pic: Unobtainium 6wd - Cantilevered, Dead-Axle, Slot-Tensioned Drivetrain

[Rob Stehlik]

Actually, my original concept called for 1/2" shoulder bolts in a slightly different configuration. I intended to have the shoulder portion of the bolt pass through the frame, and tighten the entire wheel assembly against the frame with very thick washers.

The thread of a 3/8" shoulder bolt is 5/16"-18, which in my opinion is too small to be supporting this kind of cantilevered load. Thicker washers would help some, but the thread length is only 1/2", so you don't have much to work with, probably not even enough for a heavy lock nut.

I think what you have shown could work with a larger shoulder bolt, but it looks like squirrel's method with 1/2" threaded rod would be simpler and stronger. And fine thread really helps to get things tight.

Another issue is bending of the frame itself. Putting a cross bar at the top of the angle next to each wheel would help a lot. It could be a thin wall tube with a small threaded rod running all the way through.

[Joe Johnson]

There is a lot there to love.

A lot that is cringe worthy as well, but overall, a nice effort.

Got me thinking... ...How can I use this? Maybe there is something there...

Thanks!

Joe J.

[JamesCH95]

Cantilevered shoulder bolts can take a lot of load. I used them in an FSAE car suspension rocker that saw around 5,000lbf. The trick is to get the shoulder supported and eliminate bending stresses from any threaded portions. The only serious issue I can see in your design is how the shoulder bolts are connected to the rail.



In this picture near the left-hand edge about 1/3 of the way down you can see where we used a 5/8" (1/2"?) shoulder bolt on the suspension rocker. About half of its length passed through a steel mounting point, then went through a needle bearing in the rocker, and was captured with a nut on the inside of the car. We sandwiched thrust bearings on either side of the rocker so we could clamp everything down very tightly yet still spin it by hand.

I hope you find this useful *cough* needle bearings in wheels *cough*

[topgun]

Originally Posted by Joe Johnson

A lot that is cringe worthy as well, but overall, a nice effort.

For those of us that use the various drivetrain designs posted on CD as learning tools, what do you find cringe worthy? And why?

Thanks.

[Joe Johnson]

Originally Posted by topgun

For those of us that use the various drivetrain designs posted on CD as learning tools, what do you find cringe worthy? And why?

Thanks.

Let me start by saying that I am home sick today and probably should not have used the term cringe worthy. I really don't want to pick on the design. There is a lot to like.

Here is the thing. For those who have been doing this for a long time, design is an emotional experience.

I believe that we humans utilize our emotional brains to sort through complex space of possible solutions. Our emotional brains are just really good at searching through complex system interactions.

So... ...When I see something obviously awesome, I often laugh (I recall the first time I looked up in the GA Dome I laughed and laughed as I realized how cleverly the designers used hoop strength and tension members together with compression columns to raise the higher and higher tent poles over that beautifully clear interior). When I see something not so awesome, I feel it in my stomach.

So... ...what do I think is less than awesome in the design?

I think it boils down to two conflicting functions of the 1/4 angle. When I first see it, it seems both not strong enough and too strong at the same time.

It doesn't seem strong enough to be the sole mount for the cantilever axles. While at the same time the 80-20 and 1/4 angle seem pretty heavy for what they are doing.

The thing about emotional brains is that they can be wrong. We educate our emotional brains the best we can but they sometimes point us in the wrong direction.

I would have to do the calculations to know (and I haven't).

So... ...that is probably the numb of what I was getting at (that and the connection between them draws my eye as well).

Let me close by saying I was too harsh. There is a lot to like and a lot that I would change. But a very interesting effort.

Joe J.

[topgun]

Thank you Joe for your comments.

I agree with you about the 1/4" angle. I see teams driving down to thinner and thinner metals so to see something that thick makes me wonder.

Last year we accidentally ran our .090" 5052-H32 sheet metal frame into a cement column at full speed without bumpers. We ended up with a little ding on the flange, but the structural integrity remained. Made me think we could have gone to .080 or maybe even less.

I always like to know the tradeoffs on various drivetrain designs so I can help our students understand the tradeoffs and make decisions according to what we need.

I am also constantly looking for designs that use less precision machining because I see a lot of teams without that capability. That is one thing interesting about this design.