Modifying Leaf Springs

[tony barnhill]

Okay, here's one:

I just removed the 7-leaf springs from under my race car, a '68 GT

...now, the car is pretty light (rear hatch is fiberglass w/Lexan window - heck all glass is replaced w/Lexan, doors gutted, no bumpers, only 1 driver seat, 5-gallon fuel cell, anything that even looked like a GT in the hatch area is gone, holes drilled most everywhere, no undercoating, only 1 battery box, etc, etc, etc...I mean, we lightened this baby just a bit....car's probably coming in around 1700 pounds hen finished, so:

...Wonder how many leafs I can take off the 7-leaf springs to get her to squat at a height about 1" lower than normal?)

 

[jlaird]

I know this is comming from a non GT guy, but try two. Worst case is you have to put one back.

Messed with springs in another life. And yes I saw what you took out of that machine.

 

[Matthew E. Herd]

Keep in mind that you're already what, 800 lbs lighter than stock? Just to get it back to original specs, you'd probably have to remove one spring. I could calculate it for you, if you'd really like. I just had the appropriate book open this afternoon, and it's a fairly simple calculation.

Matt

 

[Joe Reed]

Wonder what it will do to the spring rate if you remove more than one - assuming that removing one leaf will essentially make it the same as a roadster spring. Removing leaves would lower it....but would that also make the springs too soft for competition use?

 

[Dave Russell]

Re: Modifying Leaf Springs *DELETED*

Post deleted by Dave Russell

 

[jlaird]

Haha, do you supose he wants to race it as well as lower it? That's a whole other question indeed and far beyond my expertise.

 

[Nunyas]

well, he's significantly reduced the sprung wieght of the car's body... so wouldn't that have the same effect as putting in higher rate springs? I mean the spring is holding up less weight so it shouldn't take as much strength to keep the rear where it belongs... Then again, I don't know enough about springs to know how much is too much or too little for the application...

 

[tony barnhill]

See, I don't know anything about springs...I thought about dearching them...looked at my 7-leaf GT springs next to a 6-leaf roadster spring....only difference seems to be a short - about 1' - spring added to the GT unit...so, I took the 3 little GT springs out & left the 4 longer ones (that would be the same as taking the 2 little roadster springs out)...I'm going to have to rethread the spring center bolt now as there's not enough threads to tighten it down - but that's nothing.

And the new rear end setup should drop another 70 or so pounds off the weight!

I'm a firm believer in Colin Chapman's "add lightness" edict...got to to make ths old GT stay up with the faster cars in E Modified - even with the Camaro 3.4V6 & 5-speed!

...no, i don't want it to bottom up but I also don't want its butt up in the air like now.

Matthew, would you mind doing the calculations?

 

[Joe Reed]

I don't know enough about the effects of re-arching leaf springs - so I don't know if re-arching them increases the spring rate (stiffness), but I suspect not. I would think you'd want stiffer springs for competition, but how much is too much - especially since you've lightened the car? I guess the only sure way - apart from advice from other racers - it trial and error. You probably want stiffer springs on a competition car than a street car - but, then again, an oversprung car isn't what you want for the track either! Should be interesting first time out! /ubbthreads/images/graemlins/laugh.gif

I don't think the lighter rear end will be much of an issue, since that's all unsprung weight...

 

[DrEntropy]

[ QUOTE ]

I'm a firm believer in Colin Chapman's "add lightness" edict...got to to make ths old GT stay up with the faster cars in E Modified - even with the Camaro 3.4V^ & 5-speed!

[/ QUOTE ]

Tony, your stock just went up exponentially here at Chaos & Mayhem Pty., LTD.

 

[tony barnhill]

Joe - no, the lighter rear end will just affect overall weight...she's already got her butt up in the air...so, while everything's out from under her, I need to bring her down.

 

[Matthew E. Herd]

A series of questions, Tony:

First of all, what do B's and BGT's weigh, in stock form? (roughly of course). I'm curious how much heavier they are vs. spridgets.

As an aside, the equations are as follows, according to "Mechanical Design of Machine Elements and Machines" by Jack A. Collins:

Spring rate, k = (8*E*n*b1*t^3)/(3*L^3)

b1 = width of a leaf
t = typical thickness
n = 3 of leaves
L = distance between the spring eyes (in a straight line, not following the curvature of the spring).
E = Young's modulus of elasticity. Since we're dealing with steel, it's almost always 30x10^6psi. If it's not, it's still close enough for argument's sake.

Now we need to know (from some helpful volunteer) L, in inches, b1 and t in inches, and the number of leaves we're calculating for given an MGBGT leaf spring arrangement.

Guessing that b1=2" t=.25" L=36" E=30x10^6psi n=3,4,and 6, our respective spring rates (k3, k4, k6) are as follows:

k3=161 lbs/in
k4=214 lbs/in
k6=322 lbs/in

As you can see, the increase is linear with the number of leaves added (at the same thickness, width, etc). As was noted, the reduced rear weight will make the spring rate seem higher. This means that you can get away with some reduction in the number of leaves, but I would not advise going too extreme, as you will reach the bump limit more easily. Fatigue is also an issue, but I would wager that you don't have to worry so long as you don't get too extreme with leaf removal.

The decurving of springs has no effect on the spring rate, according to my book. That means that decurving by means of a hammer and anvil, as discussed in the Spridget forum, is an acceptable way of doing business that will not reduce the spring rate.

When addressing fatigue failure, to make a very long story short, I can draw a few conclusions from a quick look at the equations. The higher the number of leaves, the lower the stress. This makes sense logically, but in order to calculate precisely how much you could get away with, we'd need to consider how much bump travel we have. This would tell us how much force can be applied to the spring, and therefore whether the resulting cyclical stress is too much. My previous advice remains. Don't remove too many leaves, or you're asking for trouble!

 

[tony barnhill]

WHOA! Haven't seen such a word problem since college! I'll print it out & see what I can see tomorrow

...here's another question....if I disassemble my springs & reassemble them in reverse order, they'll be thicker on top & should act as a lowering block, taking care of my 'butt in the air' problem...will that 'soften' them also?

 

[Dave Russell]

Re: Modifying Leaf Springs *DELETED*

Post deleted by Dave Russell

 

[Matthew E. Herd]

I've heard elsewhere, for Spridgets at least, that the existing spring rate may be too high. I'm unsure if this is true, but the removal of one leaf and the decurving is another possible solution. Either way, decurving has no negative effect (except reduction in bump travel). I also hear that spring wind-up is a problem in MGB's (not so in Spridgets). While that's basically a separate issue, it may be worth considering when you think about the rules, etc, etc. E-Mod rules should be pretty lax (as I recall), although I'm not intimately familiar with them like I am Street Prepared and Street Mod rules.

 

[Banjo]

Tony,
I personally would look into de-arching the current springs.
Like you're thinking, that would lower the car and keep a stiffer spring rate that would be better for compitition use.
We have a local shop here "Elmira auto spring works" that specializes in car and truck springs. They can do it all. De- arch, re-arch, repair, fabricate, modify, and more. I took a set of 1/4 eleptics for a bugeye over to them and the next day the car was sitting right.
I feel that removing a leaf would be kinda "backyard". Nothing wrong with that, I just think you'd be better off with the stiffer spring rate for racing.
Road use would be a different story.

 

[tony barnhill]

Matthew - haven't even started working on the anti-tramp bar thread!

Banjo - I may need to look around to see if there's a shop in area that dearches springs & ask them some questions.

 

[Joe Reed]

I don't think assembling the springs with the secondary leaves on top of the primary is an option. I imagine those are installed on the outside of the arc for a reason. If installed on the inside of the arc, instead of a compression load being resisted by the leaves' resistance to flattening out, a compression load would be resisted by the leaves' resistance to a decrease in their curvature. Part of that resistance in a stock spring probably comes from the friction that develops between the leaves. If they were on top, they would be pulled apart, rather than pushed together.

Or something like that.... /ubbthreads/images/graemlins/laugh.gif

 

[tony barnhill]

Hey, Joe...haven't seen you over here in a while.

 

[Matthew E. Herd]

Speaking of that, can we discuss rear sway bars? I'm working on a package for my midget, and was wondering how the later B's had them attached. Yes, I know, my brother's 80 MGB has one, but I never realized that when I had it on a lift, and haven't looked at it since I found out. I really don't work on his car, so I'm not nearly as familiar with it. Any info (and pics) would be appreciated.

Back to springs, the reason I bring this up is that you may be better served with an independent setup, although it's a heck of a lot more fabrication. I know SM rules state "drivetrain is unrestricted." Does this apply to EMOD as well? (I'll have a look for myself).

I know this is far afield, but I also thought that the ultimate SM2 car would be an MGB with Rover V8. How hard is it to get them, do you believe they'd qualify for the rules (as far as I can tell, they should), what do they go for, and what sorts of transmissions and rears are compatible? Since drivetrain is unrestricted, I'd definitely go with an independent rear setup if I was building an SM2 car. If I recall correctly, the motor could be bored and stroked to its absolute limit, and then forced induction applied, and it would still be within the limits for SM2. I don't know how much the 216 V8 will take, but I'm sure you could build a real beast.