25 mph Cruise Control

[Newton]

Back in the '70s, when I got my first car with cruise control (a Pontiac), it would not work below about 28 mph, IIRC. This made it useless in active school zones. Around here, the speed limit is 20 mph in active school zones. So, I had to keep checking the speedometer while simultaneously watching for errant children. That was unsafe. Thankfully, the cars I owned after that one had cruise control that worked in school zones. Then, I was able to set the speed at 20 and keep my eyes outside the car looking for hazards. That is much safer. Now, it's back to the bad old '70s with the Bolt. Why did Chevy make the lowest speed for the cruise control 25 mph? There surely is not a technical reason why it couldn't work at 20 mph. My VW would go down to 15 mph, IIRC. Does anybody know why Chevy made this decision?

 

[Newton]

Any theories?

 

[Tech Admin]

I can only think that in situations lower than 25mph, the driver needs to be more alert for stops and starts. But that is only a best guess.

 

[BillM]

II had the same thought. GM apparently feels the risks of having cruise engaged at speeds below 25 MPH outweighs the benefits.

The old analog system in the 1970s used two weights inside a box that moved outward from a shaft as speed increased. Levers on the weights pulled on a slide which progressively uncovered a vacuum bleed. That bleed rate was compared with another bleed rate set by pressing the cruise button. The comparison valve set the amount odd vacuum applied to the bellows connected to the throttle by a chain. If the car slowed down, the vacuum in the bellows would increase and pull more on the chain to restore speed.

The differential equation governing the vacuum system was probably second order at best, so. There tended to be enough response damping,to causing some lag to prevent overshoot and oscillation/hunting around the set point. The response was somewhat non ideal. That lag factor may have been why GM felt the system was unsuitable below 25 MPH. The vacuum system was supposed to work down to 25 MPH, if 28 MPH, something was probably out of tune.

The electronic system in the Bolt should be looking at the vehicle speed sensor (VSS) output which is typically 4,000 pulses per mile, though I don’t know what the pulse rate in the Bolt is. The cruise control response can be digitally computed to arbitrary precision within limits of the input data. The limiting factor would be the VSS pulse rate. At very low speed the VSS pulse frequency would be too low to make precise calculations. Again, 25 MPH may have been selected as the point below which the math is not precise enough to provide acceptable control.

My guess is 25 MPH is somewhat arbitrary and based on corporate history.

 

[Joe Cherra]

II had the same thought. GM apparently feels the risks of having cruise engaged at speeds below 25 MPH outweighs the benefits.

The old analog system in the 1970s used two weights inside a box that moved outward from a shaft as speed increased. Levers on the weights pulled on a slide which progressively uncovered a vacuum bleed. That bleed rate was compared with another bleed rate set by pressing the cruise button. The comparison valve set the amount odd vacuum applied to the bellows connected to the throttle by a chain. If the car slowed down, the vacuum in the bellows would increase and pull more on the chain to restore speed.

The differential equation governing the vacuum system was probably second order at best, so. There tended to be enough response damping,to causing some lag to prevent overshoot and oscillation/hunting around the set point. The response was somewhat non ideal. That lag factor may have been why GM felt the system was unsuitable below 25 MPH. The vacuum system was supposed to work down to 25 MPH, if 28 MPH, something was probably out of tune.

The electronic system in the Bolt should be looking at the vehicle speed sensor (VSS) output which is typically 4,000 pulses per mile, though I don’t know what the pulse rate in the Bolt is. The cruise control response can be digitally computed to arbitrary precision within limits of the input data. The limiting factor would be the VSS pulse rate. At very low speed the VSS pulse frequency would be too low to make precise calculations. Again, 25 MPH may have been selected as the point below which the math is not precise enough to provide acceptable control.

My guess is 25 MPH is somewhat arbitrary and based on corporate history.