Suspension- to all of you who build cars/go-karts/buggys etc

[little_sparky]

Whats your budget man?

Ideally if I can get two shocks for or around $150 i will be stoked.

I was at first aiming for $100 but realized that this would get me something pretty ordinary if it was brand new.

For the length I am aiming for 280-320mm, I think that 305 or 320 is spot on though

Thanks, Little_sparky

 

[joshua43214]

First off, never be sorry for a long post, its what i asked the question for, to get answers! So thank you
Secondly, I really understand what you mean by this pyramid and it all makes complete sense i can't believe i didn't think of it already, but maybe the thing that lead me astray is that go-karts have shocks that are exactly vertical from the frame to the pivot point, they hardly ever make a V, do you know why this is?

Thanks, little_sparky

The go carts my boss raced all had the shocks attached to the axle near the wheels and came together at the back of the seat. They sort of reminded me of mini formula cars. Attaching them this way has the added benefit of reducing frame twist. As for why they are mounted vertically now, I can only hazard a few guesses.
Single point mounting is simpler, and avoids issues with the motor striking the shock.
Vertical mounting gives a shorter stroke on the shock, allowing for a lighter unit.
Unitized construction means less customization is required

I did the math and stuff I promised. It ran to 4 pages, so I just converted it to a pdf and uploaded the document rather than pasting in a massive wall of text.
It contains a diagram, very little math, some explanation of the math, all of the math is very simple arithmetic. The most complicated thing is a square root.
It also contains instructions on how to use the math to calculate spring rate.
It does not include math for calculating dynamic forces. It does include discussion on choosing a shock based on what is available by matching to spring rate, and a cheap and dirty method to measuring approximate dynamic forces.

Hope it is helpful.
It was a nice change from simulating complicated systems, and it keeps me fresh on simple elementary physics.

-Josh

 

[visenfile]

If you finally buy springs be careful of the junk on line. I have experience buying chinese springs which not only are not the rating advertised, but barely match the stock spring rate. Stay with know brands. Check the ratings after purchase. Take a look at "Speedway Motors" on line. I am not affiliated . Good luck.

 

[little_sparky]

This is a massive help! thank you so much guys this really points me in the right direction!

little_sparky

 

[savarin]

Hi Little_sparky,
nice to see another Zombie at work.
If your going to change the front forks then go for girders over springers.
Choppers Handbook has a lot of data regarding them.
Looking a real cool ride.

 

[little_sparky]

Hi Little_sparky,
nice to see another Zombie at work.
If your going to change the front forks then go for girders over springers.
Choppers Handbook has a lot of data regarding them.
Looking a real cool ride.

Really? why is that? is it a stiffer set-up? more stable?

the only problem with the set-up i have now is that the front tire sways from side to side like crazy when I turn, a lot of it is because my hinges are not the best and I am going to make a quick fix with my new lathe to see how they come up then but I kinda want a more complex design and something that makes people go Wow!

Also, regarding the head tube, I am going to cut the current one off and machine a new one, so what type of head tube would you suggest? a motorcycle head tube of some sort?

Thanks, little_sparky

 

[little_sparky]

Does anyone think that shocks with a 500lb rating will work? is that in the ball park or too heavy? or too light?

Thanks, Little_sparky

 

[Sharky]

Spring rates are pounds per inch. A about 500 # spring means 500 lbs to compress 1 inch. To get close, take your total weight, bike and rider with gear and divide by number of wheels. That should get you in the ballpark

 

[little_sparky]

Spring rates are pounds per inch. A about 500 # spring means 500 lbs to compress 1 inch. To get close, take your total weight, bike and rider with gear and divide by number of wheels. That should get you in the ballpark

So by that i'm guessing that 500lb is way too stiff.

I weigh 160 pounds (80kg)
bike weighs 100 pounds (50kg)
with gear add 20 pounds (10kg)

Equals around about 300lbs; divide by 3 wheels gives me 100lbs and will have two shocks, does that mean they should be a 50lb rating?

 

[savarin]

Really? why is that? is it a stiffer set-up? more stable?

the only problem with the set-up i have now is that the front tire sways from side to side like crazy when I turn, a lot of it is because my hinges are not the best and I am going to make a quick fix with my new lathe to see how they come up then but I kinda want a more complex design and something that makes people go Wow!

Also, regarding the head tube, I am going to cut the current one off and machine a new one, so what type of head tube would you suggest? a motorcycle head tube of some sort?

Thanks, little_sparky

They work really well, the trail only changes by a very small amount from full extension to full compression, (unlike all the other styles of fork) assuming all bearings are tight (no slop) they give a nice handling ride and lets be honest they look really cool. (the most important part:rofl
If your rebuilding the the head tube and the forks then why not make a new head tube to accept taper roller bearings. Trailer kits are very cheap and more than adequate for this type of vehicle.
Have a look at the girders section in chopper builders handbook for quite few designs but more to the point how to work out the correct geometry.