Brake bias vs ride height
03-31-2019, 02:27 AM
#1
Three Wheelin'
Thread Starter
Brake bias vs ride height
I was wondering how much a 2" drop in ride height changes the front/rear weight transfer during 1G of braking. Assuming I remember my free body diagrams correctly (feel free to correct me), the solution is pretty easy.
Lf = load on the front axle
Lr = load on the rear axle
sbf= stationary weight bias on the front axle, assume 35%
sbr= stationary weight bias on the rear axle, assume 65%
cgh= center of gravity height, 18" seems like a reasonable 964 number
wb= wheel base = 89.4"
M= mass of the car, assume 3,000 lbs
Ignore friction due to aero
BF= braking force, use 1G for this example
The equations are pretty simple:
Lf = M*sbf + M*BF*cgh/wb
Lr = M*sbr - M*BF*cgh/wb
They just say that the total weight on the axle is the stationary weight + load transfer (which is negative for the rear axle). Plugging in the numbers:
Lf= 3,000*(0.35 + 18/89.4)= 1,654 lbs
Lr= 1,346 lbs
The ratio of the weight on the front axle to the rear axle:
Lf/Lr= 1.23
Now to answer the question of what happens if we drop the center of gravity 2" (assume a 2" drop in ride height equals a 2" drop in cgh):
Lf= 1,587 lbs
Lr= 1,413 lbs
Lf/Lr= 1.12
That's a 9% change in the bias from a 2" drop in the center of mass.
Lf = load on the front axle
Lr = load on the rear axle
sbf= stationary weight bias on the front axle, assume 35%
sbr= stationary weight bias on the rear axle, assume 65%
cgh= center of gravity height, 18" seems like a reasonable 964 number
wb= wheel base = 89.4"
M= mass of the car, assume 3,000 lbs
Ignore friction due to aero
BF= braking force, use 1G for this example
The equations are pretty simple:
Lf = M*sbf + M*BF*cgh/wb
Lr = M*sbr - M*BF*cgh/wb
They just say that the total weight on the axle is the stationary weight + load transfer (which is negative for the rear axle). Plugging in the numbers:
Lf= 3,000*(0.35 + 18/89.4)= 1,654 lbs
Lr= 1,346 lbs
The ratio of the weight on the front axle to the rear axle:
Lf/Lr= 1.23
Now to answer the question of what happens if we drop the center of gravity 2" (assume a 2" drop in ride height equals a 2" drop in cgh):
Lf= 1,587 lbs
Lr= 1,413 lbs
Lf/Lr= 1.12
That's a 9% change in the bias from a 2" drop in the center of mass.
03-31-2019, 02:25 PM
#2
Rennlist Member
Interesting.
how does rake angle affect this?
I don't know about braking feel from simple ride height assuming rake stays equal
but, i know going more *** down absolutely changes the braking feel (positively), and allows a faster transition to throttle with trailing corner entry brake.
At least in my experience.
how does rake angle affect this?
I don't know about braking feel from simple ride height assuming rake stays equal
but, i know going more *** down absolutely changes the braking feel (positively), and allows a faster transition to throttle with trailing corner entry brake.
At least in my experience.
03-31-2019, 10:08 PM
#3
Three Wheelin'
Thread Starter
Yeah, I was surprised how much the front/rear load is effected.
Rake would just alter the static weight distribution. So the 35/65 front/rear split would move forward slightly, maybe 37/63 for instance. I don’t believe that it would effect the amount of weight transfer. Less rake would put more weight on the rear wheels, if you have too much rear braking bias then less rake would help balance. I’m not sure what you’re running, but the typical brake upgrade paths never seem to over bias the rears. That would be Bill V’s specialty.
I believe up to 3° of rake is the usual recommendation. Not sure how much is built into the RS height spec. RS front is a 50mm drop from US, and the rear is a 45mm drop. Resulting in more rake than US or RoW heights.
Rake would just alter the static weight distribution. So the 35/65 front/rear split would move forward slightly, maybe 37/63 for instance. I don’t believe that it would effect the amount of weight transfer. Less rake would put more weight on the rear wheels, if you have too much rear braking bias then less rake would help balance. I’m not sure what you’re running, but the typical brake upgrade paths never seem to over bias the rears. That would be Bill V’s specialty.
I believe up to 3° of rake is the usual recommendation. Not sure how much is built into the RS height spec. RS front is a 50mm drop from US, and the rear is a 45mm drop. Resulting in more rake than US or RoW heights.
04-01-2019, 09:38 AM
#4
Instructor
Nick, I think you have that backwards. Rake would impact the cgh, not the static balance, right?. Changing rake just raises or lowers vehicle mass without moving front or rear appreciably. However raising or lowering the rear will move cgh up or down for sure, since mass is so unequally distributed it will be much less if the front is raised or dropped.
04-01-2019, 07:37 PM
#5
Three Wheelin'
Thread Starter
The fore/aft center of gravity position changes with rake. But that’s exactly what defines the static front/rear weight distribution. The vertical center of gravity position isn’t likely to be effected very much by rake. That’s what effects dynamic weight transfer.
That is assuming you raise one end the same amount that you lower the other, and don’t change the mean ride height.
That is assuming you raise one end the same amount that you lower the other, and don’t change the mean ride height.
04-03-2019, 10:02 PM
#6
Instructor
I guess I’m picturing it differently somehow. If cgh is center of gravity (height) and the car is evenly balanced (50/50 mass distribution) the no, it wouldn’t matter. However since the mass of the 911 is heavy-rear won’t raising the back (even if the front is equally lowered) increase the cgh? If you took the motor out and put it on top of the spoiler (same weight, just a lot higher up) it would surely increase cgh, no? Average car height might be the same but average mass height above the ground would be increased.
If we use rake to lower the front mass and raise the rear mass (rather than relocating the motor) then the center of mass will move along an arc centered at our point of rotation. Since the diameter is pretty large I would expect a (relatively) large increase in the center-mass height with a small movement of cg to the front.
Maybe I am missing something though; happens from time to time.
If we use rake to lower the front mass and raise the rear mass (rather than relocating the motor) then the center of mass will move along an arc centered at our point of rotation. Since the diameter is pretty large I would expect a (relatively) large increase in the center-mass height with a small movement of cg to the front.
Maybe I am missing something though; happens from time to time.
04-04-2019, 11:50 PM
#7
Three Wheelin'
Thread Starter
Ah, I see what you’re saying. Given the geometry, I don’t believe that the center of gravity height changes nearly as much as the fore/aft position. But yes, definitely cgh would also change with rake unless the weight distribution was 50/50.
I need to go find some more backs of envelopes.
I need to go find some more backs of envelopes.