Technical How to design suspension system to obtain desired ride height?

I’m now at the point of designing the bracketry that will locate the 1/4 elliptical springs (front and rear) which will attach those springs to the axles and ch***is. I’ve determined that I want a ride of about 6” between the ground and the bottom of my frame rails. The question is how do I go about designing that bracketry and the respective mounting points, such with the vehicle when at curb weight is at that desired ride height. At the moment I do not have an accurate curb weight figure or any info on the spring rates. So, I’m ***uming I need all that info to design the bracketry, or I could just try mocking up it all up, and ***ume it will take some iterations which is what I want to avoid as the plan is to have all the bracketry laser cut. I’m not even sure what the correct terminology is for this exercise, and searches on the interweebs are all useless and bring to vids of coil overs and 4x4 four link stuff.

 

I used the speedway units with the adjustable jack bolts on my model A. And in simplified terms I mocked the car at right height with the bolt all the way out.

once I sat the car down it drop about 1 3/4 inches. I had considered planning that into the mount locations but it was nearly impossible to get the springs attached to the rear axle that way.

I lifted the ch***is to unload the springs, turned the jack bolts maybe one turn? And sat the car back down and measured. And that’s about where it wanted to be.

there has got to be 5+ inches of adjustability in these springs it’s pretty cool.

it took an afternoon of turning it a 1/4 turn and sitting it down and looking , then repeat but you get the jist.

 

put it so the suspension is at about 1/2 of it's travel, at the desired ride height. Actually, set it so it's a little off from that, perhaps give it 3" or 4" more downward travel, and 2" more upward travel.

The last thing you do is figure out the spring rate and length, after you have the car finished, so you know what it actually weighs. Because they all gain weight as you get closer to being done, and "remember" all the stuff it needs.

 

Ok. I’m guessing total suspension travel is likely no more than 5” with 1/4 elliptical springs. I’m also very limited in rear suspension travel due to the design. Frame rails will actually p*** under my Model A banjo axle tubes. Front Speedway tube axle centre will be about 10” - 12” beyond the ends of the front frame rails.
I’m cutting the stk Model A front and rear springs in “half” to make the 1/4 elliptical springs. Both front and rear spring attachment points will be below axle centrelines. Front similar to this setup.



Approximate location of rear axle. Will likely be an 1” higher off of top of frame rail.

 

You need to talk with a spring shop about spring rates for half of a leaf spring. You will likely need the total length, each leaf thickness, center pin location, and total number of leaves for a shop to figure out your spring rate once you cut them.

 

@Ziggster You have mentioned this a couple times. I really want to see the results, so I'm pulling for you.
That front attachment looks like a good idea. I could see that being able to be adjusted in and out to tune spring stiffness, but the drag link would also need to change length.
Is there a spring rate published for the one you are planning on?
Punched quarter leaf spring rate calculator in and got this
https://www.ecosia.org/search?q=quarter+leaf+spring+rate+calculator
one of the top results is a 5 year old thread here, which also happens to be the only about quarter style.
I glanced through it quickly, and think it might help you. @squirrel 's answer is probably the best, provided you can get the desired rate once it's built.
https://www.jalopyjournal.com/forum/threads/quarter-elliptical-leaf-spring-rate-formula.1183536/

 

I set em at ride height during construction
With the desired rake
Make sure the spindle height and axle centerline have the travel needed at that height
welds it up
The rest is adjusted when it’s done.

 

Yeah, I forgot that I can add/remove individual leafs to perhaps get desired height, but that of course will change spring rate.
Will have to go through those links first. Thinking I could calculate spring rate with just spring mounted to something and hang different amounts of weight off of the end and measure deflection. I think that’s what we did back in high school physics cl*** to plot out spring rate.
I could also start weighing components to get a more accurate estimate of curb weight. I was guessing it might be around 1,200 lbs - 1,400 lbs, but likely closer to 1,700 lbs or more.

 

Well, there have been a couple threads over the years about finished weights. It's a shame there isn't a single reference thread about this to help guide everyone. I know it's not exactly the same as what you are cooking up, but my T-Bucket is 1975# with my 200# in it, and @Tim_with_a_T 's is lighter.
Here he mentions 1600 pounds.
https://www.jalopyjournal.com/forum...et-build-thread.1000590/page-49#post-15703394
Remember to divide by 4 to get close to each spring's load for your spring at each corner setup, so 400-500 pounds to suspend at each corner.

 

I’d make some guesstimates where the spring height would be when fully weighted, but ***emble it with just the main leaf. If it rides too high or low when done, you can rearch the spring higher or lower to suit.

 

@Ziggster That thread here has you as one of the Characters!
It also has a mention of the T-Bucket forums and a calculation (?).
I went over and did a search. This is what I found. Haven't read it all, but might be useful to you.
https://tbucketeer.com/threads/1-4-eliptical-springs.7532/

Another discussion
https://tbucketeer.com/threads/1-4-elliptical-front-and-rear-suspension-brooklands-special.17775/

EDIT: and weights (there are others)
https://tbucketeer.com/threads/weight.17666/

 

This is always a problem, especially if you’re not using an adjustable mount like Speedway’s. I just did the rears in my ‘33 pu and basically took a wild a$$ guess. My springs are Model A cut just past center with 5 leaves to start.

My first try was pretty close. I tacked everything together and tried it. Then I shimmed the rear to get the height right and will move the main mount down on the frame rail to get the correct height without the shims. From there, I can add dummy leaves to the top of the spring stack, shim the other end for “fine tuning” and add or subtract leaves for stiffness.
You can design in adjustability at the main pad plus you can change your shackle length to fine tune a little. I took that approach with my ‘27 T (quarter elliptics at both ends). In this case, I could also flip the perches. Lots of adjustability in this setup but I didn’t want to go to all this trouble for my pickup. Getting lazy in my old age I guess. Or just impatient, I can hear the clock ticking…..

 

Thanks! That’s too funny! I also just used Google searching for calculation for 1/4 elliptical springs and it turned up the same HAMB thread! Of course, I don’t recall any of that.

 

Very nice! How is the ride with these types of springs? From what I recall reading, most folks prefer the ride quality with the 1/4 elliptical setup vs full leaf or transverse.

 

Thanks! Will try to see if I can find some type of scale to weigh the thing. I’m really curious to see what I’m at now. Just doing a quick checklist of items of just over items items I’m at around 1,550 lbs (unsprung weight)
I of course tend to overthink everything, and normally I wouldn’t be to too concerned, but this time I really want all the parts to mount the springs to be laser cut for precision, quality, and to save fabbing time. I will plan for sure to build in some adjustability in where the springs mount to thd axles, but in reality, like you have said, it must likely will be an iterative thing, and some things will likely need to be s****ed/tweaked. Maybe I fab up the first iteration, etc. and once satisfied, then get the parts laser cut. Thus probably is the best approach. Parts don’t have to look like final, but just to get location of mtg points located.

 

Not sure of your intended use for this project, but 6" from bottom of frame to ground?

 

Yeah. Oil pan will be around 5” off of ground. Use unknown. lol! Unfortunately, it will never be street legal in Quebec, so events I guess. TROG?

 

Google AI gives me this formula:

 

Building some ride height adjustability into the design is a good idea.
Here is an excellent thread by @lostone

https://www.jalopyjournal.com/forum/threads/built-my-own-quarter-elliptical-springs.1115374/

 

Using the above formula with the following parameters.

F = 400 lbs (***umes 1,600 lb (unsprung) and 50/50 weight distribution)
L = 12”
n = 3 leafs
E = 30 x 10^6
b = 1.75”
t = 0.25”

deflection = 0.49”

Wow! Does that make any sense? Didn’t check formula to ensure units all work out. Tried with 1 leaf and got a deflection of 1.475”.

 

Thanks for that link. Some really nice workmanship there. I don’t think I’ve ever come across that one before.

 

How’s the ride quality?

 

A later version of my avatar was converted to rear 1/4 ellipticals soon after the dirt track ***ociation opted for overhead engines necessitating the immediate need to get all that unleased horsepower hooked up in the form of traction, a pair of now tired tow vehicle Jeep Comanche rear springs were dissected & 'U' bolted directly below the 32 frame rails & attached without shackles to the common place for the era hefty 3/4 ton rear axle housings. Others perhaps ahead of the times, leap frogged to the 9" rear as they came available with parallel springs. Both methods proved their worth until the heavy & gangly 3/4 rears were better left with the trucks as intended, while similar lighter weight after-market off shoots for the quick change continues on to our current day. The 9" like the Q.C. proves its modern era reliability with various suspension systems employed in the form of leaf springs, coil overs, torsion bars etc. & then of course, the Quarter elliptical rallies on with new life in the world of Hamb.

 

If you are asking about the link I posted, I don't know. You would have to ask the owner...... @lostone

 

Haha! For some reason I thought it was your thread.

 

My roadster rode nice and smooth, and due to the low CG it cornered dead flat. It was a lot of fun. I never put a lot of power in it just a 0.060” over 283 with a small solid lifter cam and 1.94 heads. Several factors weren’t optimal, slow steering box, front shock angle, etc. but it was just fine. No quirky habits. The biggest problem with that car was stopping it with my preferred tires - Firestone bias-plies. Plenty of brakes, tiny contact patch and no weight.
I gathered up my books and started out trying to math out everything but, like you found out, the numbers sometimes don’t match up with common sense or our instinct. These days the whole thing can be modeled and calculated by a talented person with a computer. That’s cool but it’s not for me. I’ll just get it on the road and make tweaks to my satisfaction.
Here’s the front. I improved the shock angle before it ever hit the road. Angled springs and no shackles negated the need for a Panhard bar. It worked. Probably wouldn’t work on a big car or a car with a high CG.

The old rule holds true, longer leaf packs will ride better. These are four Model A springs cut about 2” past the center bolt. I figured that half a standard street rod 6-7 leaf spring would be close and it was.

 

Thank you @twenty8 !!

@Ziggster honestly I don't have the car up and running to say about drivability BUT I did have the car (31' tudor) all mocked up and with me and my son ( we each way under 150lbs each) it was easy to push the car up and down and honestly felt really good. From myself and my son playing around with it I have a really good feeling it's going to drive really good...

...

 

As I plug away at the front suspension, it just occurred to me that I can just lower the ch***is onto the front 1/4 elliptical spring mtg eyes, and measure the deflection. I’ll centre some jack stands where rear axle centre-line will be. I’ll then add some weight to simulate me sitting in the seat just to see what happens. Not sure why I didn’t think of this in the first place. Right now, I have 4 leafs, each about 80% of the length of the leaf that precedes it.




Also got these rebound clips from the suspension shop that made me the U-bolts. You’re supposed to bend those ears down over the sides of the clamp.



Was watching a vid on early American p***enger car suspensions from the 50s and they did mention anti-dive considerations and they mentioned the “instant centre”. Never heard of that before.

 

That graphic displays the 100% anti-squat line incorrectly (though it doesn't indicate it as such). The 100% anti-squat line doesn't p*** through the CG but through a point on the front axle plane at the height of the CG:

The 100% anti-dive line is the same thing reversed, i.e. through the front contact patch and the rear axle plane at CG height. This is true for live axles and outboard brakes. For inboard brakes, independent suspensions, or birdcaged axles, i.e. wherever you don't have a torque reaction in the suspension, you'd construct the line in each case through the wheel centre rather than the contact patch.

 

Be aware that all automotive springs will settle some after use. Leaf and coil. And I’ve never seen any data to give an exact amount. So just bolting everything together and putting all your weight on it won’t give you a final ride height. It takes more magic than that.