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MG Midget and Sprite Technical - Anti tramp bar length?

Hi all,

Does anyone know what the distance between bolt holes in the bushes of the anti tramp bars is? Or have some fitted which they could measure for me?

Any help appreciated as always

Cheers
Dave
D Prince

Ive seen some anti tramp bars I really liked that went from the leaf spring/axle polts to the leaf spring bracket...using a hymn rose joint stick

I know that dosnt help much, but its an extra idea,,,,just replace the rose joints with bushings
Prop

Dave, on a semi eliptic car the anti tramp bars should be the same length as the length of the spring between the front eye and the center pin that locates the axle and run parallel to the line drawn directly between those two points. The idea is to create a parallelogram with the spring so that there isn't any binding when the suspension moves up and down. Because the spring tends to lengthen slightly as it compresses there will be some bind unless you use some type of compensation, either a spring loaded bar or polyurethane bushings to take up some of that movement. It's been quite a while since I built my bars but as I remember the length center to center was 17" for my MKIII midget. The bars don't have to locate directly under those two points on the sprng, as long as the length is the same and they are mounted parallel they should work well. I mounted then ones on my car off the front spring mount and the lower axle clamp plate.

B Young

HI Dave,

I'm hoping to fit mine in the next week or so,

Quick measurement give 41.5 cm between bolt hole centres,

hope that this helps,
John

John Collins

Although it's a parallelogram in principle, in practice it isn't. This is because the effective length of the spring changes as the spring flattens and curves when the axle moves up and down, the centre of the arc through which the axle moves is not the front eye bolt. This means that one side of the "parallelogram" is changing length, while the other side is fixed in length, so it's not a simple paralellogram situation.

The way I have always approached it is to jack the axle right through its range and measure the movement of the tramp bar mounting point (tom the car to the ceiling, or have a few heavy helpers sit on it to allow the axle to go almost right to the bump stop).

Every inch of axle movement, plot the position of the tramp bar eye where it mounts to the axle bracket using a reference grid based on the chassis. These points can be plotted on a full scale drawing and from that the arc drawn and the true geometric centre of the arc found. This centre is where the tramp bar mount should be in order to prevent it restraining the axle movement and confusing the spring action.

In our K, we had the axle mount 3.0" below the bottom of the shock plate, directly under the axle centre. The centre of arc through which the tramp bar eye moved plotted at 2.52" below the bottom of the plate into which the front spring mounts, and 2.3" behind the back edge of the spring mount plate.

Once the chassis bracket is made & fitted, attach the tramp bar at the axle end and then as a check you should be able to repeat the jacking exercise. Offer the front eye of the tramp bar up to its mount but don't put the bolt in it. Instead, observe the alignment between the front eye of the tramp bar and the hole in the chassis bracket. As the axle moves throughout its range, these should stay aligned with each other. This was the case in the tramp bar mount on the K.

If there is more than 1mm relative movement between the two, then the tramp bar will be restraining the axle and adding effective stiffnes to the suspension. If this is the case, generally the stiffness increases with vertical axle displacement.

Less than 1mm is acceptable if there is compliant bushing in the tramp bar; if it is rose jointed then the tolerance needs to bevirtually zero or the tramp bar and spring will forever fight each other.

Apart from the suspension stiffness, this also causes fatigue and sometimes failure at the chassis bracket mounts.

Getting the right length is important, but so is getting the right height of the tramp bar mount, and it is even more sensitive to height error than length error. 1 mm difference in height affects the trueness of the geometric centre about 4-5 times more than 1 mm out in length.
Paul Walbran

So what you are effectivly saying is that a tramp bar should be made to suit the vehicle or more specific the rear spring?
I assume different spring rates and spring travel can have a big impact if the tollerances are as small as 1 mm
Onno Könemann

While it's likely that the geometry will copy from one car to another in a given model, I would always check before fitting to make sure. I certainly would not assume that shifting the mouning point of the axle end (for example, making it less than the 3" depth we used) would need a matching shift in the chassis mount.

However, I wouldn't expect spring rates to change it, as the spring dimensions are still the same. If the arc has been properly plotted then it is showing the axle position at all heights and so for all shapes of the spring between full rebound and the bump stop. But if there is a tramp bar already fitted it is easy to check whether anything has changed.

Paul Walbran

Paul's comments above are noted. Does anyone know for sure if the geometry of the Frontline kit works without binding?
M H Allen

I use compliant bushes in my a/t bar to allow for body roll (incl some sideways movement, too), which would otherwise stress the chassis and axle mount. More solid bushes would pass this roll as twisting force into the mounting brackets - this is the reason that some use rose-joints.

Care is needed with leaf-springs as they don't provide a suspension geometry - and if you introduce anything that provides a geometrical constraint, you've got to allow for unintended dynamic movement - not just the normal up/down (pure vertical) motion we think about most of the time.

A
Anthony Cutler

Yep, the wretched things insist on changing their shape and effective length all the time. Keeps us on our toes!
Paul Walbran

Thanks Anthony and Paul, message received. If I can't get a set landed here quickly and cheaply enough I'll have to fab some up myself. This axle tramp must be gone before my next hillclimb in 2 weeks.
Paul, do you have a surplus set lying around?
M H Allen

Dave P, I like the look of that 2.0 l in your car pic - tell me how difficult was it to get into the midget underbonnet ?

I am working up to a K series conversion - but this has me thinking about alternatives again..
John Barber

Wow... Paul added some interesting thoughts

But im not sure im comprehinding his thoughts very well
is paul saying with a rose hymned stick that it should be adjusted with the leaf under full load

Or does he mean the rose hymn stick mounting need to be set up hieth wise so that the bar is level when the leaf spring is fully loaded

Prop
Prop and the Blackhole Midget

Prop I think they mean that a heim joint should only be put on one end, and a compliant bush put on the other end. The heim joint will allow for small angle changes and the bush will allow for minor length changes as the axle moves.
M H Allen

Mmm... My minds eye appeartly needs glasses...hahha

Im still not grasphing the concept


I get the rose joint to allow up and down movement...its the compiant bush part im stumbling on

Would the compliant bush part look like a telescopking antena... It simply slides in and out ...aka back and forth

Or perhaps some kind of tringle lever with the top attached to the chassis and the wider bottom hooked to the stick so it can hinge back and forth as the leaf spring flexes up and down

Prop
Prop and the Blackhole Midget

Okay on 2nd thought.. Scrap my previous post .... Cause thats not right
That would negate the whole purpose of the tramp bar aka keeping the axle from moving back and forth...

crap, now im going to think about this ALL freaken night long... And that now means no freaken sleep to night ... AUGH... I HATE MY BRAIN...hahaha

Prop
Prop and the Blackhole Midget

Okay...its now 1 am

Lets just cut to the chase, at the risk of sounding completely retarded

Is a compliance bush just a fancy way of saying 2 big fat round rubber bushes with a bolt going thur it.aka

As an example the the truion link that connect the lever shock to the king pin...

Is that what.we.are talking about ??? 2 rubber with a bolt going thur it

The only other thing i can imagine a compliance bush being is a tie.rod end bolted to the chassis that the tramp rod would screw into...and that just sounds insane


If not...can someone please. Post a photo of what a "compliance bush" looks like

Cause this has really got my atten and i cant stop thinking about it....and i really dont need this to be a distraction in my already busy day

Thanks guys

Prop
Prop and the Blackhole Midget

Ohhh...here is A better example of what im asking about what a compliane bushing is ???

2 rubber bushing and a long bolt going thur the wishbones attached to the frame on the front of the midget...

is that the idea of a compliance bush ??


Thanks guys for any help

Prop
Prop and the Blackhole Midget

That's it Prop, rubber bush with a bolt going through. The rod can pull/push against that and get enough movement
M H Allen

Thanks for all your comments guys, they have helped greatly and tramping is a thing of the past! Tramp bar made with rubber bushes n' all :)



John - The Xe wasn't too much of a struggle in my case as the car already had a fibreglass front end and a ford type9 gearbox fitted.

It does need battery tray removal and to get the weight balanced I had to push the engine back quite far. Slight modification of the front crossmember to allow clearance for the oil filter (or fit a remote take off).

I had a new shortened and uprated prop made by a company based in Bristol, the inlet I made myself using a lasercut flange (eBay), the exhaust is currently a c20xe performance manifold (eBay) which has cut and rewelded to fit, though now I have this as a template I will take it to have a custom made manifold when I get round to it.

It uses R1 bike carbs so fuel system is just a facet red top near the tank going via a fuel pressure reg and filter. Correct basic fuelling for the c20xe can be achieved by drilling out the main jet on each carb with a 1.8mm drill bit. The spark is controlled by a megajolt system which uses the edis setup from early 90's fords, base c20xe ignition maps can be found as a free download online.

As the c20xe is used in rwd for kit cars and the like, bell housings and clutch setups can all be purchased from SBD Motorsport though can be expensive. It does mean though there's a lot of info for installation in a rwd format.

I have to say the car is great fun to drive, and after a year and about 5000miles very reliable. I chose to go with the xe instead of the K series as it seemed to be a cheaper way of attaining the upper power levels for a naturally aspired standard engine, but without having to deal with vvc, vtec, vvti... etc.

Hope this info is useful.
Dave

D Prince

I've had a set of the original Aldon Automotive Sprite/Midget anti-tramp bars for 20 if not 25 years.

However, while I made a hybrid locating bracket for the rear locating point (centre of half elliptic spring), I've always used the original front locating bracket. Currently I'm making (eventually I'll make a pair!) a front spring hanger bracket and decided to make it co-locate the front anti-tramp bracket. I'll also have to make completely new bars themselves as I plan to put the centre point (the eye) of the front of the anti-tramp bar exactly underneath the spring eye whereas at the moment it sits rearwad of it. In other words my new design will be exactly as B Young describes above.

UNLESS, anyone can explain why I wouldn't want to do that.

So, the new anti-tramp bar rods will be exactly the same length eye to eye, as the front of the spring eye is to the centre point of the axle. I'm planning on the anti-tramp bar to be exactly horizontal.

I understand that in theory as the axle rises and falls it moves in an arc. However, I'd expect that what actually happens is that to some extent as the axle rises the spring lengthens somewhat and so that arc isn't a perfect arc at all. However, the anti-tramp bar forces the spring movement to remain in a perfect parallelogram as the axle rises which restricts the front half of the spring extending it's length somewhat and thereby creates a stiffer spring action for any given spring. It's possible to negate the restriction of the anti-tramp bar by using as large and soft bushes at each of its ends.

It occurs to me that if in fact the front eye of the anti-tramp bar is higher than the rear eye of the anti-tramp bar the anti-tramp bar will move in an arc that can match the lengthening of the spring as it compresses with travel. Is that what Paul is explaining?

Is my understanding correct?



Daniel Thirteen-Twelve

Daniel

It's along the right lines. For us, because we plotted the movement of the rear tramp mount and then found the centre of its arc from a full scale drawing, it was a case of "the center is where the centre was measured to be".

Having done this exercise we found that the front mount bas behind the front spring mount. This makes sense as the spring shortening effect makes the radius of the arc a bit shorter than the actual length of the spring when it is flat. I think our front mount is a bit closer to the spring ("higher") than the rear mount, but don't know whether raising it a bit would compensate - intuitively I'd say no as the tramp bar would still be longer than the arc radius.

What we did find out when we tested for sensitivity to variation in the front moiunt position was that the vertical position was critical, almost +/- 1mm, and that the horizontal fore/aft position was less so by a factor of about 5:1 (for axample we had to move the mount forwards 10mm produced the same effect as shifting the mount 2mm vertically).

We built our tramp bar front mount on the spring mount bracket as you are intending to do. The car isn't here for me to photograph at preesent, but plus or minus some scruffy artwork, the diagram shows its layout.
It is not to scale and I can't remember the static orientation of the tramp bar and spring, whether they are level up or down - but the front mount is a bit closer to the spring than the rear.

solid black = spring mount bracket
blue = spring
red = tramp bar
thin black = outside plate of tramp bar mount, welded to spring mount plate
grey = inside plate of tramp bar mount, also welded to spring bracket, with rearward extension for additional support
green = supporting bracket bolted to inner arch gusset plate (on original shock mounts IIRC)
purple = brace between outer and inner plates

Paul Walbran

This thread was discussed between 15/08/2011 and 07/05/2012

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.