backwards, how can it bind?

Of course, it will tend to bind under power. Is that what you were meaning?

I know naaathing.
--
Olly
Oliver Stephenson

Oliver-
No, I'm afraid not. It's a simple matter of geometry. As the spring flexes upward, the axle moves toward the rear because spring flattens, hence the need for the shackles at the spring's rear mounts. The fixed-length antitramp bars then try to tilt the axle, distorting the leaf springs and causing them to bind. The greater the flex of the springs, the greater the bind. Without antitramp bars the axle can be similarly distorted by the torque of the axle, the antitramp bar merely limits it. However, if you stop and think about it, it's obvious how they will effect ride quality over bad pavement. As I pointed out in my previous posting, the RV8 made use of tapered single-leaf springs which were more tolerant of this force than the multileaf springs of the MGB.
Steve S.

Okay, I understand about the axle moving backwards as the spring is compressed (I'm currently furiously doing some crazy maths to calculate by how much it moves...) but if you install a fixed-length bar of a *different* length to the front half of the spring, at an angle, then that too will tend to move the axle backwards.

True, it's not a perfect mimic but since delta-theta (the change in angle at the front of the spring) is relatively small I think that an approximation could be made that will allow negligible tilt on the axle, certainly lowering it to an amount that is insignificant compared to the tilt caused by, say, a little bit of throttle.

Of course, I've never done this before so I defer to your better judgement. I'm just doing this for brain excersise really :o)

ttfn,
--
Olly
Oliver Stephenson

Steve has the rough ride bit correct, though I'm not sure that a single leaf spring would be immune to the twisting effect. Anti-tramp bars are only good for inline accelleration; they cause problems in cornering because the twisting of the axle due to the different positions on the respective arcs of the bars is transfered to the chassis. Leaf springs without solid bars will bend to accomodate the axle twist. Someone on the V8 list provided a site for for traction bars that acted on a sort of bellcrank pivot to bear on the tops of the springs to prevent wind-up. I would think that unless you are doing the fabrication yourself the composite springs, which require no modifications, might be a bargain.
George B.

George-
The word that I used was "tolerant", not "immune." You're dead right about the effects of the forces generated by the axle when the wheels are at different planes. Unless the mounts are well-designed and properly installed, the forces can overstress and rip out the bracket and do serious ($$$$) damage to the structure of the underside of the car. As you said, they're practical for straight line driving only, like at a dragstrip. Can you explain your comment about the composite springs?
Steve S.

Steve,
Oliver said that he couldn't afford composite springs, so he wanted the RV8 anti-tramp bars. I was addressing that; though some of the advertising for the composite springs indicates that they are less prone to wind-up.
George B.

Hang on, hang on... my inability to afford composite springs and my desire to replicate the RV8 design aren't really connected. I can't afford the RV8 springs, or any other kind of composite ones, full stop.

I've now come to the conclusion that I can only fully control all the variables if I make the bars myself. However, I will probably try to buy a set of bolts and bushes for the RV8 setup so that I have something at least approaching a standard for when I need replacements.

The people I use for the metalwork that's beyond our workshop are very reasonable and I'm pretty sure that I can get everything I need made for less than £150.

For those that are interested, if you take a leaf spring of curved length C, and install it so that it is d high (height being the vertical distance between the mid-point of the two mounting ends and the middle of the spring) then the horizontal length L of the spring (straight line from end to end) is:

L = C.cos ((sin-1 ((pi.d)/C)) / 2)

as long as d is not greater than C/pi.

Obviously if you divide L by two, you get the horizontal distance between the front mounting point and the axle. Handy, no?

That's just the first part of the maths... I'm too lazy to type up the rest of it right now :o)

ttfn,
--
Olly
Oliver Stephenson

Oliver-
Because the axis of the mounting points of the different ends of the springs lie on two different horizontal planes, d must be perpendicular to an inclined plane that intercepts the axis of the mounting points. Thus d is inclined, not vertical. However, I'm sure that you're aware of this and merely mispoke yourself. If you can run the math well enough to predict the total rearward movement of the axle when the suspension reaches its compression limit, you could design a telescoping antitramp bar which would function well enough without binding the leaf springs. To my knowledge there is no such design commercially available on the open market, so your hours of designwork might net you a tidy little side income, especially if you could interest the MGOC in it.
Steve S.

Steve,

Yes, you're right I didn't bother talking about the incline between the two end points of the spring - instead I considered them to be at the same height for ease of explanation.

A telescoping bar? Surely that would defeat the object of having the bar, as it would cease to take any of the thrust load :o)

What I assume you meant is to have some kind of a cam or other means of 'extending' the bar as the car moves downwards... this seems like an expensive option, and perhaps unneccesary, but experimentation will no doubt prove me wrong.

Now, what is K (the young's modulus) for a pair of RV8 rubber anti-tramp bar bushes? :o)
--
Olly
Oliver Stephenson

Guys, you're making a big deal of this binding thing. Take a look at a rather basic but well-manufactured traction bar kit for Bs designed and sold by British Automotive in CA (theirs only work with composite springs, but you'll get the idea ... to work with multi-leafs you'd just lose a bit of ground clearance). I've found them to work extremely well and they do not bind the springs ... or I should say any such binding isn't perceptible ... but they greatly crispen accelerating and decelerating. You just set the pre-load to a fairly neutral setting and that seems to take care of it.
Bob

Oliver-
Telescoping antitramp bars have been around for US muscle cars for ages. One half of the bar is tubular while the other half has two diameters, the smaller of which fits inside the tube. This provides some free travel to allow for the necessary variance in length as the axle moves fore and aft, yet permits a solid stop, thus limiting spring wrap without the binding that is inherent to a fixed-length antitramp bar design.
Steve S.

Steve,

Ah I see... they're basically loose until you put on enough power to slide them all the way home? And presumably this distance isn't as far as the axle would normally move under hard accelleration.

Makes sense but I think I'll stick with a standard bar. If it doesn't work I can always change it later.

The trouble I'm having at the moment is choosing bushes, as RV8 ones just aren't available... current choice is shackle bushes, but they don't have a tubular insert 'cos the shackle bolts up against itself. Hopefully I can find some suitable tubing, otherwise I'll have to use front (of the spring) bushes which are a bit on the large side. I also looked at midget top trunnion bushes but these have a very big hole through the middle and would need a 1/2" bolt.

Bob... composite (or steel single-leaf) springs are immune to binding. The binding is caused by the increased load between the leaves and their interleaving that is brought about by the axle being tilted, thus preventing the leaves from sliding against each other and effectively shortening the spring. However, I'm sure British Automotive's design doesn't tilt the axle much (they say they can remove the bolts at full rebound and bump conditions) so it would work fine if it just had a bit more clearance.

On the clearance point, even if I'm really conservative in terms of distance between the spring and the bar when at rest, there's still no way it will be below my exhaust system, so I don't think the bars will hit any sleeping policemen.

ttfn,
--
Olly S
Oliver Stephenson

This thread was discussed between 29/06/2002 and 01/07/2002

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