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Discussion Starter #1
now that we have figures (see thread "shaft strengths"), what do we do with them? calculate traction torque limits for our rig. we need weight on axle (total weight with driver, passengers, tools, etc. x max weight transfer-usually .90 when rock crawling), coefficient of friction (COF), and tire radius(diameter divided by 2) to calculate this. some useful COF's are:
.1-.3 for mud
.3-.4 for hard dirt
.70 for dry rock
1.0 spike load on dry rock
1.0-1.5 sticky compound tire on rock?
take weight on axle and multiply by COF
multiply this by tire radius
divide by 12 to get traction torque limit.
this is the most torque your rig can deliver before tire slips, regardless of horsepower. if you have a lead foot on rocks use a high COF in you calculations, maybe 1.5 with stickies.
a few examples: my rig with driver at trail weight of 3380lbs. and 39" baja belted's: 4943 lb/ft. on rock
2500lb samurai with 31's: 2906 lb/ft
2500lb samurai with 35's: 3281 lb/ft

Premium Member
1,389 Posts
Good tech. This is the same calculation I use. I always go .9-1.0 for the COF as even in mud the worst case is when you catch something solid like a root. Which brings us to the topic of shock loads... I feel a safety factor of 2-3 is about right. I also feel that worst case is about half the vehicle's weight is on one rear tire (beyond that I am easy on the throttle to prevent a roll).

Bottom line is, I divide the total weight by two to get the max load, then multiply by two to cover shock loads, and end up in the same place you do.
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