Discussion
I have heard various things about using ABS from the donor car in your kit.
WK? are building a Marlin 5exi fitting the ABS, so it shouldn't be a problem I start to think . . .
. . . yet I hear that ABS is calibtrated to the car it is fitted to and therefore fitting it to a car that is half (or less) of the weight of the donor screws with these settings and a best, stops the ABS working effeciently.
What is the truth?
WK? are building a Marlin 5exi fitting the ABS, so it shouldn't be a problem I start to think . . .
. . . yet I hear that ABS is calibtrated to the car it is fitted to and therefore fitting it to a car that is half (or less) of the weight of the donor screws with these settings and a best, stops the ABS working effeciently.
What is the truth?
No figures out the deceleration rate of the wheel, when it exceeds a threshold it will free up the pressure (for a particular wheel or a diagonal depending on the system) for how long it free's up the pressure depends on the brakes fitted to the vehicle and weight etc. Yes it probably would have to be tuned for the vehicle.
A couple of years ago I ran across a great description of how modern ABS actually works. Its written for the benefit of someone upgrading ABS brakes but it explains the dynamics really well. It's long but worth the read if you're interested in the technical details.
www.stoptech.com/whitepapers/abs_bigbrake_122701.htm
You can skip down to the section "ABS Control In Super-Slow-Mo".
Basically, modern ABS systems are much more than 'is the wheel completely locked up (slip rate of 100%) and therefore needs to be released'. Now they start taking action on slip rates of maybe 5%; i.e. one wheel is going 63 mph but the car is probably going 66 mph.
The ABS doesn't have any way of independently measuring the car's true speed over the ground so the ABS estimates it based on what it knows from the wheel speeds and, (crucial to your situation), its knowledge of the car's maximum ability to slow down. If your car was capable of slowing down at a maximum of 1.0 G and it calculates that a wheel is now slowing 1.4 G, (figured from change in wheel speed vs. time), then that wheel must be starting to slip and it's time to start intervening. So it will cut back brake fluid to the 'slipping' wheel(s) until they are decelerating within correct thresholds again. Correct for the donor car, not for the 5exi.
In a nutshell, it seems that the ABS will not want to let the new car decelerate faster than the car it was designed for. So using ABS from a heavier car on a much lighter car will keep you from realizing the increased maximum braking potential of the new car. Also, this is all assuming that the rest of the braking system is stock, changing other components would play even more havoc with the system.
Disclaimer - I am not an expert in this, just info I have gleaned from reading the above linked and other technical articles about ABS.
www.stoptech.com/whitepapers/abs_bigbrake_122701.htm
You can skip down to the section "ABS Control In Super-Slow-Mo".
Basically, modern ABS systems are much more than 'is the wheel completely locked up (slip rate of 100%) and therefore needs to be released'. Now they start taking action on slip rates of maybe 5%; i.e. one wheel is going 63 mph but the car is probably going 66 mph.
The ABS doesn't have any way of independently measuring the car's true speed over the ground so the ABS estimates it based on what it knows from the wheel speeds and, (crucial to your situation), its knowledge of the car's maximum ability to slow down. If your car was capable of slowing down at a maximum of 1.0 G and it calculates that a wheel is now slowing 1.4 G, (figured from change in wheel speed vs. time), then that wheel must be starting to slip and it's time to start intervening. So it will cut back brake fluid to the 'slipping' wheel(s) until they are decelerating within correct thresholds again. Correct for the donor car, not for the 5exi.
In a nutshell, it seems that the ABS will not want to let the new car decelerate faster than the car it was designed for. So using ABS from a heavier car on a much lighter car will keep you from realizing the increased maximum braking potential of the new car. Also, this is all assuming that the rest of the braking system is stock, changing other components would play even more havoc with the system.
Disclaimer - I am not an expert in this, just info I have gleaned from reading the above linked and other technical articles about ABS.
I don't know a great deal about it but can categorically say that ABS systems ARE "mapped" for a particular application just like an engine management systems. I work for a company that converts mass-produced cars into taxis / wheelchair-accessible vehicles so I deal a lot with the base vehicle manufacturer and they've mentioned the above to me on more than one occasion! If it was me, I wouldn't try to fit it on a car with different overall weight, weight distribution, spring & damper rates and centre of gravity height.
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