Really confused about traction for sumo

[Conscripted]

Good evening everyone. I've decided I'm going to try to build a mini sumo for the ROBOGAMES next yearhttp://www.robogames.net/. I was in the process of reading every post on these forums about sumo and I'm really confused about traction. I read this.....

erm.. i did think this also, the larger surface area the more friction.
also 203:1 is a large gear ratio, ive got one of these gear boxes also and they are very powerful, i would be impressed to see anything force it the wrong direction against the gearing (they have a LOT of resistance when the motors are turned off). which brings us back to the wheels. larger surface area more friction.
i would keep with the design personally, but get as close to the weight restrictions as possible

Im afraid, im going to have to disagree with the bolded text. Surface area does not affect friction. friction = uN = umg. Mass, gravity and the coefficient of friction between the two materials affect friction.

If you drag mass block 2 x 2 x 1 accross a table, the 2 x 2 face of the block will have same friction as the 2 x 1. i did a lab like that in my physics class lol <3

Pressure though is affected by area <3

if i am wrong plz correct me :O

If surface area doesn't affect traction and tracks don't provide an advantage why does a bull dozer have tracks and not wheels?
It's the best pushing thing that I can think of....

[madchimp]

tracks spread the weight of the vehicle over a much larger area reducing pressure preventing such vehicles from sinking in and getting stuck some tracked vehicles actually tread lighter than the average man when you figure the pounds per square inch/cm

[SmAsH]

i have seen you snooping around the old sumo topics
madchimp is right that the tracks increase the surface area the vehicle is on.
hence why people lie on nails instead of standing...
friction mostly depends on the weight on the robot and where it is, along with the materials that are touching.

[Conscripted]

Ok. So friction and traction are the same? And just so I understand it. Say I take a brick and place it on a one inch square piece of rubber or what ever. If I measured it's static friction to be X and then I used a 4 inch square piece of the same material and did the same thing I would still get the same X? So why does everyone I've seen use wide tires for their sumo bots? And why do drag racers not race with the donut spare?

Confused......
Conscripted

[SmAsH]

nope, traction is the amount of grip something has on something else.
friction is just an amount of force exhibited on two objects moving against each other in the opposite direction
they are moving.
most people who use sumo bots use wider tires because there is more surface area for the tire to grip with.
the wider a tire, the more traction it will have (within reason).
drag racers have wide rear tires so they can get a decent grip when they start instead of just sitting there
spinning their wheels.

[Soeren]

Hi,

To get more traction, use a softer material tire.
If you use a softer tire, you need to enlarge the footprint (are of contact), assuming that you will allow the same amount of "collapse" of the tire.

Remember though, that traction is loss of power in a dynamic setting, or we would all use tires sticky as molten caramel.