5 ms·

> And a colleague of mine with a late 90's Volkswagen also proved that the throttle slams shut if he rides the brakes while requesting engine power with the gas

by pedrocr 12y ago

> And a colleague of mine with a late 90's Volkswagen also proved that the throttle slams shut if he rides the brakes while requesting engine power with the gas pedal. Maybe automatic cars do this but when driving a stick shift a common advanced driving technique is to heel-and-toe on downshift so that you can rev up the engine to the correct RPM to not upset the car. That requires revving up the engine under braking. As far as I know this is still possible in most cars. The usual guarantee is that the brakes are always specified to be able to overpower the engine even at full throttle. So even if you have a stuck throttle for some reason you should always be able to safely stop just by standing on the brakes, even if it takes you a little longer. Naturally in a manual car you should just stand on the brakes and clutch for emergency braking and then the engine is completely disconnected no matter what the throttle is doing. [1] https://en.wikipedia.org/wiki/Heel-and-toe https://en.wikipedia.org/wiki/Heel-and-toe

serf 12y ago
blipping the throttle while clutched to match RPM between the forced road speed and given engine RPM during a gear shift to alleviate weight-shift(heel-and-toe) is still possible on modern cars, but that is not the technique that is being mentioned. the technique being mentioned is more akin to 'left-foot braking', a technique used to pivot the weight balance from the rear to the front in order to induce certain driving characteristics that may be beneficial in a turn. It is a common technique to balance out the inherent understeer of a front-wheel drive car to a more neutral balance mid-turn in an effort to reduce lap times. It's quite common in rally.[0] and as was said, many cars disallow left-foot braking now; with the worst responses triggering semi-permanent CELs (CELs which require mechanic intervention, as opposed to being clear by more drive-cycles). Probably not a bad idea, as it's an advanced technique that quite easily upsets a car mid-turn, and is often just compensatory for an ill-setup race car. [0]: http://en.wikipedia.org/wiki/Left-foot_braking http://en.wikipedia.org/wiki/Left-foot_braking
- sliverstorm 12y ago
  You're mixing up techniques. Heel-toe is so called because you are using your right toe to operate the brake and your right heel to operate the gas pedal (at the same time) while the left foot operates the clutch. The technique is used in racing during corner entry to allow the driver to rev-match downshift while under hard braking. This way they are already in the optimal gear as they release the brakes and initiate the turn
- pedrocr 12y ago
  > the technique being mentioned is more akin to 'left-foot braking' Both heel-and-toe and left foot braking require revving up the engine under braking. The difference between the two is if the car is in gear and the clutch engaged at the time. Maybe modern cars have enough sensors to know if the engine is actually driving the wheels so they can disable the throttle under braking in those situations.
  - kw71 12y ago
    > Maybe modern cars have enough sensors to know if the engine is actually driving the wheels so they can disable the throttle under braking in those situations. This is possible, any car with traction control has wheelspeed sensors for all wheels, and manual shift cars have a pushbutton switch on the clutch pedal arm (for enabling the starter, or for a car with electronic throttle, for cancelling the cruise control.)
    - pedrocr 12y ago
      Those are not enough. You need a sensor in the gearbox itself to know if you are in gear. Otherwise you can have the clutch engaged and the wheels spinning but not have the engine connected to the wheels as naturally happens in the middle of a double clutch shift.
      - kw71 12y ago
        Couldn't it work by checking that crankshaft speed increases remarkably without any corresponding increase in wheel speed? I don't know how racetrack drivers do this, but when I downshift, the crankshaft has some opportunity to fall a little while I get the gearbox in neutral and engage the clutch again. Then the crankshaft speed rises to 4000 rpm or more.
kw71 12y ago
It takes more than 1 s for the throttle to shut, so it's still possible to spin up the gearbox in neutral to downshift. My friend's VW was a manual shift, some variant of Golf, and he did encounter this behavior on a track day.
- pedrocr 12y ago
  Yeah, a time lag is probably a good solution for this. Only kill the throttle if brakes are applied and the throttle has been open for >Xs. That would allow most shifting techniques without issue but wouldn't work for left-foot braking.
  - kw71 12y ago
    I found a test plan for this on the old 5 series [1] BMW say the engine should return to idle speed "immediately." The wiring diagrams imply that for this car, the electronic throttle was only applied to cars with automatic gearboxes and traction controls, the benefit here being that the electronic throttle can act to limit engine power during gearbox shifting or during intervention of the traction control. So it looks like VW was rather thoughtful to introduce this delay, when there is a manual gearbox. Wouldn't the left foot braking be more useful in a car with a really light front end, like MR2 or other mid-engines? [Edit: My 5 series is a manual shift with traction control, with a v8 engine that never got electronic throttle on this series. There is a second throttle plate ahead of the usual throttle that the traction control will close during intervention.] [1] https://www.bmwtechinfo.com/repair/main/741en/images/7410265.jpg https://www.bmwtechinfo.com/repair/main/741en/images/7410265...

by pedrocr 12y ago

serf 12y ago

blipping the throttle while clutched to match RPM between the forced road speed and given engine RPM during a gear shift to alleviate weight-shift(heel-and-toe) is still possible on modern cars, but that is not the technique that is being mentioned. the technique being mentioned is more akin to 'left-foot braking', a technique used to pivot the weight balance from the rear to the front in order to induce certain driving characteristics that may be beneficial in a turn. It is a common technique to balance out the inherent understeer of a front-wheel drive car to a more neutral balance mid-turn in an effort to reduce lap times. It's quite common in rally.[0] and as was said, many cars disallow left-foot braking now; with the worst responses triggering semi-permanent CELs (CELs which require mechanic intervention, as opposed to being clear by more drive-cycles). Probably not a bad idea, as it's an advanced technique that quite easily upsets a car mid-turn, and is often just compensatory for an ill-setup race car. [0]: http://en.wikipedia.org/wiki/Left-foot_braking http://en.wikipedia.org/wiki/Left-foot_braking

sliverstorm 12y ago
You're mixing up techniques. Heel-toe is so called because you are using your right toe to operate the brake and your right heel to operate the gas pedal (at the same time) while the left foot operates the clutch. The technique is used in racing during corner entry to allow the driver to rev-match downshift while under hard braking. This way they are already in the optimal gear as they release the brakes and initiate the turn
pedrocr 12y ago
> the technique being mentioned is more akin to 'left-foot braking' Both heel-and-toe and left foot braking require revving up the engine under braking. The difference between the two is if the car is in gear and the clutch engaged at the time. Maybe modern cars have enough sensors to know if the engine is actually driving the wheels so they can disable the throttle under braking in those situations.
- kw71 12y ago
  > Maybe modern cars have enough sensors to know if the engine is actually driving the wheels so they can disable the throttle under braking in those situations. This is possible, any car with traction control has wheelspeed sensors for all wheels, and manual shift cars have a pushbutton switch on the clutch pedal arm (for enabling the starter, or for a car with electronic throttle, for cancelling the cruise control.)
  - pedrocr 12y ago
    Those are not enough. You need a sensor in the gearbox itself to know if you are in gear. Otherwise you can have the clutch engaged and the wheels spinning but not have the engine connected to the wheels as naturally happens in the middle of a double clutch shift.
    - kw71 12y ago
      Couldn't it work by checking that crankshaft speed increases remarkably without any corresponding increase in wheel speed? I don't know how racetrack drivers do this, but when I downshift, the crankshaft has some opportunity to fall a little while I get the gearbox in neutral and engage the clutch again. Then the crankshaft speed rises to 4000 rpm or more.

kw71 12y ago

It takes more than 1 s for the throttle to shut, so it's still possible to spin up the gearbox in neutral to downshift. My friend's VW was a manual shift, some variant of Golf, and he did encounter this behavior on a track day.

pedrocr 12y ago
Yeah, a time lag is probably a good solution for this. Only kill the throttle if brakes are applied and the throttle has been open for >Xs. That would allow most shifting techniques without issue but wouldn't work for left-foot braking.
- kw71 12y ago
  I found a test plan for this on the old 5 series [1] BMW say the engine should return to idle speed "immediately." The wiring diagrams imply that for this car, the electronic throttle was only applied to cars with automatic gearboxes and traction controls, the benefit here being that the electronic throttle can act to limit engine power during gearbox shifting or during intervention of the traction control. So it looks like VW was rather thoughtful to introduce this delay, when there is a manual gearbox. Wouldn't the left foot braking be more useful in a car with a really light front end, like MR2 or other mid-engines? [Edit: My 5 series is a manual shift with traction control, with a v8 engine that never got electronic throttle on this series. There is a second throttle plate ahead of the usual throttle that the traction control will close during intervention.] [1] https://www.bmwtechinfo.com/repair/main/741en/images/7410265.jpg https://www.bmwtechinfo.com/repair/main/741en/images/7410265...