Wooders
17-11-2003, 12:38 PM
SY02 posted the following on JU, and the thread was immediately locked under claim of trolling...However I think there is some healthy discussion points in the idea (right or wrong).....so what's your spin on this:
"OK...for all of those that have "upgraded" their rear brake systems you may wish to chime in on this thread.
First off...you have not increased your vehicles braking ability...you have made your vehicle more dangerous.
Brake systems are engineered so that the front brakes will ALWAYS lock up first - the front brake force will surpass the maximum traction available of the front tires. This is good since when your front brakes lock you simply continue to slide forward in a semi-controlled situation.
What happens if your rear brakes lock first - all hell brakes loose (pun intended!) Think about the instability of the vehicle - all this mass pushing forward on the front tires...the only thing keeping the rear end from trying to swing around and pass the front is the lateral stability provided by the REAR tires since they maintain some traction. The rears break loose and the rear will come right around.
Rear brakes contribute no more than 20-25% of a vehicles total stopping ability anyway. You lift the vehicle and fit larger tires and the contribution of the rear brakes becomes even less. SO ONCE YOU RAISE THE CENTER-OF-GRAVITY YOU SHOULD LESSEN THE REAR BRAKES STOPPING ABILITY TO RETURN SOME BALANCE TO THE BRAKING SYSTEM.
There exists a firm relationship between braking bias (distribution front and rear) and the ratio of wheelbase to height of center-of-gravity (CG). Shorter wheelbase and higher CG equals more front bias - we can all see this can't we? More front bias equals less REAR braking ability. Less rear braking ability equals less rear brake force necessary for the rear tires to surpass the maximum traction available.
I know you guys are all waiting for the numbers so let's rip:
F=uN
F (force required to lock up tire)
u (tire coefficient of friction)
N (weight [normal load] placed on tire)
A stock Jeep has basically a 50/50 static weight distribution. Upon braking the dynamic shift approximates 80/20 with the bias moving forward. OK so far? We will say the Jeep weighs 4000 lbs and the tires are ATs with a rating of .7 for the coefficient of friction.
Stock front brakes
F = .7 * (4000 lbs * .8)
F = 2240 lbs (and the tire locks up!)
Stock rear brakes
F = .7 * (4000 lbs * .2)
F = 560 lbs (and the rear tires lock up - not likely)
Now, let's look at an "average" lifted rig with 4" of lift and 33" tires. The CG has been raised so the braking bias will be increased in the front and reduced in the rear (remember the relationship between the wheelbase and the height of CG?) Still a 50/50 static distribution but...now the dynamic shift is 90/10. And since you are running MTs (with less on road traction) with a rating of .6 for the coefficient of friction you are really hurting:
Stock front brakes on lifted rig
F = .6 * (4000 lbs * .9)
F = 2160 lbs (and the tire locks up!)
Stock rear brakes on lifted rig
F = .6 * (4000 lbs * .1)
F = 240 lbs (and the rear tires lock up - not likely)
You see the force required to break the rear tires loose on the lifted rig (240 lbs.) is nearly half of what the stock rig requires to break loose (560 lbs.) If the stock rear brakes are good enough for 560 lbs. they are good enough for half that!
And some of you consider increasing the ability of the rear brakes on lifted rigs a good thing?
"
"OK...for all of those that have "upgraded" their rear brake systems you may wish to chime in on this thread.
First off...you have not increased your vehicles braking ability...you have made your vehicle more dangerous.
Brake systems are engineered so that the front brakes will ALWAYS lock up first - the front brake force will surpass the maximum traction available of the front tires. This is good since when your front brakes lock you simply continue to slide forward in a semi-controlled situation.
What happens if your rear brakes lock first - all hell brakes loose (pun intended!) Think about the instability of the vehicle - all this mass pushing forward on the front tires...the only thing keeping the rear end from trying to swing around and pass the front is the lateral stability provided by the REAR tires since they maintain some traction. The rears break loose and the rear will come right around.
Rear brakes contribute no more than 20-25% of a vehicles total stopping ability anyway. You lift the vehicle and fit larger tires and the contribution of the rear brakes becomes even less. SO ONCE YOU RAISE THE CENTER-OF-GRAVITY YOU SHOULD LESSEN THE REAR BRAKES STOPPING ABILITY TO RETURN SOME BALANCE TO THE BRAKING SYSTEM.
There exists a firm relationship between braking bias (distribution front and rear) and the ratio of wheelbase to height of center-of-gravity (CG). Shorter wheelbase and higher CG equals more front bias - we can all see this can't we? More front bias equals less REAR braking ability. Less rear braking ability equals less rear brake force necessary for the rear tires to surpass the maximum traction available.
I know you guys are all waiting for the numbers so let's rip:
F=uN
F (force required to lock up tire)
u (tire coefficient of friction)
N (weight [normal load] placed on tire)
A stock Jeep has basically a 50/50 static weight distribution. Upon braking the dynamic shift approximates 80/20 with the bias moving forward. OK so far? We will say the Jeep weighs 4000 lbs and the tires are ATs with a rating of .7 for the coefficient of friction.
Stock front brakes
F = .7 * (4000 lbs * .8)
F = 2240 lbs (and the tire locks up!)
Stock rear brakes
F = .7 * (4000 lbs * .2)
F = 560 lbs (and the rear tires lock up - not likely)
Now, let's look at an "average" lifted rig with 4" of lift and 33" tires. The CG has been raised so the braking bias will be increased in the front and reduced in the rear (remember the relationship between the wheelbase and the height of CG?) Still a 50/50 static distribution but...now the dynamic shift is 90/10. And since you are running MTs (with less on road traction) with a rating of .6 for the coefficient of friction you are really hurting:
Stock front brakes on lifted rig
F = .6 * (4000 lbs * .9)
F = 2160 lbs (and the tire locks up!)
Stock rear brakes on lifted rig
F = .6 * (4000 lbs * .1)
F = 240 lbs (and the rear tires lock up - not likely)
You see the force required to break the rear tires loose on the lifted rig (240 lbs.) is nearly half of what the stock rig requires to break loose (560 lbs.) If the stock rear brakes are good enough for 560 lbs. they are good enough for half that!
And some of you consider increasing the ability of the rear brakes on lifted rigs a good thing?
"