Tall tires with low frames... what suspension?

I started a thread the other day that started out as a spring interference thread and turned into a mess of info and ended with.... "what the hell do I do now??!"
http://www.jalopyjournal.com/forum/showthread.php?t=105111

So, I thought I would just start over by getting right to the meat and potatos...

What the hell do you run for rear suspension if you have tall tires and your frame has a low ride height (4 to 6 inches)? Let's see what you've done!

I'm looking at running Coker/Firestones (750-16 = 31.48" diameter inflated) and dropping the frame down low, but the suspension geometry is all screwed up! So I need new ideas!

 

Run the frame under the axle?

 

You did the most important part by stopping to rethink things. If you just lower a car an inch or two, the original suspension usually allows it. Beyond that, you are building from scratch in a manner of speaking.

Step one: set the car at ride height.
Step two: set the wheels/axles where they go.
Step three: build the right suspension geometry in.

In your case, a raised perch and rear rails are probably in order. (rear Z)
That's pretty serious frame work. Either study really well before hand or find someone to take you under their wing and guide you. This is really an area where a skilled welder will be needed. If you can't buy one off with beer, get out the cash...

The ch***is outline you have in your last thread is right on.

 

I built a 'Torque-Arm' style of rear suspension for mine. My Rod sit's 3-4" off the ground.

 

That, ladys and gentlemen, is how you optimize floor space in a car that's only 3-inches off the ground!! BURLY TOO!!! WOW!! Nice work! Have you tried any major burn-out yet?? How did it feel? More importantly, how does it ride? One more question, do you have a shot from the rear looking forward? Thanks AHR

Thanks everyone for not freaking out on me for restructuring the question and reposting. The same thing was being covered (sort of) in the other thread, but I thought it was losing focus on the topic at hand, so I started again...

Keep the input flowing!!!

 

Well, I'm not finished with the car yet, so I cannot comment.
But, NASCAR and BUSCH cars all use this same basic design, and I'm sure you have seen the burnouts on the Victory laps, and.....well....we know they handle great at 200+MPH !
Here a couple of snap shots from the rear.

 

AHotRod, does that work kind of like a swing arm? Meaning, is the axle solid to the two arms?

 

YES, that is correct. The trailing arms attach solid to the rear axle. You use a locating bar from the pumpkin to the chasis to control side-to-side motion, and spherical rod ended adjustable AFCO coil orders. The triangluation of the trailing arms give great flexability to the axle motion.

 

It's as close to a swing arm as your going to get in a car. Its also called a trailing arm suspension, early '60s Cheby trucks used the same basic setup.
If you run the springs behind the axle it doesn't take as much spring to control the axle, that's commonly called a daytona mount.

Its a good setup, handles well, and can be made to ride well also. AHOTROD. thats a sweet setup. Does your front U joint land even with the front trailing arm mounts? That's the ideal setup.

 

Why, YES it does ****** !

 

Somehow I knew you had your **** together.

 

Ah-ha! I knew I wasn't crazy, thinking Chevy trucks had something like that...

 

Question for you guys...

I was talking with a guy at P&J's this morning and they say I don't need a panhard bar with their set-up? That made sense to be because the P&J ladders are basically triangulated. Wouldn't running a panhard create bind in the ladder bar's movement???

Also... why couldn't a triangulated 4-bar or standard 4-bar be used with a transverse spring?

 

AHotRod,
One more thing... what is the verticle distance from the centerline of your axle to pivot point centerline?

 

You still need a panhard with P&J style ladder bars. The bushings will let the rear move a LOT, especially under load. Given said flex, a panhard won't bind things up.

 

I'm not certain of what your asking?

 

triangulated four link

 

yea, but your using bags, he is gonna use a partially dearched transverse leaf, mounted infront of the axle, which will likely stick the center of the spring right about where the triangulated links need to be.

?'s to scooter-

Why are you so set on mounting the spring front of the axle? Why not behind? that would free up a bunch of valuable suspension real estate.

Are you set on the transverse leaf? My modified will be sitting at about 6" @ the rear. I'm using a quarter elliptical setup. Doesn't take up anymore room than a standard 4 link.

I think the trailing arm setup will be your optimal setup for what you seem to want- low ride height, minimal interior space loss, and of course, a quality suspension design. That is unless you are willing to consider other spring choices.


BTW - Jakdup - your truck is ****ing sweet

 

should/could he just use something like the early 70's gmc pickup trailing arms?
i know where i can get him some nice ones...

 

he could, but they will likely be way too long. I'm not sure how long the stock chevy truck arms are, but I'd guess in the area of 4 feet. I, personally, would do like AHotRod did, and build my own. That way they are exactly how long you want them. They can be made fairly easy using 2x3 tube. AHotRod's appear to have adjustable mounts at the axle, though its hard to tell from the pics, may just be my eyes playing tricks.

 

Mostly for looks. I'm trying to hide as much of the frame as possible and I like the look of a quick change all up in your face. All I want to see are the things that make it work.... I.E. Axle, suspension components, the lever shocks I'll be running and a nice spun aluminum tank... kind of hard to explain the image I have in my head, I guess

Once again, partially looks, but I would like there to still be that Model A "element" in the suspension when you look at it.

I think these are looking like the best option right now. I'm thinking that either I need to make them from scratch or modify the hell out of some speedway ladder bars (P&J are too expensive to just cut up and alter)... and yes, they're going to have to be short. Probably between 30 and 36 inches long or I'm going to have some problems with the floor, seat mounts, all kinds of stuff.

Hillbillyhell...
I need a panhard huh? DAMN! One more thing to try and figure out where to mount... wait though... there is a nice tight little mount available that mounts to the front flange of the quick change, right behind the yoke. I can make that work...

So, someone please give me the break down on why a 4-bar won't work. I was thinking because of how much front-rear axle movement there is when it swings up. Is that the reason? Does it just load a transverse spring weird?

WOW! You guys have provided some great info and it's helping a ton!! Keep it coming! I feel a lot closer then I was at the beginning of the week!

 

I don't mean to highjack, but this is such an excellent discussion, I can't resist: Isn't the '64-'72 GM A-body rear suspension similar to the NASCAR/'60s truck suspension as well? I have a '68 LeMans 10-bolt that appears to have mounting points for trailing arms on the bottom of the pumpkin. Could those be used with homebuilt trailing arms and a Model A spring for something like this?

 

You can use a 4link if you use equal length upper and lower links that are of reasonable length, so the arc that the axle moves in through suspension travel is pretty wide. The problem with the 4 link in your case is that the spring will likely be in the way of the upper link.

as can be seen is this pic(stolen from Harrison)


the outter portion of the spring is right about where you need to run the upper link.

Although it may be doable... It is going to be one of those things where as soon as you mock up the body set the axle and spring in place you will know right abaw wether or not it'll work.

 

I'm considering the 4-bar with a panhard now actually (I'm indecisive )...

4-bars + 1 panhard + 1 transverse spring + 2 shocks = "it works great"

This set-up is VERY similar to what I have going on... tall tires, low frame, transverse spring


Working out the cleanance issues might be the only hurdle... there are creative ways around that though...

Thanks for the info JW!

 

I think these are looking like the best option right now. I'm thinking that either I need to make them from scratch or modify the hell out of some speedway ladder bars (P&J are too expensive to just cut up and alter)... and yes, they're going to have to be short. Probably between 30 and 36 inches long or I'm going to have some problems with the floor, seat mounts, all kinds of stuff

I'm not an engineer, but I've read several places that those arms are so long because if they are any shorter you get wheel hop and bouncing during braking. Maybe someone could correct me, I would just hate for you to have to design and build anything twice. Learned this the hard way with some radius rods for a tube axle. I hope this helps.

 

You might also consider using split rear wishbones to build your trailing arms from. They're pretty compact, and look right. You do want to keep them as long as possible for the reasons you mentioned, in addition, the longer they are the less front to rear axle movement you'll get during compression and rebound.

 

For what it's worth I went back and looked at a book I had on suspension design. It said the minimum lenght for trailing arm suspension was 50". This is not for 4 or 3 link, but any time you only use 2 arms like the NASCAR stuff. The guy before me was right on as well. The longer the arms the less forward and backward movement of the pumpkin you'll have. Again, hope this helps you out.

 

Fifty inches? That's like half the wheelbase on some hot rods!

 

Agreed, the longer the trailing arm the less forward to rear movent there will be front to back and would reduce stresses in a spring stack for sure.

Here's a question I should be asking.... How much suspension travel should I really be planning for? If the car is only 4 to 6 inches off the ground then obviously I need a good safety factor figure in there. I have to make an ***umption that no matter what I use that rear axle needs to articulate and allow "X" amount of travel for each wheel....

Okay, maybe this is too much for a Friday... or are we supposed to be sharper on Fridays??

Strongly reconsidering 4-bars as long as there isn't too much suspension travel and the geometry is correct.

Maybe someone can shed some light on the bushings used with the bones. What's used and how much do they flex? I know a lot of engineering plastics have great lube properties and provide good ware resistance, but don't offer a lot of flexibility for things like panhard bars and things.

This thread is really becoming a great "tech" thread and I think it will be usefull...