Frame design question

[Fawteen]

Okay, I got that 12 horse Briggs running pretty well and it's time to start thinking about the frame.

This will be a pretty low speed cart, at least initially, rigid frame, no suspension.

I'll be using the front axle/steering/wheels off a Craftsman lawn tractor, and the transaxle from the same thing for the rear axle.

That fixes the tread width at about 34 inches.

I realize that wheelbase will depend to a certain extent on fixed factors like engine-to-transaxle position and seat-to-pedals requirements, but is there an optimal ratio between tread width and wheelbase that I should shoot for?

 

[skid20]

Well logic says that 34" wheelbase+12 horse briggs+ sharp turn= broken neck.

I don't think there is really much of a ratio between tread width and wheelbase so much as there is a ratio between falling over and staying upright. If it's going to stay geared low like a rider you'll probably be ok, but if you are going to step up the speed I'd shoot for a 40" wheelbase but that'll probably hender the use of your transaxle unless you have a machine shop resize those axles.

 

[frederic]

The longer the wheelbase, the more stable (less bouncy) the vehicle will be. Compare a Jeep Wranger to a Lincoln Town Car.

The wider the track, the longer the suspension arms can be, and the less steering alignment changes will occur during suspension travel. Compare an MG or Triumph to a Corvette.

Generally speaking a ratio of 1.6 is considered "good", meaning that the vehicle is 1.6 times longer than it is wide however I'm not sure how that applies to go-karts. Looking around this website, it seems most to almost all of the projects used other factors in deciding how long and wide their kart should be, and in the end have great results so I would put less value on the 1.6 "normal" ratio used in the automotive world. I mention it because it's at least a starting point.

I would view your kart as three sections - front suspension, "cabin", and rear suspension/powertrain. Figure out what measurements are needed for the front and rear areas, then measure out the space you need in the middle to sit in it by yourself if a single seater or with a passenger for a two seater - then you have your ballpark measurements to tweak your design from.

Or, you could do what I did. I found a long piece of square steel tubing and decided "hey, that looks about right" then designed around it ha ha ha.

 

[Fawteen]

Frederic, that's just the sort of info I was looking for, thanks.

I'd pretty much decided on the modular approach to determining dimensions, but I wanted a ballpark figure to shoot for.

This is a low-buck deal for my grandson to putt around in, don't plan to deal with suspension or speeds much over 10 or 15 MPH.

Next hurdle is to come up with some sort of brake.

 

[Tommy Boy]

Good luck on the brakes.

A good number of the transaxles you'll find out there are designed around the small, woefully insufficient puck brake that's part of the transaxle assembly. The crap thing about this is that models of transaxle you'll find under a Craftsman rider like the Foote 4300 series and Peerless MST models are wide track, but don't allow much by way of room between the end of the housing and the axle stub itself once the wheel and tire assembly is mounted. Yes, there's a 2-3 inch spacer. But no, the tire and wheel by design eats up much of that space. Which in turn means only a very small diameter rotor will fit, which in turn means limited surface for a caliper if you can even find one to fit in the first place, which also in turn means rapid overheating and significant brake fade.

Hate to be a poopface about this, but I'm trying to get my head wrapped around the exact same issue on a Murray I'm speeding up. Short of extending the axle or essentially jackshafting the transaxle and extending the frame length to get the free space to allow for brake and caliper, I'm scratching my head in a bad way.

In other words, let me know if you get something figured out!

 

[frederic]

A brake is fairly easy and can be a simple disk attached to the rear axle with a caliper around that. That caliper can be lever actuated and pulled in by a pedal that pulls a cable between the two or if things work out right a long rod can go underneath instead of the cable.

Or, use hydraulics. Once can by official go-kart hydraulic brake parts or acquire parts from a "tiny" car in the junkyard.

Regarding the chassis, you'll notice that the front suspension and rear transaxle ride below the stamped steel mower chassis.

You could, in theory, mount those parts above a chassis you create yourself out of angle iron, 1" square tubing, or ordinary black pipe you can purchase fairly inexpensively at home depot, Lowes, or other home center.

1" OD black pipe sells for about $12 here in NJ and you get eight feet of it. To build an average-sized one person kart you'd need about four pieces, assuming you want some kind of structure rather than a simple rectangle with wheels sticking out.

Not everyone likes to work with black pipe because it's round - and most things that attach to a chassis is not round. Not a big deal, but something to think about.

 

[Tommy Boy]

A brake is fairly easy and can be a simple disk attached to the rear axle with a caliper around that. That caliper can be lever actuated and pulled in by a pedal that pulls a cable between the two or if things work out right a long rod can go underneath instead of the cable.

In THEORY.

In practical practice, given the scenario provided by the original poster and accounting for the high possibility of the transaxle set up he may have, there's a legitimate problem in the spacing required to mount a disc and caliper to the axle stubs, those I mentioned in the previous post. So not to come off like a jerk, but in the case of riding mower transaxles, there's a lot of instances where it isn't a click-bang solution. Trust me, I wish it was.

 

[frederic]

I don't think you're a jerk at all - discussion helps people and allows us to learn something, even with opposing views.

With a differential in the transaxle two brake rotors would be necessary to ensure that power isn't "differentiated" over to the wheel without the brakes. The link between them can be something as simple as a "T" fitting on one caliper and an elbow on the other.

I think the challenge is where to put the rotors... like you said there's not a lot of room on the axle stubs however most automobiles have less "stub" to play with and some cars have massive brakes on them - particularly the front. They can do this by making the brake rotors shaped like a hat instead of a simple disk.

I guess what we recommend depends on how much space is between the wheel and the transaxle, what brake parts are going to be used (or at least considered), and what metalworking capabilities the original poster has available to him.

Most of the riding mowers I have seen at have at least an 8" diameter wheel, which leaves 7.50" of available diameter inside the wheel. That's probably just enough room for a 5.5-6" rotor plus small caliper, either an official go-kart part or maybe from a motorcycle.

Yes, I would agree, it's not a "bolt on" but I really don't think it's super difficult either.

 

[Fawteen]

Thanks guys.

I'm planning to build the chassis out of 1-1/4" square tubing, going up to pick it up today.

I have MIG/TIG/Stick capability in my home shop, and access to a lathe and vertical mill at work, so most metal working is not a huge issue for me.

Tommy Boy is right about the clearance. With the negative offset on the Crapsman wheels, there's essentially zero clearance between the rim and the transaxle. Being a vertical shaft application complicates things a little too. Can't picture a jackshaft configuration that works, but haven't spent a lot of time thinking about it.

While typing this, I had a random thought: What if I direct-drive (via a centrifugal clutch) the input to the transaxle, and then chain-drive from the T/A to a live axle? I could adjust the ratios as necessary between the T/A and the axle, it'd give me plenty of room for a brake, and most of the extra length would be offset by mounting the engine directly over the T/A.

I'm guessing the T/A would probably self destruct from the higher RPM tho. Just going from memory (dangerous at my age...) the motor-to-T/A pulley setup looks like about a 2:1 reduction.

Hmmmmmm. Also just occurred to me that I might be able to reverse the wheels...

 

[frederic]

Another option is to skip the transaxle entirely and buy a go-kart live axle that's the same diameter as the axle shafts on your transaxle so the wheels fit. Probably 3/4" or 1" OD. You can get this axle at whatever width you want also if that's a concern.

Then you could use a jackshaft and have plenty of width to place the sprocket and brake rotor as it suits your design, and gear as you like.

I was originally going to use a mower transaxle for my son's "backbone kart" however once I started taking it apart I realized "something else" would be better and more flexible.

The drawback to a live axle and a centrifugal clutch setup with a gasoline engine is you don't have reverse. That's one of several reasons why I am going electric with my kart. It will be easier to back out of shrubs than yank. Since my son's four, I know this cart will be kissing shrubs quite a bit ;-)

Another option is to comb ebay and craigs list for a mower transmission rather than a transaxle as that would solve your desire for multiple gears as well as reverse. Typically they have a belt drive in from the engine and a chain drive from the transmission to the axle. If you're lucky you might find a complete riding mower that has this, and you can use the rear axle/transmission/chain/belts from that, and all you'd have to do is arrange them to your liking on your new chassis.

In your shoes, I'd probably go with my second suggestion, assuming I could find a free mower on craigs list or a cheap one on ebay. Or better yet, sitting at the curb!

 

[Tommy Boy]

Here's an idea for ya, Fawteen.

For starters, the idea of using the transaxle as a right angle gear box is solid. But you don't need the centrifugal clutch. You got all the clutch you need in the idler, and if you're feeling especially froggy, can set up a dual idler clutch like you'll find the racers use. But that's not the point of the post.

The bane of transaxles is the pulley swap. Yes, you can rework one, fill it with gear oil and reseal it, but the stresses and higher RPM needed to get the sexy speed you're looking for from the transaxle itself is what'll bust one to bits. So we come to your idea, the same idea Frederic hit on, and one I've been kicking around for quite awhile myself. Lengthen the chassis, set a live axle, and KEEP the transaxle, only using it as a RAGB and jackshaft all in one. With this, you can reduce RPM input to the transaxle, making up the difference in the ratio from axle stub to live axle to pinpoint your speeds. You can still have speed control, as the drive sprocket on the axle stub will still be influenced by the gearing in the transaxle. You'll effectively lengthen the chassis and by virtue of the live axle, can increase the width of your track. And as you guessed it, you're a disc and caliper away from greatness as you'll have all the room you need with your new set-up.

It only poses a couple of questions. One, how long is too long? If you have the transaxle I think you do, it's a Foote 4300 series with an excessively wide profile, essentially putting your axle stubs out in front of the tires on your live axle. It's not as pronounced as the MST-205 on my Murray, but wide enough to pose concern. Two, and this one is the $64,000 question...

Just how fast do you want to go?

 

[Tommy Boy]

Here's an idea for ya, Fawteen.

For starters, the idea of using the transaxle as a right angle gear box is solid. But you don't need the centrifugal clutch. You got all the clutch you need in the idler, and if you're feeling especially froggy, can set up a dual idler clutch like you'll find the racers use. But that's not the point of the post.

The bane of transaxles is the pulley swap. Yes, you can rework one, fill it with gear oil and reseal it, but the stresses and higher RPM needed to get the sexy speed you're looking for from the transaxle itself is what'll bust one to bits. So we come to your idea, the same idea Frederic hit on, and one I've been kicking around for quite awhile myself. Lengthen the chassis, set a live axle, and KEEP the transaxle, only using it as a RAGB and jackshaft all in one. With this, you can reduce RPM input to the transaxle, making up the difference in the ratio from axle stub to live axle to pinpoint your speeds. You can still have speed control, as the drive sprocket on the axle stub will still be influenced by the gearing in the transaxle. You'll effectively lengthen the chassis and by virtue of the live axle, can increase the width of your track. And as you guessed it, you're a disc and caliper away from greatness as you'll have all the room you need with your new set-up.

It only poses a couple of questions. One, how long is too long? If you have the transaxle I think you do, it's a Foote 4300 series with an excessively wide profile, essentially putting your axle stubs out in front of the tires on your live axle. It's not as pronounced as the MST-205 on my Murray, but wide enough to pose concern. Two, and this one is the $64,000 question...

Just how fast do you want to go?

 

[Tommy Boy]

Double post. My bad. Computer is acting wonky.

 

[Fawteen]

For now, I'm going to go with the T/A and just jigger the pulley ratios a little. This one is kind of a test bed for carts in general. If it works out okay, I'm thinking I'd really like to have a shifter cart. Watching for a deal on a murder-sickel engine now...

Discovered a little screwup today. Got the main frame welded up and mounted all the bits. As I was measuring and fitting for the front axle/steering, I watched very carefully to make sure the tie rod and steering arms all cleared. In fact, I watched SO carefully, I completely missed that the dang tires will hit the frame rails well before full lock on the steering.

Dang.

I'm toying with the idea of extending the spindles to move the tires away from the frame. I have the technology to do it, but I'm not sure it's a good idea structurally.

Choice B is to whack the front off the frame and move the mounting points in a couple of inches.

Choice C is to deal with a larger turning radius.

Decisions, decisions...