Frederic's Backbone Kart

[frederic]

Backbone chassis
From Wikipedia, the free encyclopedia

"Backbone chassis is a type of an automobile construction chassis that is similar to the body-on-frame design. Instead of a two-dimensional ladder type structure, it consists of a strong tubular backbone (usually rectangular in cross section) that connects the front and rear suspension attachment areas. A body is then placed on this structure. It is almost a trademark design feature of Czech(oslovak) Tatra heavy trucks (cross-country, military etc.), but this type of chassis is also often found on small sports cars. It also does not provide protection against side collisions, and has to be combined with a body that would compensate for this shortcoming."​

Not the typical go-kart construction method but I'm giving it a try, primarily because of the raw material I acquired for free. One of my neighbors tossed out this massive simulated weight bench / stepper machine years ago and it's been sitting in my basement unloved, so it's been declared "raw material". It's comprised mostly out of 3" square mild steel tubing with a 1/8" wall.

I am using a Lincoln SP125T flux-core wire-welder out of laziness. I would have used tri-mix and non-flux wire but the regulator wouldn't seal in the bottle so after 20 minutes of futzing with it I decided chipping off "pig snot" was less aggrevating. I couldn't find my tig torch and my oxygen tank was almost empty as what little I had left I used to cook one of the rims off the transaxle, so gas welding was out as well. The path of least resistance was to use the 120V welder and simply go slow for maximum penetration and seemingly ugly welds. My test welds on scrap survived the 20lb sledge test so even though they're splattery and ugly, I know they're strong.

 

[frederic]

Basically the dimensions of the chassis with wheels should end up being within my 4'x8' maximum. I decided on these measurements because that's what I can "drive" into the bed of my F350 crewcab and have enough room to close the tailgate, just in case my son wants to take this ride-on toy to the park, a friend's or what have you.

The tires are 18x9.50-8, though two are brand new "knobby" Carlyles and the other two are semi-used Sears with a turf-tread pattern. Ideally they'd be all the same however free overcomes many faults. The rims are all the same size and have a 3/4" keyed snout in the center. I have a clever plan for the front wheels/spindles and I'll share those pictures once the chunks of aluminum arrive - hopefully this week.

See, I'm not just an idealistic lurker... I actually build stuff!

The most difficult part of building the a-arms was boring a 1" hole at the end that sticks out, to receive a machined 1" diameter bushing with a 5/8" fine thread. This bushing is what the heim joints thread into with a jam nut. I think it's picture three on this post? Anyway, I modified my pipe notcher (fishmouthing jig) to clamp directly to the a-arm and using a framing square I nudged it into place as I tightened it. Doing it this way, with the a-arms attached to the frame, allowed me to bore the holes on all four a-arms directly opposite from one another thus assuring any misalignment of the a-arm triangles due to shrinkage (which happens during welding) is compensated for. Best I can tell with my limited measuring tools, I have the bungs for the heim joints within 0.100" of "ideal" across the width and height of the chassis. I can adjust the position of the heims with the jam nuts to reconcile that down even more, and I'll worry about that much later on.

The last picture is a peerless transaxle I got for free along with the three tires. The left tire on the transaxle is narrower and will not be used but the rim is good and will replace the rim on the right side of the transaxle which I had to remove using an oxy torch - it warped really bad from my having to "cook" it to a cherry red condition in order to allow the steering puller to yank it off. So, I have enough rims and tires.

I'm not sure if I will use the transaxle in whole or part yet. I might narrow it significantly and make an independent rear much like the front. I might rip out the differential and machine a mating pinion gear. I might throw it over the fence and arrive at a different plan. Not sure yet.

 

[Kaptain Krunch]

Looks good, I like the frame design, i've seen that in sand rails before and really liked the idea. What is the power source going to be? and when are you going to make one for dad :biggrin5:

Also, looks like you got a nice shop there, lots of toys.

 

[lemegacool]

lots of toys and no space like me

****ing nice frame! keep up the good work!

 

[frederic]

Looks good, I like the frame design, i've seen that in sand rails before and really liked the idea.

Thanks! I'm all for simple, and a long square pole with some stuff hanging off it is simple

What is the power source going to be?

• I have a couple of choices. I have an EZGO golf kart motor in unknown condition that I got in trade a while back.
• I also have a Reliance 5HP 48V motor which I have yet to find in my attic.
• I can get a working golf cart 4HP 48V motor and corresponding dead controller in trade as well.
• I can buy for $200-ish 4-5HP DC motor off ebay easily enough.
  
  

  and when are you going to make one for dad :biggrin5:

  
  Notice the size of my chassis ;-)
  
  

  Also, looks like you got a nice shop there, lots of toys.

  
  Yeah, I am into toys of many kinds!
  
  

  lots of toys and no space like me
  
  ****ing nice frame! keep up the good work!

  
  Thanks! There is a point where you can have too many toys for a given space... at least I got all the car/truck engines to the back of the garage before starting this project. Though a 460 block makes a good sawhorse ;-)

 

[frederic]

Cracked open the transaxle this afternoon, and wow, what an ugly mess. Bad seals, nasty grease that doesn't come off with wiping, scrubbing, and scraping, twisted up washers, keepers and other small bits. The chain between the jackshaft and the PTO shaft has enough slack on it that I can insert a phillips screwdriver shaft between the sprockets and the chain and still rotate the assembly all the way around. I won't complain however as I got the thing for free and most of these innards won't be necessary.

Why?

With electric drive, I can adjust the speed of the motor as well as reverse it - so all these intermediate gears, chains, and other crap is to be removed. The last picture shows what is truly necessary for my purposes, assuming I keep this stuff housed in this case. Narrowing it will be a challenge as the differential is so far offset to the one side that the longer axle shaft side will be a problem - there's no room for a bearing to be integrated. The short side isn't a problem.

It's easy to see what will break however... it's the cheesy fingernail-depth splines on everything, and I'll have to address this at some point as my son gets older and wants more power.

The differential is a nice piece - massive pinions and spider gears, so this by itself may be usable even if I trash the rest. The first three pictures were as it was acquired, the latter four pictures were after an hour of aggressive 4000psi power washing. Still some of the grease lingers and I'll have to dissolve it with a strong solvent. Not sure what I'm going to do about the lawn though, this ugly goop actually stained the grass blades.

 

[Kaptain Krunch]

The best thing to get rid of that grease is gas. All the transaxles i have cleaned i simply filled the cases with a little gas and scrubbed away with rags and toilet paper. Since you a nice mill and lath and such, im sure you could machine a new differential housing, with bearings for the output shafts, you would have to be very accurate so there isnt too much side to much side to side slop, but i think it would be worth it.

 

[frederic]

The best thing to get rid of that grease is gas. All the transaxles i have cleaned i simply filled the cases with a little gas and scrubbed away with rags and toilet paper.

That's a great idea, and I tried it out and it worked VERY well on the internals. I simply dropped them into a large coffee can full of old gas dumped out of various lawn equipment I haven't used this season due to the rain and viola, the goop came off very easily. I only had to wipe dry upon removing them, with occasional brushing where C-clips go and such.

Since you a nice mill and lath and such, im sure you could machine a new differential housing, with bearings for the output shafts, you would have to be very accurate so there isnt too much side to much side to side slop, but i think it would be worth it.

If I were to machine out a differential housing I'd use measurements of the original and simply make it smaller all over with beefier axle supports closer to the differential's ring gear so it can be narrower overall. Coincidentially I found in a drawer today six tapered bearings that have a 0.751" inner diameter. I'll be looking for the oversized seals maybe tomorrow when I have more time. The oversized seals go on the side of the bearing facing the lubricant, and seal well enough that even a lightweight oil won't seep past for years to come.

I'll have to think about making a new housing. I might be able to weld up the housing case halves with support blocks inside, flycutting the mating surfaces then cutting out the necessary slots, grooves, and what not.

Measuring up the old one to an accuracy of 1/1000" is easy. I'll think about this for a while.

 

[frederic]

I got sick of cleaning transaxle parts and pondering a homemade differential housing so I decided to start mocking up the spindles.

Picture 1: Using a length of 5/8" rod, I'm illustrating the kingpin inclination angle. According to my digital protractor this is a 12.71 degree angle. The caster is 3 degrees on the nose, and camber is -3 degrees.

Caster impacts "self centering", i.e. the vehicles desire to straighten out on it's own.

Camber impacts how much tire touches the ground when the chassis is leaning and/or cornering.

Kingpin inclination angle impacts both but is usually associated with the chassis's ability to roll.

What I have here is "reasonable" for a small, lightweight vehicle with an amateur driver, my four year old son. I'd cut the kingpin angle in half if I were to drive the kart often, mostly on asphalt.

Picture 2: I wanted to get a feel for clearances inside the wheel so I made a disgusting "mock" spindle out of two 5/8" diameter bolts welded head to head with a piece of 3/4" rod sticking out for the mower wheel to push onto. As I thought, the clearances are going to be too tight for the wheel center to be centered on the spindle with both rod ends inside the wheel. I was 99% sure this would be the case and this simple mock-up proves it.

Picture 3: Here is one of the chunks of aluminum that will eventually become a spindle, and next to it is a same-size chunk of fencepost that I will make mock-ups of various spindle designs. Wood is cheap, easy to cut on a miter saw, and if I screw it up I can hack off another 7" or so off the 8' fencepost I found lying in the far back corner of my yard. Waste not I say!

picture 4: The mock spindle is now in place with wrong size bolts simply because they were conveniently located on the garage floor near the kart. I shoved a piece of 3/4" round stock into the hole I drilled in the front of the spindle to support the wheel.

Picture 5: With the wheel installed, I was able to verify my sketch and corresponding math was correct - the wheel doesn't touch the spindle at all and there is plenty of room.

Picture 6 & 7: While the wheel clears the mock spindle, the upper rod-end hits the spindle and the bottom rod-end (and the lower a-arm) hits the spindle as well, so I'm going to have to make short conical spacers or hand-shape the spindle to a more rounded shape with the angle grinder. I'm inclined to do both, actually.

 

[Raywelder]

Im looking forward to seeing this one.

 

[TOO FAST]

Kingpin inclination. This is the angle of the kingpin? This is usually angled so the line drawn through it would end at the
center of the bottom of the tire right? Isin't this done so when
the tire steers it pivots in the center rather than "scrubbing"
in an arc? Or am I all wet on this

 

[frederic]

Im looking forward to seeing this one.

Thank you, it's nice to get feedback!

Kingpin inclination. This is the angle of the kingpin?

Yes, spot-on!

This is usually angled so the line drawn through it would end at the center of the bottom of the tire right? Isin't this done so when the tire steers it pivots in the center rather than "scrubbing" in an arc?

You're not wet at all - having an imaginary line between the two pivots of the knuckle hitting dead center of the contact patch puts "scrub" at zero and the contact patch rotates and deforms around the center of the patch. A great ideal to design for however as the suspension articulates, other factors come into play quite noticably - camber, caster, spring rate, damper (shock absorber) rate extend and contract, weight of the vehicle, balance of the vehicle front to back, left to right, center of gravity, suspension travel limits, how tight everything is, and so on.

Based on some of the unchangable, annoying parameters I'm working with (particularly a puny, 8" wheel), I have a lot of limitations on some of the magic I could in theory implement. So, this will scrub a bit and there is little I can do about it unless I reprioritize the characteristics of the front end in a different order than I have.

This is one of the reasons why I made a cheesy, wooden "actual size" model of the spindle - this allowed me to test my math and see what I'm actually getting in real life. For me, this proved out well enough that I'm going to machine up the chunks of aluminum with this design, with minor modifications to correct minor issues I discovered while mocking it up. Here were the important things I designed for, in order of importance to me:

• At no point in suspension travel will I have positive camber.
• Maximize ride height.
• Self-centering.
• Minimize scrub at full suspension compression rather than at rest.
• A-arms are to be unequal length and non-parallel.
• Jam all of this into a 8" rim with enough clearance even if the one bearing gives out.
  
  The first picture is the drawing of the suspension with the viewpoint being in front of the car, at eye level.
  
  The large grey "patchwork" rectangle represents the 18x9.50-8 tire, the big white square in the center of the tire represents the wheel (or rim), the "beehive" represents the spindle with threaded hole top and bottom for the rod-ends and the two tapered bearings/machined cups/long axle stub going through. It's difficult to see as I didn't draw the bearing detail but the inward end of the axle stub is larger in diameter than the axle itself, which "captures" the inner wheel bearing. The outer bearing is captured with a flat washer and in turn, held in place by the hollow shaft inside the wheel, which in turn will be held on with a washer and an aircraft nut.
  
  This axle shaft does not rotate in relationship to the wheel, but instead rotates in relationship to the spindle because of the two bearings that will be installed. This eliminates the need to use the ID of the wheelshaft as a bearing surface which means the wheel and the axle stub will last a very long time and never wear in such a way that I can't get the wheel off. Sure, it might "stick" due to rust but not due to oddly wearing parts which are much harder to deal with post-trauma. Yes, there will be a woodruff key slot machined onto the axle stub to ensure the wheel doesn't rotate on the shaft.
  
  The green lines are the a-arms at rest, the red line is the kingpin incination angle at rest, and tbe blue line you can barely see that starts where the bottom of the red line meets the tire's edge (contact patch) and goes to the right, is the scrub dimenison - about 3-1/4" assuming the tread is actually 9-1/2" wide. The knobby tires I have are slightly less and the turf tires are a hair more - I averaged the two tread widths for my diagram "just because".
  
  Picture two of this post is the final process of making spacers that will go between the rod ends and the spindle, to ensure clearance between the body of the rod end and the spindle itself while the wheels are turned towards lock, left or right. This is a parting tool I made myself working hard. The material is mild steel.
  
  Picture three is the upper spacer installed in the upper position to demonstrate it's purpose. It is 0.257" thick. The lower is 0.300" thick, because there is more spindle to clear.
  
  Picture four is with both spacers installed and unfortunately the bottom rod end still hits the spindle "a little" so I'll be shaping the final spindle made of aluminum to ensure clearance. A quick visit to Mr. End Mill solves this problem quickly. The spacers are to reduce the amount of shaping I have to do to the spindle as well as slightly increases the distance between the two rod ends.
  
  Aside from having to deal with the cramped space in and about an 8" wheel, the other issue that was tough is that to get a decent ride height the upper a-arm had to be parallel to the ground, and the lower a-arm extends outward to meet the bottom of the spindle. This is where the scrub comes in "at rest", but once the suspension is compressed the scrub lessens in relationship to how much suspension compression there is.
  
  I wish the two craig's list ads I responded to would let me know if they have the items still available, or not. One is for a $125 electric golf cart which would give me a bigger motor, controller, live axle and some batteries (and a variety of other accessories) as well as a "free" riding mower which looking at the picture has rear tires that closely match one of the pairs I have.

 

[freakboy]

Is every post that you put on the forums filled with very helpfull information?

 

[TOO FAST]

I'm adding suspension to my kart right now and doing something
very similar to your set up but not quite as sophisticated. Because I only want about 2 to 2.5 inches travel, I am using heim
joints as all locations. You wouldn't think there is enough movement there but when you have a 18 inch rod out it is plenty
for my application. Problem is there is no easy way to know
where the kart will sit with me on it before one begins
Thanks for the helpful post, I found it really informative

 

[Kaptain Krunch]

Looking good!

 

[TOO FAST]

Let me ask you something else. If I incorporate king pin inclination, I won't need much castor right? Since the angle on
the king pin will already cause the kart to "lift' on turning? My
castor will be built in so I don't want a situation where it is too
tough to turn off of center.

 

[frederic]

Looking good!

Thanks!

Is every post that you put on the forums filled with very helpfull information?

If you (and others) prefer I can instead offer one or two word answers that essentially equate grunts ;-)

Because I only want about 2 to 2.5 inches travel, I am using heim joints as all locations. You wouldn't think there is enough movement there but when you have a 18 inch rod out it is plenty
for my application.

With that little travel you can mount the rod-ends vertically or horizontally. Horizontal installations offers more lateral strength than vertical installations because more of the ball comes in contact with the race while under forces - forward and back.

Problem is there is no easy way to know where the kart will sit with me on it before one begins
Thanks for the helpful post, I found it really informative

Math!

(or)​

Using a cheap bathroom scale ($10 at K-mart, Wal-mart, places like that) measure the weight on each tire, one at a time. Place the scale under one tire, then install same-height blocks under the other three. Sit on the kart, facing forward as if you were driving, and while you're holding still have someone read the scale. Repeat for all four wheels.

With these four measurements you can figure out a lot of things.

Necessary spring weight for each wheel.
Weight bias left to right.
Weight bias forward and aft.
Total weight.

Once you figure out your spring mounting method and in turn your spring pivot angles and in turn you can calculate out what the spring rates need to be.

 

[freakboy]

i prefer the paragraphs of use full information that is well writen, well thought out! i was wondering if you know how to make a simple but easy to make 3 or 4 speed transmision? im trying to figure this one out still. any help is appreciated and either pm me or put it in my thread so im not gunking yours up. thats if you know a simple way!

 

[frederic]

I was just teasing you.

Regarding your transmission I don't know your arrangement or plans so if you could point me to your thread/post about this subject I'd be happy to respond best I can if I have any ideas.

The easiest way might be to use a bicycle sprocket set off a 12-speed bike along with the derailleur. The derailleur is cable actuated and using those parts might be simplest, or one could fabricate a lever to move the derailleur instead of a cable.

If you can machine gears and such, you could make a basic "slip-catch" system like most non-hydrostatic riding mower transaxles are based on.

This is nothing more than a slew of differently sized gears on a large diameter shaft, with a woodruff key that is machined to the shaft's surface everywhere but the ends. One end receives a collar that is slide up and down the shaft, and the other end of the woodruff key inserts itself into a keyslot machined into the hub of the gear, thus engaging that gear and that gear alone.

We can also talk about syncros, dog clutches, and other more complicated things but I'd guess that's far more complicated that you'd want to explore.

I believe some mowers have the transmission seperate from the differential housing - belt drive in, chain drive out?

 

[TOO FAST]

And after you answer freakboy let me ask you this. If my spring
shock combo is rated a 540 lbs, does that mean if I angle them
at 45 degrees the rate changes to 270 lbs? .. 67.5 degrees equals
405 lbs etc..