Designing a 4 trailing arm go kart.

['69 baja]

I just got back from the steel yard and a home depot with 2 1/2" tee fittings and 4 huge C brackets 3/8" thick with a 2" gap for only 3$ each.That are going to serve as the trailing arm brackets and as the spindle brackets..Along with some 3/4" round schedule 80 piping that I will use as bushings for the trailing arms to pivot on in the brackets.

The bump steer may be a problem depending on the style of tie rods i make.I plan to use 1/2" rod ends(Heims) with the standard go kart pit man arm setup and some 1/2" threaded rod.I'll look into what the fl250 odyssey's use and what the setup is on my baja bug because neither of them have much bump steer from my testing.And worst case scenario I'll have 2ft of unneeded tubing that I'll find some use for.

Any design flaws for the rear or does it seem strong enough?Thanks for the help.

 

[Doc Sprocket]

Rear's pretty straightforward. You'll probably want another crossmember to tie the two sides together, looks like you've only planned for one. You want that puppy stiff.
You're probably already aware of it, but- Your diagram has the engine running backward. Your axle sprocket will have to be located to the left side of the kart. If you're planning a jackshaft, ignore this. Cheers!

 

['69 baja]

Yea i actually didn't know which side the sprocket on the motor comes out on.I have little experience with motorcycle or small engines but put a 1600cc flat four in front of me and i can do a complete rebuild.

Speaking of engines what would be a good sized dirt bike or quad engine for the thing.The frame only weighs about 35lbs with the rear trailing arms mounted so I estimate less than 150 with engine so about 375lbs total weight with me in it.I was thinking a 200cc 4 stroke with 4-6 gears or a 125cc 2 stroke with gears as well.Would those be good choices or would something bigger be more preferable?I'm planning 16" to 18" knobby tires with a 1" live axle.

Edit:The front end brackets have only a 2 1/2" gap so I'll be forced to do a design like this

Instead of an H arm it will just be 2 pieces of tubing welded together lengthwise.Will that still be strong enough?

 

['69 baja]

Just got done finishing the front trailing arms and welding the rear cross member.Every weld point has a bolt backing it up in case the weld breaks(unnecessary but I'm still iffy on my welds,only on my second pound of welding wire.)

I have the shock mounts and steering figured out but I'm out of steel(31ft so far).I'm up to $80 for everything.

Any weak points or areas that need support or gussets?Anyone know a better way for the tee fittings for spindles,as the fittings and the trailing arm bushings seem like a weak point on the kart.Thanks for any input.

Also I'm not too sure what my crappy camera phone is picking up on the C bracket for the spindle but there's nothing on it.

 

['69 baja]

The measurements as of now are 25 3/4" chassis width,84" from the front bumper to the end of the rear trailing arm,and from spindle to spindle is 48 1/2".A bit bigger than I planned but still pretty light at around 45lbs.

It will have a standard bell-crank steering with heim joints on 1/2" or 5/8" threaded rod.And i'm leaning towards a 125cc 2 stroke with a 6 speed manual.#42 chain and a 4 lug rear with a single disc brake.For the front a 3/4" grade 8 bolt for either a bearing hub with 4 lug as well or just a center ball bearing wheel riding on the bolt.I'm thinking 16" to 20" tires.

The final build with likely be different than that(a good craigslist deal may change it lol)but that is the plan so far unless you guys have some better ideas.

 

[modelengineer]

Make it 3 dimensional if you want it to last. The way it is it will flex a lot and eventually crack at the welds IMO.

Here are some pics of the swingarm I made.

The bracing is mostly 30x30x2. The main rails are 50x20x2.4 IIRC. While this is overkill in your application, some 1" tube to make the structure 3 dimensional, and triangulated, will make it MUCH stronger.

 

['69 baja]

Well i got an engine and it came with a dirt bike lol.Its a '84 Honda cr125 with 5 gears.The bike itself is rough but the engine was rebuilt not to long ago and looks fairly clean.

Since he was a friend of mine I got the whole bike for 60 bucks including a Comp coil over shock with reservoir.And i plan to use the hydraulic disc brake as well.

I'm not to sure about how much power the buggy will have with this engine.I have seen some specs for the engine but cant find any exact HP specifications.My research puts it at around 24hp but that seems a bit too high for a 125cc.

I'll try to get some pictures up tomorrow.

 

[Doc Sprocket]

Actually, I found a spec page for the 1983 cr 125r. I'm not sure of the engine difference, but it says it has a 6-speed. Anyway, rated hp is 31- here's the page.
http://www.bikez.com/motorcycles/honda_cr_125_r_1983.php

 

['69 baja]

Thanks that would be nice if it had 31 hp and I just checked it today it has 6 gears.If the engine works out and does run it looks like this thing is going to be pretty fast.I'm going to have to triangulate the rear end for sure and for the front it's built about as strong as i can get it with the current design. If it doesn't hold up I will have to redesign the front and add some single or dual A arms.

I just ordered some shocks for it today,I don't know if there any good but there cheap enough that I wont care if they don't hold up.If they aren't adequate for the rear I'll use the high end dirt bike shock and run it to the center support of the swingarms.

https://www.surpluscenter.com/item.asp?item=28-1721&catname=wheels

 

[Doc Sprocket]

I was surprised at the HP rating, too. Have a friend that has one just sitting around, thought I'd do some homework on it. When I saw the numbers, I just about spit. I really didn't think it's be more that about 20 myself. Makes me really think about my next project. Problem is, I already have the engine (20-22hp) and it's hard to justify another cost...

 

['69 baja]

Yea my friend actually has two just sitting around but he intends to start on his own custom buggy this summer.He's got a plan for a 2 seater dual A arm mini rail.But I'll have to see how well my buggy is gonna go before we know if it'll be powerful enough for a 2 seater.

 

['69 baja]

Made some good progress over the weekend.The shocks arrived(looked incredible for 12 bucks). So I started on the shock towers and their supports.

I'm planning a L shaped support for the front shock towers mount because it is a lot of stress on a single bushing and grade 8 3/8 bolt.I finished up all the welds since i took the picture and cleaned them.Believe me the welds look better in person,a cell phone just doesn't take good enough pictures.

The chassis sits 7 inches off the ground and i can compress the suspension all the way to the ground and it still has more travel.I estimate it to be around 8 inches in the front and 8 to 10 inches in the rear.Which is about double what i planned and thought it would be.It's got more travel then my baja bug and its wider,I don't believe it's a go kart anymore it's a mini rail lol.A budget one at that I only have $180 in it including the engine.

 

[T-man]

this is interesting. with this set-up you'll most likely want to have your tie rods going under your floor. Also with this the caster angle will be constantly changing...I like stuff like this. but what I'm afraid of seeing is in the event of flying down a curb is the shocks collapsing and the tie rods getting pinched inbetween your arms and the frame...unless you have a differant plan.

 

['69 baja]

Yes i intend to use a bell-crank steering with the pitman arm below the frame.Are you talking about where the shocks mount up to the shock towers?If so i am going to weld in a piece of angle iron on the shock tower to support the other side of the shock bolt.

If your talking about the front trailing arms themselves, I don't know how to make them any stronger than the way it is now,I'm open to any ideas to make it stronger.It's been tested to 500lbs(my dad and i stood on it)and it took it no problem with no flex whatsoever.

I have these additions planned for the front.I'm hoping they will be enough.

 

[T-man]

that angle iron support looks like it will do the job ... but if I might make a sugestion. Remove the bolt that is welded, and drill a hole strait through the angle and the tower, so you can slide a bolt through the angle-shock-tower and put a nut on the other side. this would be better in case you want to change the shock.

 

['69 baja]

The shock tower bolt is not welded it goes through the shock tower itself and through a pipe that is welded to the tower as a type of bushing or spacer.The bolt will go through the tower,the spacer,the shock and then out of the angle iron with a nylock nut to tighten it..The angle iron is just a L support to have the shock mounted dual shear.

 

[T-man]

oh ok.. It looked like the bolt was welded to the tower ( what happend to the picture ?? )

 

[That One Guy]

I agree with toystory, but it looks really good. Suspension should be bombproof.

 

[c02cutter]

I have to say that the design is an idea, but will not work well. Every time the suspension moves you will have opposing interaction on the tie rods which will oppose each other making stearing a bit of a pain to say the least. Bumps will cause the wheels to turn against each other everytime. The tie rods will be connected at the stearing point at a stationary point, the wheels will be able to float on the suspension pivot. the tie rods willl change lenght and every comperssion of the suspension causing the bumpstear but really over bumpstear. They will work against eachother every time the suspension is compressed. If the suspension compersses equally the tires will toe in or out depending on if you are front or back with the pivot point for the wheels.Which is true also with a standard A arm suspension, but is much more controlable. This is the same in a standard VW suspension is a bit more complicated than just trailing front suspension, there are more pivot points in the suspension to overcome the bumpstear. The old bugs are prone to multiple front end problems due the configuration they use in a performance application. A go kart is a performance application, exspecially in your configuration. The front is going to be really unstable for riding, and a basic pain to drive. It will work, but you will not be happy at all with the results.

My own 2 cents, just telling you how it will be.

 

['69 baja]

I do agree that it will be a pain to steer.It is more of a proof of concept right now.If the bump steer is dangerously bad I can just cut the front off and go with a dual A arm design with minimal cost and minimal changes to the steering system.

Any ideas to help counteract the bumpsteer as my original plan was only meant to get around 3-4 inches of travel not 7 to 8 inches?I'm thinking the height of the pitman arm and the angle of the tie rods will determine the how much bump steer there is.

I will add a steering dampener to help keep it steady on rough terrain.