12" of Travel from a pushrod suspension

I am working on designing a pushrod suspension that will have 12" of travel for the ASME moon buggy competition. Is this even possible in a small space? I am having a little trouble with it. Help please!

12" of wheel travel? No problem. Check out any of the dirt bike rear suspensions for ideas. Be prepared for some eye popping spring rates, though.

But those aren't pushrod style are they?

It's a simple ratio problem. You know you need 12" of suspension travel and you have X amount of packaging space. Maximize your available amount of spring size (compression distance) within that space. Now you know you have 12" of suspension travel and x" of travel at the spring, make a linkage to accomodate that ratio.

Bryce

Why does it need to be pushrod?

If it absoposilutely has to be a pushrod setup, look for an old Suzuki 'Full Floater' from the early '80's for inspiration. This Trek mountain bike setup is similar, but the Suzuki setup didn't have the triangular swingarm.

Lots of dirt bikes have 'rising rate' linkage designed to do exactly what you want and it doesn't necessarily have to be mounted directly to the swingarm. Basically, you want your pushrod close to the inside pivot, then have it go to your bellcrank then to the shock. With the right ratios, you can get 12" of wheel travel with 2" of shock travel. The thing is, you will have such a high leverage ratio it's going to need stupid stiff springs.

So basically I have to run huge spring rates to get 12" of travel. I don't know how well that will work since we want to more float over the obstacles and not get air born. Any other suggestions?

You're not making any sense. A "huge" spring rate isn't huge when there's a ratio reducing it's effectiveness at the hub. You obviously have some design constraints you're not mentioning OR you don't understand how a typical pushrod system works. The good news is, no matter how bad you screw up, you can't get a moon buggy air born because there's no air on the moon.

Bryce

The shocks we are using have 2.75" of travel. So to get a 12" of travel we need a ratio of 4.634:1 at the bellcrank. The problem I am have is figuring out the spring rate desired. The spring rate I am getting using the numbers we have is between 555-672 in which to me seems really high.

How are you computing the spring rate? Do you have a desired wheel rate or natural frequency?

The buggy weights about between 90 and 100lbs and we have two riders of say 150 lbs. That puts 100lbs on each wheel when the riders on the buggy. Our team lead wants the suspension to be fully compressed when there is a 350lbs of force on the wheel.

OK-

350lb/12in = about 30lb/in wheel rate.

12/2.75 = 4.364 MR, MR^2 = about 19. 19*30 = 570.

So , you're right on the money. Luckily, those are very normal and commonly available rates for automotive coilovers, so if you have room for 2.5" shocks you can use those. since it's a 100lb buggy with 2.75" shock travel, I doubt you will have room. That's a tight constraint. I don't know a thing about this competition - are you designing for moon gravity? I don't see it holding up on Earth, that's for sure. And that MR won't make shocks happy on this planet either.

Yeah that is what I am having trouble figuring out. I can't help but think the only way round this is more shock travel. And these are designed to go on a coarse at the NASA Space Musesum in Huntsville.

OK, I see this is human-powered, which helps since speeds will be low. Still, more shock travel would be very nice.

That is what I am thinking and I really not sure 12 inches of travel is nessecary esp. considering we have 13 inches of ground clearance.

What if you set up the suspension for mad droop travel and limited up travel? This guy has a system that allows the springs to drop out but still be contained. I'm having a hard time explaining it so just click the link. Also, that guy has some serious fab skills. I recommend reading the whole thread, or at least just looking at the pictures of his work.

If you have 13" of ground clearance with the 100 lb. vehicle unloaded, you are going to lose at least 1/3 of that (possibly more) when you park the passengers on board. If you cut down the wheel travel from 12", then I could see this being a problem. You'd lose a lot of droop and that can be very important on an off road vehicle. Check out some of the rock crawler sites and see how they use that monster travel to get an idea of what I'm talking about.

I wouldn't be overly concerned about the shocks other than that the compression and rebound damping will need to be pretty stiff as well. If this were a high speed event (desert racing, etc) the shocks could overheat pretty quickly, but I don't think you are going to have that problem.

Designing for moon gravity should only entail dividing the Moon weight by the earth weight to come up with the overall spring rate (wheel rate) needed and then solve as usual. Of course, that spring rate wouldn't work well at all here on Earth.

BTW: if the suspension is fully compressed with 350 lbs on each wheel that's a total of 1400 pounds (350 lbs x 4) but the total weight of the vehicle is 400 pounds (2 passengers x 150 pounds + 100 pound vehicle) that seems awfully stiff for a vehicle that's supposed to go slowly over large obstacles. I would think a more reasonable wheel rate would be 200 pounds (175 x 4 = 800 pounds) or thereabouts.

Yes, obviously some retuning of the suspension would be required on the moon :) But the short shock travel and stiff rates are not a big deal. It's all about the motion ratios. Don't get spring rate and wheel rate confused, I think that's what you're doing. The short shock stroke mostly becomes a problem when you're trying to damp small movements, I think.

You don't need to go with 2.5" springs, you can get 500-600 lb rates in 1.9" springs (I have 500s on the front of my Locost) and I think bike shocks tend to be in that area as well.

thatsnowinnebago wrote: What if you set up the suspension for mad droop travel and limited up travel? This guy has a system that allows the springs to drop out but still be contained. I'm having a hard time explaining it so just click the link. Also, that guy has some serious fab skills. I recommend reading the whole thread, or at least just looking at the pictures of his work.

wow, that's awesome. that build is starting to give me a chubby. thanks for the link!

Keith wrote: Don't get spring rate and wheel rate confused, I think that's what you're doing.

Ditto.