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MG Midget and Sprite Technical - Composite semi-elliptic springs
|In the effort to reduce the weight of my race Spridget I have prucored a set of composite rear semi-elliptic springs. These were manufactured by Flexa-form in South Carolina, a firm with much experience in the manufacture of said items for heavy stock car racing applications. I wonder if anyone in the UK (Daniel 1312) or elsewhere has expereince with these? I have been told that as they come they test at about 175 lb/in spring rate, and I am awaiting the specs on reducing the thickness (milling machine required) to bring the rate down to about 125 lb/in, from a fellow who has a modified set. They have to set up (ideally) with parallel anti-tramp bars, upped and lower, with the axle housing set on the spring but not tightly clamped too it (else the spring will break). I have upper anti-treamp and will be fabbing up a set of lowers. Just seeking pointers, experience etc. As always I am in Maryland USA, not Alabama.|
I remember talking about these here on the forum, but i dont think anyine has gone that direction...i think everyones opinion was mixed....this was last winter iirc.
I think it was a woman named harvey ... Last name ...that was building a midget race car in an aircraft hanger... Mmm she may have even purchased... But i dont recall exactly
holy buckboards batman, the stock rear springs are only 75 ~ 80 lb/in
maybe if you slit those composite springs in half, lengthwise, you could get them down to around where they need to be for decent handling (or else, add a few hundred pounds of ballast in the boot, or let most of the air out of the rear tires and use the sidewalls for suspension instead)
|The issue was that no one was manufacturing these springs for Spridgets I believe.|
The only ones available were I thought MBG springs that were nuch wider?
Are these that you have the correct width for a Spridget?
|Robert (Bob) Midget Turbo|
We ran a pair of composite springs in the back of our Comaro dirt car and they lasted for two full seasons of racing till the rh. one broke after a giant impact with the fence. They were 175 lb springs in a 2200 lb car so you will definatley need to give yours a shave.
The secret on mounting them is to run a single short(8"-10')steel leaf between the comp.spring and the diff fixture. This spreads the load out along the spring further instead of stressing the spring at one point Our thing had about 650 hp so your springs should last forever. Just to compare - When we ran steel springs , if the track go choppy you could wreck a pair in one night's racing
Hope this helps Willy
|Thanks, for the tip, Willy. Re: Norm's comment - these rears are to be matching the very stiff front of the car, which is currently set up with 600 lb/in coils and a 7/8" ARB. 1/2" ARB at the rear (adjustable). I have no more than about 1 1/2" suspension deflection at the front under veryhard cornering (hard enough to rub through the top half of the 'H' in Hoosier on the sidewall of the cantilever slicks I'm running). The whole point of going to the composities is to save the 35 pounds or so to be realized by this change (trying to get down to 1550 pounds with car, driver + kit, car with fluids as it comes off the track) and maintain its current excellent handling balance. The handling is my only advantage over the VWs and Hondas I have to race against. Also going to a Quaife diff to replace the welded I have been running (at least for the 3.9 gear which is the optimal ratio for my home track, Summit Point). To Robert Turbo: these springs were manufactured in the US in a limited production run, yes, they are for a spridget -|
|Wow... 600 lbs front coil springs|
Im not sure what to say.... I hope you post a photo
How will this set up effect the over / under steer of the car.
Are you refering to each spring individually or as a pair,....
Aka... You said the front coil spring is 600 lbs....do you mean each coil is 600lbs or is each coil spring is 300 lbs and your doubling it to 600 Lbs because there are 2 coil springs at 300 lbs..
Sorry im just a little lost in my understanding
|That's 600 lb/in per spring. Competition fronts range between 400 and 600, and I've heard some guys in the UK run 700 lb/in. To make a spridget scoot around corners you want it to stay as flat as possible. My car is pretty neutral, a little oversteer before the tires heat up, managable understeer in high speed corners when the tires are up to temp. If I could figure out how to post pdf copies of race photos I would -|
One needs to keep in mind that the spring rates being quoted are the spring rate and not the rate at the wheel which is an important distinction. As the spridget wishbone has around a 2:1 ratio between the spring and the wheel you end up with the spring moving half what the wheel does and the spring rate is reduced in the ratio also, the end result in this case is that the wheel rate is a 1/4 the spring rate, assuming the 2:1 ratio, so the stiffness increase is proportional but the wheel rate value is only around 125lbs/in with a 600lbs/in spring.
|all that math aside, going from 271 lb/in to 600 in the front, and from 75 lb/in to 125 in the rear means that they must have pretty smooth race tracks!|
It also means that I know almost nothing about racing. No wonder, when my friend offered me a ride in his racing mini I thought that it rode like sh*t and he thought it was great.
It also reminds me that another friend had told me he'd strap in as hard as the harness could go, in his Ralt, but after a few laps they would always start to feel loose, because of the tremendously high g forces squishing him into the (unpadded) seat.
Norm "racing is not much like normal driving, I gess" Kerr
Nice stiff Spridget you have there
I tried a 7/8 bar on mine once and found it a bit too pushy for me
I ran 550 front, swapped between 3/4 - 13/16 bars
3/4 for hillclimbs and 13/16 for track and one extra leaf in std springs in the rear - never measured them but I would guess about 100lb This was in a very light Midget and gave the same results as you mention
I found with 1/8" toe out the understeer at high cornering speed and on initial turnin to a corner could be eliminated but it causes a bit of a loss of high speed directional stability which could be scarey at times - I think a little understeer is the better option
Interested in seeing a pic. of your car -- Willy
This thread was discussed between 17/10/2011 and 19/10/2011
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.