Designing a very simple gravity powered cart

[saturnv7890]

mine had full length threaded rod for axle shafts. you can get that stuff pretty cheap from the hardware store. and that will also reinforce the 2x4 so a 4x4 isnt nessesary

I like this idea!

So would I attach the rod with u bolts?


and for the wheels, I'll just go to home depot and buy some sort of cheap rubber wheels, see how they hold up, and then maybe buy nice go kart wheels for my later gravity racer!

 

[porsche930dude]

yup small u bolts should work good. just use big washers so they dont pull through.
these wheels are very common and iv got them for as low as 6$ a piece. They dont hold up for gas powered carts but for yours they will work great

 

[saturnv7890]

Where did you find the wheels for $6 a piece?? I know they're on HF for 10, and the groove thread version (which I suggested as an idea) is 8 or 9...

Ok i've redesigned this accord to the new ideas.

Note the new brakes... actually thats the only change I made. I have two brakes in case I need more stopping power, or one fails. How should I attach them to the frame? Would some thick lag bolts work?

 

[freakboy]

thick lag bolts and washers should do.

 

[porsche930dude]

i got them from tsc tractor supply company http://www.tractorsupply.com/webapp...10551_10001?cm_mmc=SEM-_-Google-_-brand-_-tsc
they often have a sale on them.
As for the design. my cart had the driver seat set forward a bit and the back had a small platform that we usto use for a passenger or pusher. it worked great. also would be better for weight distribution

 

[Benjo]

you could use a long bit of 4x2 with skateboard wheels and axle. Just lie on top of it and lean into the corners to turn.

 

[porsche930dude]

also for the back axle you should have a couple 45degree boards to triangulate it. either that or just screw a piece of plywood over it which will double as the seat

 

[saturnv7890]

Ok well I'll quickly redesign this (for porsche's ideas), I appreciate all the help!

Here it is!

I can post a nice 3 view drawing with full dimensions later, but now its 55" long, and the back of the seat is 40" from the front, making the center of gravity about 32" from the front, which is just under 60%, making it a 60/40 weight distribution which is pretty decent.

Oh, and that's a note for myself. I realized the the brake handles are 12" apart... and thats too narrow.

 

[saturnv7890]

Update! I went to home depot, got all the things l need except for screws since I still need to take a look at the plans and see how much and of what lengths l'll need.

I got 2x4's but they're out in the garage. I have 8 big nuts so I can lock them together, and there were no 5/8 nylon locking nuts. Also, I'm gonna order the HF wheels because there aren't any better deals.

 

[porsche930dude]

looks good. i dont know how heavy your rider is going to be but i think id go with a 2x6 for the main beam instead of a 2x4 they dont make lumber like they usto. and thats like the hardest piece to replace if you had any problems with it cracking. also i think id put the brakes inbetween the drivers legs id think you get a bit better control like that and also its comfortable for everyone. if you have a small kid riding it they might not have such an easy time reaching for the brakes and pulling them. also if your only able to pull one for some reason youll still get even pressure. one more thing you said 40" to the back of the seat have you considered the stretch your leg will have to make in a turn? if youve got a 4' wheelbase youve got to reach an extra 15" or so. I suppose that will be somewhat of a guessing game until you get it all layed out. i suppose you can always adjust the seat or even make a few holes up front to adjust the axle back and forth

 

[saturnv7890]

Following your advice, l redesigned!

I redesigned it to use steering wheel steering. This allowed me to put my feet in front of the front axle, giving proper 50% weight distribution. I designed this to have nearly perfect distribution, maybe biasing the rear by <5%. Also, this shortened the wheelbase considerably, now 46".

I'm also considering giving it a little caster angle by putting a wedge between the front axle and the frame.

And I did actually sit in a seat on the floor and put my legs on a 2x4 to see what felt comfortable for me, which is where I came up with the 40".

Note the steering mechanism in the drawings. I didn't put the pulleys or rope in because they would be a royal pain in CAD, but the pulleys go near the front steering brace on either side. The rope goes from one end of the axle, through one pulley, around the steering column (a few times), then back through the other pulley and to the other side of the axle.

I also added another 2x4 under the main frame for more support. This will be glued and screwed so the whole thing will act as a 4x4. This is actually stronger than a 2x6 because its thicker in the vertical direction, sorta like the idea behind I-beams. Also, I'm 150 lbs.

The brake location (moving it forward or backward) is gonna be decided once everything starts to come together, but I followed your advice putting them between my legs, this looks a lot better!

Frame:
http://img689.imageshack.us/img689/6241/cartframe.png

Frame+Everything (no wheels):
http://img109.imageshack.us/img109/577/cart.png



With steering rope (note the rope is hiding a little in part of the drawing):
http://img44.imageshack.us/img44/8100/42360660.png

 

[porsche930dude]

wow thats cool ! certainly complicates things though. your gonna have to get the ratio for the steering acceptable usually in a car its about 3 turns lock to lock. you could adjust that by how far the rope is positioned outward on the axle. that could pose a problem with leaverage on the wheel if your planning on a small one. also that pulley system will really have to be well designed and somewhat failsafe. if it comes off the pulley things go bad real quick. definatly looks like it could work though. On the foot steering version youd have a rope beween the two front wheels and the driver to help keep it stable and double as a pull rope.

 

[saturnv7890]

Well my idea to adjust the steering is to change the size of the shaft by using a hole saw and making a ring that goes around the shaft.

I'll probably want shaft with a diameter of 2-2.5" because that should pull the rope about 19-24" with 3 rotations which should be about right. (I used math in real life!)

And I figure if the steering fails I can just bring my feet back and steer it like that.

 

[Rickracer]

Both ropes will need to come off the top of the shaft, (not just wound around), or it won't work, and if you plan 3 turns lock to lock, you will probably crash your first time out. "Karts" have no more than 120° to 180° lock to lock, that quick response is necessary.

 

[modelengineer]

Seriously, go with the foot steering. Also, you should be sitting basically on the back axle to make it as short as possible.

 

[saturnv7890]

Both ropes will need to come off the top of the shaft, (not just wound around), or it won't work, and if you plan 3 turns lock to lock, you will probably crash your first time out. "Karts" have no more than 120° to 180° lock to lock, that quick response is necessary.

Well I'll make sure to find a bigger hole saw!

and here's a better diagram of how I'll wind it around. You can see it's only wound around more so the steering shaft has plenty of friction:

 

[drscope]

Check out the "cars" page, the "videos" page, the "gallery of photos" page - heck, just check out the entire site!

These guys are hard core down hill racers!
http://www.downhillmisfits.com/

 

[saturnv7890]

Check out the "cars" page, the "videos" page, the "gallery of photos" page - heck, just check out the entire site!

These guys are hard core down hill racers!
http://www.downhillmisfits.com/

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And, I've been working on the cart. It's coming along quicky. I still need to find a seat and a steering wheel, and I'm waiting on the wheels to be delivered.

Other than that, only a small things remain which I'll take care of tomorrow... the pulley system, and the tall steering support.

I'll take some pictures tomorrow!

 

[drscope]

yup small u bolts should work good. just use big washers so they dont pull through.
these wheels are very common and iv got them for as low as 6$ a piece. They dont hold up for gas powered carts but for yours they will work great

There are 2 big problems you will have with these wheels! First, the bearings SUCK! These are made for walking speed!

We have used these on our down hill cars and the bearings won't last 1/2 mile at speed.

The other problem is the tires. Cheap tires SUCK! They get out of round very quickly and you will find yourself riding on something that feels like it has cinder blocks for wheels.

The bearing issue can be corrected easily. A good high speed bearing is about $2, so if you are using these wheels, figure another $4 per wheel for bearings.

We still use these wheels on a few of our loaner cars, but they have better tires on them and better bearings. It's still way cheaper then buying a good wheel and they work pretty well that way.

Just make sure you check the bolts often to make sure they don't come loose.

One other problem is the wheel halves don't align well. So you need to be really careful when you start tightening them together. make sure they are aligned and you might need to bump them around a bit to get them right. If not, you end up with one out of round and wobbly wheel.

 

[drscope]

One other thing...
The all thread is a good idea to keep your frame rails together, but it is not good for an axle!

All thread works well in tension, but any side load is going to want to bend it.

Also, the threads will tear up the inside of your wheels and then you get slop between the axle and the wheel.

Slop between the axle and the wheel makes for one evil handling, scary riding cart!

You would be MUCH better off with 5/8 rod for your axles!