CNY_Dave Posted December 17, 2007 Share Posted December 17, 2007 They could but.... Its impossible to modulate a wheel with no traction. The wheel with traction has the mass of the car behind it. Once you loose that traction, you just have a spinning mass of 100 lbs, where as the brake system normally (for arguments sake) modulates a 800 lb mass per wheel. nipper OK, so current ABS controller tech can't cycle fast enough? How does the traction control unit know what pressure to send to the caliper- they just use something that matches most situations? Dave Quote Link to comment Share on other sites More sharing options...
nipper Posted December 17, 2007 Share Posted December 17, 2007 has nothing to do with cycling. It is the mass fo the car. It takes very little brake pressure to stop a spinning wheel, as opposed to stopping a car. Example (dont do this). Jack up one wheel and spin it by hand. Stop it by hand (carefully). Now Push the car to the same speed and try to stop it without using the brakes. Much more force is required. I hope that helps to explaine it. ABS and Traction control use the wheel speed sensor (along with some others, but we will keep this simple) to decide what to do. If one wheel is locked up, and the brakes are applied, it releases the brake untill it catches up with the others, then re applies the brake. If the wheel is spinning on acceleration, it can either apply that one brake, or cur the power output of the engine (for subaru it applies the brake i do beleive) Companies are getting away from playing with throttle response and using the brakes more. nipper Quote Link to comment Share on other sites More sharing options...
CNY_Dave Posted December 17, 2007 Share Posted December 17, 2007 has nothing to do with cycling. It is the mass fo the car. It takes very little brake pressure to stop a spinning wheel, as opposed to stopping a car. Example (dont do this). Jack up one wheel and spin it by hand. Stop it by hand (carefully). Now Push the car to the same speed and try to stop it without using the brakes. Much more force is required. I hope that helps to explaine it. ABS and Traction control use the wheel speed sensor (along with some others, but we will keep this simple) to decide what to do. If one wheel is locked up, and the brakes are applied, it releases the brake untill it catches up with the others, then re applies the brake. If the wheel is spinning on acceleration, it can either apply that one brake, or cur the power output of the engine (for subaru it applies the brake i do beleive) Companies are getting away from playing with throttle response and using the brakes more. nipper Yes, but if the traction-control braking cycle time was fast enough (faster than it would have to be for ABS, I'm guessing) it could pulse the brakes at just the right intensity/cadence to make the braked wheel spin at any desired rate- the desired rate would be whatever the wheel with traction was doing (or match actual vehicle speed measured some other way). The cycle times would have to be shorter, because the control-loop feedback would be faster, because the effective inertia of the braked wheel would be much less (as you say) but there's no technological issue, just perhaps a cost issue. It would be almost like a stepper motor control, but doing the opposite, of course. This'd prob. be more useful for going slow over slick/ugly stuff rather than saving your butt if you're going too fast over slick stuff. Now 'could be done' is a lot different than 'anyone has' or 'its paractical to do', I know. Dave Quote Link to comment Share on other sites More sharing options...
nipper Posted December 17, 2007 Share Posted December 17, 2007 Yes, but if the traction-control braking cycle time was fast enough (faster than it would have to be for ABS, I'm guessing) it could pulse the brakes at just the right intensity/cadence to make the braked wheel spin at any desired rate- the desired rate would be whatever the wheel with traction was doing (or match actual vehicle speed measured some other way). The cycle times would have to be shorter, because the control-loop feedback would be faster, because the effective inertia of the braked wheel would be much less (as you say) but there's no technological issue, just perhaps a cost issue. It would be almost like a stepper motor control, but doing the opposite, of course. This'd prob. be more useful for going slow over slick/ugly stuff rather than saving your butt if you're going too fast over slick stuff. Now 'could be done' is a lot different than 'anyone has' or 'its paractical to do', I know. Dave Keep in mind that for subaru, and other car companies, this is all first generation stuff. Thge next generation may use an entirely differnt system. nipper Quote Link to comment Share on other sites More sharing options...
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