Drive Train help

[Samk]

Our team had decided to use 8 wheels, all of them 8 inch andymark wheelchair ones(non pneumatic), as to be able to turn, the center 2 on each side were supposed to be lower by .1 of an inch. Some mentors brought up a concern that we would be too tippy with this design, we were under the impression that this is common to do. Any thoughts on whether to drop the center two or not? Also, we got all the blocks mounted, and found that though the shafts were true, most of our wheels had significant wobble, and they are brand new straight from andymark. Any thoughts on how to combat this? as it seems to cause major problems when chain is put on.

[MrForbes]

Dropping the center wheels makes it so that the robot will only be on 4 wheels at a time, which allows it to turn. It does not really make it "tippy". Having a high center of mass is what makes a robot tippy.

[Daniel_LaFleur]

Originally Posted by Samk

Our team had decided to use 8 wheels, all of them 8 inch andymark wheelchair ones(non pneumatic), as to be able to turn, the center 2 on each side were supposed to be lower by .1 of an inch. Some mentors brought up a concern that we would be too tippy with this design, we were under the impression that this is common to do. Any thoughts on whether to drop the center two or not? Also, we got all the blocks mounted, and found that though the shafts were true, most of our wheels had significant wobble, and they are brand new straight from andymark. Any thoughts on how to combat this? as it seems to cause major problems when chain is put on.

In a drop centered wheel configuration you will have some rocking. As far as being tippy, that is more a function of the Center of mass than if you have a slight drop center (although the drop center does exacerbate the issue). many teams use a drop center approach to 6 & 8 wheel designs effectively.

Wheels should not wobble, and you need to find out why yours do. I'd suggest looking at how your shafts fit through the hubs and make sure that your bearings are pressed in properly (assuming dead axles since you mentioned chains).

[Pat Hart]

Are you driving in the wide or narrow configuration. If you are driving wide the added distance may allow you to turn without lowering the center wheels. In the narrow configuration you are more likely to need a lower center wheel.

I am not sure how your blocks are mounted but if possible try testing by shimming the middle wheels with washers and find what turns best for your robot. Wheel spacing and center of gravity location can have an effect, so not all robots handle the same way

[Akash Rastogi]

With a .1" drop and a well balanced robot, you should not have troubles with being tipsy.

Make sure you drop your centers enough (would recommend .125") and balance your bot.

[Samk]

we are driving in a narrow config, with dead axles. all wheels are powered

[JB]

In narrow config I would say anything from .125 to .1 would be good for 8 wheel drop center.

Our team usually does a .0625 drop, but that is because we found with a 6wdc .125 rocked to violently when accelerating from stop.

[Samk]

Thanks for all the replies! I'll show the team this and see their thoughts