Long Travel Off Road Go-Kart Suspension

[tyler71385]

I am starting a project similar the one these guys started. thread800-129475 Just to get my wheels rolling in this field. The only differance is where they went with a on-road suspension, I want to do the same thing with an off road suspension. Push-rod/rocker style. Power will come from a 600cc Street bike engine, and tires will be of ATV sizes. It will probably be a single seat design too. My problem is that the resources I've come across primarily relate to on-road suspensions and I need to know if the same principles will work for off-road as well.

Thanks,
Tyler

 

[BioMax]

I have personally built a vehicle for a customer like you are talking about and I can tell you that there were a few geometry problems related to the customer wanting the cart "lifted." If you really want to cart to work in the dirt it needs to be "neutral" so to speak. That is that the suspension should "sit" at about half travel. What type of front and rear suspension are you going to attempt?

 

[Fabrico]

You might check out the Honda Odyssey or Pilot buggies. Otherwise you can look up the real thing and perhaps scale it down.

 

[tyler71385]

BioMax: I think I will use a dual a-arm pushrod and rocker front suspension. For the rear I'm not as certain other than the fact that it must be indpendant. I thought about using a trailing arm with a pushrod and rocker setup, but then thought of using a dual a-arm in the back also. Any help you could give on design or any ideas y'all have would be appreciated.

Thanks guys,
Tyler

 

[BioMax]

I've seen a few different variations of rear suspension that were tollerable at best. Any true independent rear suspension would require a diff of some sort fixed to the frame with CV's (or some other system, maybe Rhino?) in order to drive each wheel. I think that an, IRS Volkswagon style rear trailing arm would be easiest to tune (and build) and if done right can offer a small amount of camber gain as well as a roll center that is above ground.
The front suspension can be adressed in several ways. Are you running front brakes? Are you starting with a quad spindle or building your own? How much travel are you dreaming of? Keeping the front and rear suspension close to the same travel is a good idea. I wouldn't run any more than 4" more travel in the rear. That means that you may need to figure out the rear end first in order to see how much travel you end up with. Then set up the front with a few inches less. Too many cars have front and rear suspensions that aren't even close (in inches of travel.) If you plan to really tune this thing to work, heed my warning.
As for the rocker link that just drives the shock, as long as the geometry is right on the links there's not much concern. You could set up the links to have a rising rate so that the spring and valve rate would get stiffer as the suspension goes into "bump."
I'm still not exactly sure what info you are looking for. I hope I'm heading in the right direction.

 

[patprimmer]

IRS travel will be limited by axle or half shaft length and maximum CV joint angle.

A narrow track vehicle will be quite limited. I think a wide track Buggy can get about 23" travel with inboard CVs, and a bit more with outboard CVs.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BioMax]

I agree. He will only be able to get about 8 to 10" if he is lucky. BUT on the other side of the coin, a builder friend of mine took a trophy truck out testing with sensors on all of the wheels and found that at race speeds the truck didn't use any more than 16" of the 30" of available wheel travel. I'm sure that a properly designed (and tuned) "short travel" cart could work well.

 

[Fabrico]

BUT on the other side of the coin, a builder friend of mine took a trophy truck out testing with sensors on all of the wheels and found that at race speeds the truck didn't use any more than 16" of the 30" of available wheel travel.

He was likely referring to up travel as that's approximately where they sit. Otherwise he must have been going awfully slow, wasn't where a TT is designed to go, or having some other problems. The TT's I work around average 24-27" front and 30-36" rear and use every bit of the travel. And BTW, it's possible to get significantly more than 23" of highpower, reliable travel from an IRS system.

 

[patprimmer]

I played with them around 1980, so they have obviously progressed somewhat from there.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[BioMax]

I'm not saying that Trophy Trucks don't use all of the travel, but more that they don't always use all of the travel. The truck I'm talking about was originally designed with 28" in the front and 32" on the rear and was limited to 25" and 27" and got faster doing it. There will always be something to hit in the desert to use any amount of available travel. I'm just trying to point out that you don't need 30" on all four corners for a truck (or golf cart) to work well.

 

[tyler71385]

Hey guys,

The initial question is about finding design resouces that outline design principles of off-road suspension. And design ideas and problem(opportunities) for building a relatively long-travel buggy style go-kart. I'm not trying to cut anybody off or be rude. I just need some help finding some resouces.

Tyler

 

[BioMax]

I have not found any information in writing, or otherwise, about long travel suspension. That is why I was asking as many specifics about your front suspension ideas so that I could offer some sugestions as to how I would aproach the project.
The cart that I did was a simple rear swing-arm and a-arm front, but as I said, it was too tall to ever really work well. The rear did work decent.
Building a rear independent will certainly cost more, but would be a nicer finished product.
Let me know what you are looking for and I will try to point you in a general direction.

 

[tyler71385]

This buggy is a lot like what I want to build. Any formulas or books you can point me to would be greatly appreciated.

Thanks for all your input guys.
Tyler

General overview:

50-60 inches track width
72 inch wheel base
12 inch ground clearance
12 inch bump travel
6 inch droop
16-18 inches total travel wishful thinking all around
at least 600cc sport bike engine possibly 1000cc-1300cc

 

[GregLocock]

Basically you are aiming at very neutral curves, that is, you need your toe and camber gain gradients to be small, and your geometry to be very 'parallel'. Small changes in your hardpoints away from the 'ideal' will have a big impact at the extremes of travel.

So, you need to make the arms long, and draw it out at teh extremes of travel.

Sorry, i haven't got any particular values in mind.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

Those numbers just don't add up. A car that size cannot put 16-18" of wheel travel to use. A 60" track with 72" wheelbase could not effectivly use more than perhaps 10-12 inches of wheel travel at most. If you plan to go fast you will need 30-50% droop. But that kind of speed and droop requires that everything else be right, otherwise it will just turn into uncontrolled body roll. This means race type shocks (perhaps even by-pass type), rear sway bar, and exceptional springing. Static ground clearance of around 6" would be more realistic with 1.5-2" at full bump.

Even at 12" travel you will need to, as mentioned above, keep the control arms as long as possible. This cannot be done effectively without a very narrow differential, custom hub units front and rear, moving the CV's out, and a lot of attention to steering.

It can be done but to be effective and/or go fast at all you will literally have to make a minature of the real thing.

You might check out

http://www.race-dezert.com

or one of the other desert racing sites.

 

[BioMax]

I have to agree with the both of them. I visited a friend of mine today that builds Rhinos and they are running about 12" on all four corners. They have one Rhino with 14" in the rear with the trailing arm type suspension, but are limited by the CV "plunge." The rear diff from one of those might work, but I think I would build my own carrier in order to get the axles as long as possible.

I don't see any probs with a little more camber gain (in bump) then usual as well as a little positive camber in droop. Of course that is my opinion, but I have had good luck with it in the dirt. It also would help, a little, with CV bind.

There is a place in Rockfor, IL that can help with CV's and axles, but you're on your own with the center diff.