Sunday, December 11, 2016

Rear End Development

If you saw the rear axle in my 3D model, it shows that the drive sprocket and the brake are positioned symmetrically opposite on the centerline. This is an attempt to balance the axle while spinning. As it turns out the brake is much heavier than the drive sprocket, so realistically this wont happen...I need to recalculate the rotational velocity of the axle and the moving parts to find a center, or add ballast to the drive sprocket.

The drive sprocket is aluminum in this case and weighs about 3.5 lbs. The brake is heavier and weighs about 8 lbs. Its not a critical item at the speeds this vehicle will move, but if I decide to 'level up' and make a full car out of this design I will need to balance the axle to keep the axle from feeling the effects of torque steer. Without a 'cush drive' on the rear axle, the tendency to torque steer is increased, and having such a short length of chain isn't helping. The longer the chain the more tendency to stretch though, and lose power.

Here is how the back end came together.

its only wood...
\When I first put the engine in place, my idea was to sit the block on top of a plate of steel or aluminum above the axle. As things progressed, I realized I could sink the engine lower and get less height over the rear body cover. What you see here is me removing the bulkheads from the back end to make room for the new engine placement, and axle position. I drilled a single hole here to get started, then made a hasty retreat to regroup and come up with a better plan.

At this stage you can see the idea of where the drive sprocket wants to live (left-hand-side) because the Torque Converter jack sprocket lives there. This pic also shows how low the engine sits in the body, and relative to the axle. The engine bearing sits almost at the same height as the rear axle (just a bit lower) which is perfect for my rotational-momentum forces. If I get it right, the back end will accomplish a minor gyroscopic effect keeping things more balanced at high speeds than normal.

I did not want the sprocket to be too far away, and I did not want the axle behind the engine. This meant I had to modify the stock design of the torque converter. This isn't the first time having to do this, here's a pic of the previous kart's TAV2 and how it was modified. James M did some trimming on it with his magical milling devices:

Torque Converter Mods
Most Torque Converters come with a mounting plate that is designed to have an axle behind the engine. Frankly I think that sucks. In any case the solution to my problem is to compromise. In order to have a clean path for the chain, the casting material on the torque converter needs to be cut away which you see in this picture. The path of the chain needs to be clear of any sharp standoffs, the chains tend to flop around when braking, and accelerating abruptly or even at the same time. We chop alot of chains without having a chain tensioner - another thing that I will talk about later.

New Brake Disc
Fast forward a bit to the latest mods. You can see that the chassis has some wood structure added to follow the shape of the body, which required some creative trimming. Point being the brake disc as it sits now is PERFECT at a whopping 8.25" and .25" thick. This guy will handle the heat and provide some serious stopping power. I picked up this bad boy from MCP which is in Ohio.

You may notice a bit of oil staining and wood burn marks in the area that the brake sits. This is because previously to this photo I had a different brake setup, which consisted of a $%!& brake system from something called a 'hammerhead' go kart. Needless to say that did not work. No need to bore you with why - unless you have one and are trying desperately to make it work. PM me and I will give you a history lesson. (eventually i may write a post-mortem on that thing).

And here is a detail view of what happens when you buy 'off-the-shelf' parts and need to retrofit.

This isn't the ideal situation for mounting a flange bushing, bare threads in wood with a few washers are not my idea of a solid mount. So more than likely I will need to put a tie-rod under the chassis once I get everything located. For now this is a temporary fix to get everything lined up. Plus I dont want the tie-rod to destabilize the rear end chassis until some of the other bulkheads are in place, it will cause the engine and back end to sag. - more to come on that story later.

 Here's a rare 'top view' of the rear axle and engine layout. I say rare because I dont see too many builders exhibiting the layout of the engine to axle, in a construction view like this.

Here is a word picture of the bits on the axle from right to left. A1.5" aluminum tube axle has the following parts connected to it:

  1. 1" snap-ring for 1.25" axle
  2. 1.25" split locking ring collar
  3. 1.25"-ID : 1.375"-OD x 3" wide Hub-to-axle spacer
  4. Cart Wheel (no keyway)
  5. 1.25" split locking ring collar
  6. Aluminum 8" axle spacer tube, 1.25"- ID
  7. 1.25" flanged set-screw axle bearing (no keyway)
  8. 1.25" split locking ring collar
  9. .25" x 3" key stock
  10. 70-tooth 'split' aluminum sprocket with keyed 6-point locking hub
  11. 1.25" split locking ring collar
  12. 1.25" split locking ring collar
  13. .25" x 3" key stock
  14. 8.25" MCP brake rotor with keyed 4-point locking hub
  15. 1.25" split locking collar
  16. 1.25" flanged set-screw axle bearing (no keyway)
  17. 1.25" split locking collar
  18. Aluminum 6.5" axle spacer tube with notch for keyway
  19. 1.25" split locking collar
  20. .25" x 1.25" key stock
  21. 1.25" ID Martin sprocket Sintered-Metal hub adapter w 4 through bolts
  22. Cart Wheel (no keyway) with 4 holes drilled for the 4 through bolts
  23. 1.25" split locking collar
  24. 1" snap-ring for 1.25" axle
And here is a different view of the same thing:

 In both of the above pictures it shows a chinese hydraulic brake that I found at a local lawnmower fix-it shop, that is common on minibikes and micro ATV's. I mounted it poorly and it didn't give me the brake pressure I was expecting so I changed modes to the original MCP caliper.

But you get the idea. The problem with the ZJ caliper is that its meant to be fixed to a tube flange with 2 flange bolts very VERY close to the rotational part of the brake. I had to modify the hub and the brake disc to try and get it to work, but failed. The MCP brake is superior in every way for this application - more to come on that soon. -CW

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