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MG MGB Technical - Ride Height

I need to reduce the front ride height of my MGB GT after the renewal of the front springs, what is the ratio of ride height measured from the centre of the wheel to the chrome side strip compaired to the actual unladen free standing spring height, I remember reading that the ratio is 2:1, that is 1 inch reduction of the spring height reduces the ride height by 2 inches.

Appreciate the Members input,regards Trevor Burnett
t burnett

Year? I've not heard of a formula (not that that means anything). Probably the only thing you can do is compare free height specs to actuals, both can be found here http://www.mgb-stuff.org.uk/ridetext.htm#springs and here http://www.mgb-stuff.org.uk/ridetext.htm#rideheight.

For my V8 the spring free height was 9.75" (as opposed to a spec height of 9.32"!) which gave a settled (5k) height of 14.5" right and 14.625" left. But the loaded height of a spring i.e. how much it compresses under load, for any given vehicle weight, depends on the number of coils, wire thickness, and hardness. The 2:1 may well simply relate to how the vehicle height changes with the loaded height, i.e. a function of the leverage i.e. how far from the pivot the centre of the spring is, compared to the centre of the tyre patch. The actual change you get by changing or modifying the springs will depend on the spring characteristics as above. You will also see from the springs pictured in the first link that the top and bottom turns are flattened against the next turn, you can't simply chop a turn off, the spring would buckle under load.
PaulH Solihull

Thanks Paul, you and I seem to be the only people discussing this, maybe there are a number of Members waiting with bated breath at the outcome of our discussion.
My V8 conversion with V8 springs and with almost no road time(car is at the trimmers) has a front height of 390mm(15.35 inches)I would ideally like to reduce it to 360mm(14.17 inches)
My current plan is to put some miles on the car before deciding what to do, the springs might settle like yours did and this settled height might be what I want.
BUT it would still be great to know what the ratio is.
t burnett

There is no ratio of unladen spring free length to ride height.
The only meaningful thing with regard to springs is "length at specified load" - a book number, or "length at actual load" - a car number. These could be calculated from "unladen free length" and loaded weight at each wheel, IF you had an accurate figure for effective rate at the wheel. But, "rate" as usually quoted is for the spring only, not accounting for suspension geometry.

There is an approximate geometric ratio of installed loaded spring height to ride height. It is the ratio of "effective position of applied spring load" to the distance between lower A-arm inner and outer pivots. But, "EPASL" is a thing that varies with the angle of the A-arm, so you can only approximate it, as the central axis of the spring or spring pan when the spring is at 90 deg to the arm. The "effective spring rate" also changes with the arm angle, being greatest when the spring axis is square to the arm and upper seat. Many suspensions utilize this to give an effective rising rate as the suspension nears full bump.

For an MGB, ride height change will be near 2x loaded spring height change, assuming the A-arm is horizontal (spring ends parallel to each other) at that loading.

About all you can do to figure this out is to compare spring length at a load somewhere near the installed load, about 1000 lb for an MGB. If you know the rate and free length, you can calculate the loaded length for a given load, if they aren't lying.

FRM
FR Millmore

Would a spring manufacturer have the knowledge and experience to advise? I know when I had some spring and shock absorber problems ( Combined, motorcycle use) A firm here in Australia were very helpful and made me up the correct combination. One might think this an extremely expensive way to go, but guess what, Quoted replacement from the official motorcycle dealer $2000 oz, from Koni $500 the pair! They ask all sorts of questions, what will be the use, weight of rider, tyre combinations. They will put together car applications also. I would be most surprised if I posed the MG question to them if they could not oblige? Mike
J.M. Doust

This thread was discussed between 05/09/2011 and 13/09/2011

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