Summer Project 3 - Omni/traction wheel diameter dilemma

[marccenter]

Dear CD,

For Summer Project 3, I purchased some 6 inch, VEX OMNI and traction wheels. I added the wedgetop to the traction wheels (1-1/2 inch wide) and have found that the wheel diameters are different by a half inch or so. So where does this leave me? It seems like if I put these together in a 6WD configuration using a KIT chassis with pre-drilled holes that the OMNI wheels will not make significant contact to the ground (carpet). My options seem to be:

1) Don't use VEX OMNI and traction wheels together.

2) Re-drill the KOP chassis holes to raise the traction wheels center higher.

Is there another way? How have you solved this problem? Am I missing something here?

[kramarczyk]

Before anyone can really make helpful suggestions we need to know what is your goal for this chassis. Additionally, where were you planning to use the two types of wheels? My preference would be traction on the center and one end with a pair of omni on the other end, but there are other configurations that folks run (omni on all corners, etc). Finally, what are you planning to drive on (this is related to the first question)?

Absent all that info, my best suggestion would be to assemble it as you planned, try it out and see what you notice before modifying anything.

[marccenter]

Regarding the 6WD configuration I was planning on running both OMNI wheels in the front and having the middle and rear pair of wheels be the Traction wheels.

[msaunders]

Dear CD,
Is there another way? How have you solved this problem? Am I missing something here?

No you are not missing anything. The past two years we have done a Kit Frame, 6WD-no center drop-traction on back, traction on center and Omni's in front.
This past year we used VEX Traction and Vex OMNI's.

Ended up milling new holes, pretty sure we did all of them.
We bolted two rails together and milled them as a set.
After doing a set for a practice drive train, the competition set came out pretty good.

[marccenter]

We bolted two rails together and milled them as a set.

Can you share the dimensions you used?

[msaunders]

I have submitted a request to the CAD group.

[kprzewodek]

Here's a link to our chassis drawing set. Hope this helps.

http://a360.co/11JKreu

[kprzewodek]

Tried to reply a few times and it's not going through. Now I wonder if it has something to do with the external link I attached. Here it is again with direct pdf attachment.

Hope this helps

[marccenter]

Hope this helps

Thanks. I'm surprised to see only 1/8 inch difference? In the interim I had a chance to measure the wedgetop tread (good 0.25 inches) and find a dimension somewhere (McMaster - 0.31 inches). I would have expected some dimension between those. With 6 inch traction wheel radius being 3.25 - 3.31 and OMNI right at 3.00, I would have expected the traction wheels to be raised to maybe 0.75 and the OMNI's at 0.50. Did you use the smoother, roughtop tread?

[msaunders]

We used the standard rough top tread from Vex.

Keep in mind that some of the rubber part of the tread fits inside the wheel edges (from the picture I see you are aware of this) and that the tread will get compressed a bit by the weight of the robot.

As I recall there were a few discussions when this dimensions was being determined.

.. I took a look at your image and it does not appear you have the tread fully seated in the rim all the way around.
Did you loosen the bolts holding the two side together, then lay in the tread and retighten the two halves together?

Where are you getting the dimension for the 6" traction wheel?
http://content.vexrobotics.com/vexpro/pdf/217-2589-1in-Drawing-20130115.PDF

After further recall, we ended up deciding that the traction wheel with tread has a radius of 3.125 and the Omni as 3.00.