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MG MGA - Front springs

Some of you saw my previous threads on Camber and Height of Heater shelf measurement etc. My problem is that my pushrod car i am working on is sitting too high at the front end (Hence my questions on height measurements last week). I have 9" free height front coil springs (with, what appear to be the right number of coils and diameter as per Barneys data) but i have too much gap between front wheel arch flange and top of tyre. engine & transmission are in and front & rear body tubs are sitting on the chassis - only pillars, doors and bonnet are not in situ, but their combined weight will not pull the car down much lower on the front end. I have a dimension of 700mm from heater shelf to ground, yet the norm appears to be range 630-650mm, with just one at 670mm. When i removed the springs (as per Barney's method)to measure their height, i further had to disconnect the track rod to let the spring pan drop sufficiently to remove the spring - this implies to me that whilst its dimensions may be correct, its spring rate is not, hence it does not compress as much as original spec, hence the extra height. Attached photo shows dimension from ground to underside of cross member as 15 & 3/8 inches or 390 mm. Can some of you guys give me your dimensions so i can get a better idea of the norm, then i can consider options.
regards
Colin


C Manley

Wow, you're way up there. Your tape measure appears to be at 15-3/4" height to the end of the cross member. My car is only about 13-1/8 inches from pavement to that point with 165-80-15 tires, and I still have the proper three fingers space between top of tire and fender arch.
Barney Gaylord

I forgot to mention i am running on 165/80 radials. Also, Barney's data shows free length of 8.88" for pushrod car springs, so i have an extra 0.12" in length but that's not enough to raise the height to its current extreme.

Barney,
Yep, way too high - i'm only left with the wrong spring rate as the cause.

regards
Colin
C Manley

Moss UK show 3 springs and i have asked for free length information (they do not cite spring rate). Brown & Gammons do list a 7.75" 600lb spring, so this may be a way forward. Still open to other views and opinions.

regards
Colin
C Manley

Yup. See coil spring spec's here:
http://mgaguru.com/mgtech/suspensn/fs113.htm

Notice that all MGA have the same Static Laden Height of 6.6 inches for coil springs (all the same ride height). The first 5000 or so MGA 1500 had softer (and longer) springs with spring rate of 348 lb.in. All the rest had spring rate of 480 lb/in. The Twin Cam springs are longer (free length) because the engine is heavier.

Stiffer 600 lb/in coil springs would be for competition use. To have original ride height these springs must have shorter free length. For Static Laden Load of 1095, a 600 lb/in spring would compress 1.82", so to end at 6.6" loaded it would need free length of 8.42", nearly 1/2" shorter than the standard spring.

A small change of wire diameter makes a large change of spring rate, so you need to measure the wire diameter with a micrometer to be sure what you have in hand. Also count the free coils and measure free length. With this information you can calculate the spring rate and spring deflection (using formula from some engineering handbooks).
Barney Gaylord

This gets better.... The wire dia. of offending coils is 0.54", free length is 9.2 (seems like Twin Cam springs?). However, when fitted and loaded, my guestimate of height of spring from inside of lower spring pan to the underside of the crossmember (allowing for the tapping plate)is getting on for 8" - almost 1 & 1/2" too high. Don't have free height info from Moss yet on their spring offerings, but i think the B&G spring will be my way forward at the minute.
regards
Colin
C Manley

Colin

Just thinking outside the box here, did you tighten up the suspension bushes with the weight on wheels or with the wheels 'hanging'? If the latter, could this be causing the problem in that the bushes will not twist enough with the weight on wheels?

Steve
Steve Gyles

Experience says you simply cannot predict what will happen to a newly assembled car in the shop once it is driven a bit. A part assembled car - "only pillars, doors and bonnet are not in situ, but their combined weight will not pull the car down much lower on the front end." - has a whole lot more bits to add than you list here; there is friction and settling to come. Even or especially jacking the car up and letting it down can cause 2" or more of static ride height change. Some of this can be settled by bouncing on the car and rolling it to and fro, but not all (and not repeatably).

Your previous questions indicate a certain amount of flailing around re measurements and procedures. I found the "chassis twist" statement in the camber thread a bit odd - this is nearly unknown on MGA. What was the resolution?
Severe damage or repair requires accurate checks of the chassis, measured per WSM, not random approximations measured to sheetmetal bits from wobbly floors.

The only sane path is to accurately measure and record the data for a pair of matched springs. Finish the car and drive it down a bumpy road, preferably 50 miles or so. Then, without jacking the car up after driving it onto a flat reference floor, measure the actual ride height. Record all items like fuel, water, tyre pressures, actual loaded rolling radius, tools, spare wheel, crap in the car. etc., so that you can duplicate it for later comparison. Compare these conditions to published data, which is often murky (dry weight, "full" weight, Kerb weight, etc.). Published dimensions on ride height for alignment purposes frequently have little surprises like "with 150lb on the floorboard in front of each seat" - not necessarily agreeing from one year's books to the next despite the cars being identical. Then if necessary, compare your known installed spring data to other possible springs. Go from there.

FRM
FR Millmore

I'm with FRM here

When I broke a front spring, I replaced both to make sure the car stayed level. When I put it on back down the floor it was a good 2 inches higher with the new springs than with the 48 year old originals. There was a noticeable increase in the gap between wheel and front wing.

A few days later I was out following another MGA in the country and hit a big bump in the road. The noise was as if a Grand Piano had been dropped from a great height. On reaching our destination, the ride height had reverted to the same level as the old springs (well maybe a half inch higher)

So settlement will be a big factor, driving it will cause everything to settle down.
dominic clancy

Dominic's story is a common one, the result of the springs not being correctly seated at the top mount inside the crossmember, or rarely, in the bottom pan.

FRM
FR Millmore

FRM

Yes, I remember mine going 'boing' on my first proper drive out. The wife thought the car was about to disintegrate. At the time I assumed it was the springs. I did check to make sure nothing was broken.

Steve
Steve Gyles

This thread was discussed between 27/06/2012 and 28/06/2012

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