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

[little_sparky]

Okay i am really sorry in advance to everyone who is sick of me and will probably (hopefully not) waste their time reading this but i have been asking everywhere and researching but no forum is as knowledgeable and as active as this one, absolutely none, so I will cut to the chase and i promise that i will delete this thread as soon as i get an answer, once again i am really sorry.

Okay so if you have or haven't seen my other thread on beefing up a rear axle it kind of relates to that. i have a bike turned trike called a Gladiator chopper and i plan on putting a motor on it with rear suspension but i was wondering what size rating i should buy for the shocks (how many pounds)? The motor, + the clutch, + battery = 17.5kg
Frame = 7kg
I weight around 75kg
total = 100kg

So here is my tricycle;



and here is how the rear suspension set-up will look, from the corners of the verical 45 degree angled tubes, (coming from the red square tube(seat)) coming down on a close to 45' will be where the shocks will be mounted;



I will really really appreciate any help and if anyone doesn't appreciate this thread, let me know and it will be removed straight away I really really don't want to be a pest because i love this forum

thanks
little_sparky

 

[outsider347]

Sparkey
maybe you can find some info here

http://www.diygokarts.com/index.html

ed

 

[davidh]

this will be an interesting project to follow. don't delete it whatever you do. q ? electric / pedal assist ?
where did the swing arm design come from ? that may hold the answer to your immediate question.
i don't have an exact answer however modifying a similar unit would certainly be a place to start. .

 

[jpfabricator]

First off never feel you are a pest or a bother, if someone isint intrestd in what you post they dont have to read it.
If I were you I would be looking out for a mini dirt bike, 70cc size comes to mind, and salvage the rear shocks off of it. The last mini I owned had adjustable collars that eould compinsate for added weight.
The geometry also has a major roll in this setup as well, if you angle the coil over shocks out more the swing arm has more leverage, and vice versa.

Jake Parker

 

[JimSchroeder]

Interesting project. One point that sticks out is the front suspension. You may want to do a search and look for "Earls type suspension" as used on motorcycles. On the Earls they link the two arms going from the forks to the axle which off loads the shearing effect on the front axle. A broken axle at speed could be an issue.

Jim

 

[joshua43214]

In my second career (on my third now), I was a fancy Euro-car tech, and one of my bosses actually raced go-carts.
I am not a co-cart expert by any stretch of the imagination, but I can (hopefully) set you in the right direction on a few points.

First, shocks do not support weight, weight is supported by springs (saying this just to make sure it is clear, forgive me if you already understand this). A gas strut can both support weight, and provide damping, but they do not do well in hostile environments.
Judging by your drawing, the 100kg mass is what is called un-sprung weight. In other words, this mass bears directly on the wheels and is only indirectly supported by the suspension at the hinge point.
The mass you need to dampen with the shock, is your body mass plus part of the engine mass plus spring weight. Trying to figure out how much dampening you need on paper will be about impossible even if you have a good math background. I have a math degree, and I would have to do a bit of research on engineering and motion to solve the problem, and I would probably get the wrong solution.
Consider, you go over a pot hole, the shock needs to prevent the wheel from falling into the hole so that the trike stays level. So a strong shock is desirable. On the other had, when you go over a bump, you want the shock to move very freely so the motor assembly can pivot up or the wheels leave the ground. Expensive variable rate shocks do this automatically using internal valving.

Using two shocks that mount at the back corners of the engine frame, and come together at the top of the seat back will help a lot. You get a sort of pyramid formed by the shocks and the seat support. It is hard to describe why this is good in words, but I will give it a try;
Imagine two shocks spaced apart at the base, and coming together at the top, forming a triangle. Now push down on the top. As you approach the base, the vertical force you are applying is translated into horizontal force on the shock in a wedging action. The closer you bring the top to being level with the base, the lower the amount of force is required to compress the shock. When the top approaches level with the base, the vertical force approaches zero (in mathspeak). This is the same motion that occurs when you hit a bump. The opposite hold true when raising the top of the triangle formed by the shocks.
Ever notice that off road vehicles have shocks that point toward the center of the vehicle? This is why.
In a set up like this, the springs should be parallel to the vertical motion. Coil-over shocks can be used, but they defeat the design since they increase in force as they are compressed.

Sorry for the long post that makes it sound like the problem is impossible. Don't give up hope, there is a way.
I have to run down to the lab and do some stuff. I made up a crude diagram, and will work up some math using simple physics and calculus, and reduce the solutions to basic algebra. I will also make some suggestions on how to solve the problem with out math. I might not get back to it til late tonight or tomorrow, my young lady is coming over this evening

-Josh

 

[little_sparky]

this will be an interesting project to follow. don't delete it whatever you do. q ? electric / pedal assist ?
where did the swing arm design come from ? that may hold the answer to your immediate question.
i don't have an exact answer however modifying a similar unit would certainly be a place to start. .

Thank you very much, it will be powered by a 6.5hp honda GX200 (i did order a 5.5, but they sent the wrong thing), but that is what i thought, find a unit that is somewhat similar and copy from that.

First off never feel you are a pest or a bother, if someone isint intrestd in what you post they dont have to read it.
If I were you I would be looking out for a mini dirt bike, 70cc size comes to mind, and salvage the rear shocks off of it. The last mini I owned had adjustable collars that eould compinsate for added weight.
The geometry also has a major roll in this setup as well, if you angle the coil over shocks out more the swing arm has more leverage, and vice versa.

Jake Parker

Thank you for your support Jake! I own a 100cc bike and the suspension on that feels very soft and easily compressable so I would want something close to that if possible but i have no way on finding out its rating, the bike is a honda CRF100F. Also do you know a common rule for the angle of the coil?

Interesting project. One point that sticks out is the front suspension. You may want to do a search and look for "Earls type suspension" as used on motorcycles. On the Earls they link the two arms going from the forks to the axle which off loads the shearing effect on the front axle. A broken axle at speed could be an issue.

Jim

I agree with you here Jim, I began building this bike 4 years ago when i was just young and at that stage i thought that it was going to be pedal powered but as I got older my mind changed and the suspension (front especially) has worried me a considerable amount, i have really looked into making my own springer fork suspension as seen on 'The Chopper Builders Handbook' (hope i am aloud to say that), it looks something like this;

Judging by your drawing, the 100kg mass is what is called un-sprung weight. In other words, this mass bears directly on the wheels and is only indirectly supported by the suspension at the hinge point.
The mass you need to dampen with the shock, is your body mass plus part of the engine mass plus spring weight.

Using two shocks that mount at the back corners of the engine frame, and come together at the top of the seat back will help a lot. You get a sort of pyramid formed by the shocks and the seat support. It is hard to describe why this is good in words, but I will give it a try;
Imagine two shocks spaced apart at the base, and coming together at the top, forming a triangle. Now push down on the top. As you approach the base, the vertical force you are applying is translated into horizontal force on the shock in a wedging action. The closer you bring the top to being level with the base, the lower the amount of force is required to compress the shock. When the top approaches level with the base, the vertical force approaches zero (in mathspeak). This is the same motion that occurs when you hit a bump. The opposite hold true when raising the top of the triangle formed by the shocks.
Ever notice that off road vehicles have shocks that point toward the center of the vehicle? This is why.
In a set up like this, the springs should be parallel to the vertical motion. Coil-over shocks can be used, but they defeat the design since they increase in force as they are compressed.

Sorry for the long post that makes it sound like the problem is impossible. Don't give up hope, there is a way.
I have to run down to the lab and do some stuff. I made up a crude diagram, and will work up some math using simple physics and calculus, and reduce the solutions to basic algebra. I will also make some suggestions on how to solve the problem with out math. I might not get back to it til late tonight or tomorrow, my young lady is coming over this evening

-Josh

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

 

[Terrywerm]

Little_Sparky, as the others have said, you should not be apologizing! And I certainly hope that you do not come back and delete this thread, as that would be a shame for certain! Please keep in mind that even though this site is called the 'Hobby-Machinist' it does not mean that we cannot enjoy non-machining topics and projects. I know for fact that we all indulge in other projects from time to time, and as a result may have some questions once in a while. What better place is there to come and ask questions??

As for your build, the idea of using coil-over shocks from a mini-bike or motorcycle seems to be the best way to go in my opinion. It appears that your engine and power train will be unsprung weight, so they will not be so much of a factor as your own weight. Mini-bike or mini-cycle coil-overs shocks may be a bit too small, but I would think that a set of them from an older endure style bike might be just the ticket. They also have the notches in them so that you can adjust them for weight and ride.

I did take the liberty of moving this thread to the bicycles and motorcycles forum though.

 

[chuckorlando]

You can get some cheap shocks off a kids mini buggy for cheap. What I would do if I was you.... On the upper and lower mounts, make 4 or 5 mounting points on each. Ideally your shock will be at 45* to the arm at full bump(compressed)For the best rebound. but you can kinda cheat. Keep in mind the more straight up and down th shock the stiffer it will feel and the more layed down it is the softer it feels. So with multiple choices you can play with the shock angle to find what you like.

I run walker evans clicker shocks and springs. They come off a sled and go for about 500 a set before springs. I got them on ebay for 100. Clickers are where it's at because you have the ring to help adjust the spring and the clicker adjust the shocks valving.

Shock valving is a science in and of it's self

 

[chuckorlando]

Whats your budget man?