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?
25 mph Cruise Control
- Thread starter Newton
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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.
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.
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
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Tech man your reply makes no sense
MidRom
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Here is an Google AI answer that I copied and pasted here ...
"Cruise control systems are typically designed for use on highways and open roads where maintaining a constant speed is safe. Using cruise control at slow speeds in urban areas or heavy traffic can be dangerous as it may not respond quickly enough to changing road conditions or obstacles".
Rob Anthony
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FWIW- my Bolt can engage CC at 24mph. Use it on backroads where 25mph is the limit.
I don't like AI's answer because CC is not supposed to "respond quickly enough to changing road conditions or obstacles". The driver does that and he can do it faster if CC is engaged and he has his foot hovering over the brake pedal. Doing that eliminates the lag time used to move his foot from the accelerator pedal to the brake pedal. Having used CC in school zones with my VW, I feel that the kids were safer when the CC took care of the speed limit while I devoted 100% of my attention to watching for them and other hazards (with my foot hovering over the brake pedal). I guess I will just have to live with this problem unless Chevy decides to fix it with a software update. I'm pretty sure that is all it would take. I should add that, when driving the Bolt in school zones, using "one pedal mode" has a big advantage when you need to slow or stop when a hazard appears because deceleration is much stronger while moving one's foot from the accelerator pedal to the brake pedal (compared to normal mode).
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Joe Cherra
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Joe Cherra
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Everyone should know not to use cruise control in snow or sleet be it 19 mph or 24 miles per hour. If you need to change course with cruise on you are fighting against cruise control
I framed the question a little differently to Google AI snd got back two salient points.
1. Cruise control uses rolling friction, which is constant, plus air resistance which is proportional to effective frontal area times velocity squared. Energy removal by wind resistance is thus proportional to velocity cubed. You can see at higher speeds braking effect due to wind resistance rises dramatically due to the cubing of velocity. Cruise control uses those passive braking effects to remove over speed. At low speed there is little wind resistance leaving primarily rolling friction to remove over speed. Rolling friction alone is insufficient to remove over speed. A solution would be for the cruise system to apply braking - it could on a Bolt by using regeneration. Apparently the designers did not want to add in braking factor to the basic cruise system. They probably do on the more expensive adaptive cruise. The designers simply chose to disable cruise at low speed when passive braking is insufficient for reliable operation.
2. Engineering by lawyers. Low speed city driving is more likely to suddenly present unexpected obstacles and designers want the driver fully in control of all aspects of vehicle control. It also helps absolve the manufacturer of responsibility in the event of a crash. The driver can not try to blame the cruise system because it was not in operation at the time of the low speed crash.
1. Cruise control uses rolling friction, which is constant, plus air resistance which is proportional to effective frontal area times velocity squared. Energy removal by wind resistance is thus proportional to velocity cubed. You can see at higher speeds braking effect due to wind resistance rises dramatically due to the cubing of velocity. Cruise control uses those passive braking effects to remove over speed. At low speed there is little wind resistance leaving primarily rolling friction to remove over speed. Rolling friction alone is insufficient to remove over speed. A solution would be for the cruise system to apply braking - it could on a Bolt by using regeneration. Apparently the designers did not want to add in braking factor to the basic cruise system. They probably do on the more expensive adaptive cruise. The designers simply chose to disable cruise at low speed when passive braking is insufficient for reliable operation.
2. Engineering by lawyers. Low speed city driving is more likely to suddenly present unexpected obstacles and designers want the driver fully in control of all aspects of vehicle control. It also helps absolve the manufacturer of responsibility in the event of a crash. The driver can not try to blame the cruise system because it was not in operation at the time of the low speed crash.
Joe Cherra
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Not Bolts! "L" transaxels brake as soon as your foot leaves the pedal. If you're in "Cruise" the paddle will activate the "L". No danger about lo speeds.
I don't recall ever having a problem with overspeed while using CC in school zones but I live in flat land. I don't agree with the policy of having CC work only at speeds above 25 mph. Does anybody know if this policy applies to all GM vehicles? Wikipedia says some German cities have a 19 mph speed limit in residential areas. It doesn't say anything about school zones. That may be why my VW's CC worked at 20 mph. My Mazdas' CCs have also worked at 20 mph.
Joe Cherra
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Joe Cherra
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Just the facts. My Bolt Idles at 3 mph.I walk at 2.7@ mph. That avg. Pedestrian walks @ 2.5 - 3 mph. There is no good rational for minimum cruise control speed @ 24 mph!
GM just wants to cover their ass
GM just wants to cover their ass
One other problem with driving the Bolt in a school zone is that it is quiet. ICEs make noise that changes with vehicle speed. The noise helps the driver know if speed is increasing or decreasing. So, if the driver is traveling 20 mph at the beginning of a school zone, he has a clue about his speed in the school zone from the sound of the engine. So he doesn't have to look at the speedometer as often as the driver of a Bolt. Time spent looking at the speedometer is time not watching for children. If we can't have CC, maybe we should attach playing cards so the spokes of our wheels rub against them to make noise.
Joe Cherra
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Joe Cherra
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My bolt wines. I can actually hear it and the pedestrians that I pass look my way to find out what the noise is about. I think it is due to when the EVS first came out the hearing impaired in blind were complaining that there was no noise from the EVS.
ClarD
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When I take my dog for a walk, a passing Bolt always catches his attention. He is able to hear my wife pulling into our driveway when we are in the front room and the car is about a house away.
He isn't interested in other EVs, which I find to be louder at 15 MPH than most gas cars.
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