High Speed Braking

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High Speed Braking

Postby windu34 » April 27th, 2016, 6:14 pm

I have been having some trouble with braking my vehicle. My run time is about 1.5s and I'm using a brushless setup. I have been having some trouble with the vehicle "fishtailing" and curving when the vehicle starts to decelerate. The vehicle brakes using the motor which is located in the rear. We have tried adding weight above the rear wheels, increasing the coefficient of friction between the wheels and the floor, as well as lowering the brake power, but all have not had a significant impact on the problem described. The vehicle travels straight during the acceleration period, it is only near the end of the braking that the vehicle experiences this effect. Any suggestions? Thanks
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Re: High Speed Braking

Postby Bazinga+ » April 27th, 2016, 6:53 pm

windu34 wrote:I have been having some trouble with braking my vehicle. My run time is about 1.5s and I'm using a brushless setup. I have been having some trouble with the vehicle "fishtailing" and curving when the vehicle starts to decelerate. The vehicle brakes using the motor which is located in the rear. We have tried adding weight above the rear wheels, increasing the coefficient of friction between the wheels and the floor, as well as lowering the brake power, but all have not had a significant impact on the problem described. The vehicle travels straight during the acceleration period, it is only near the end of the braking that the vehicle experiences this effect. Any suggestions? Thanks

You need the same amount of distance to accelerate as to deccelerate, and if these problems are only present during decceleration, then you just need to decrease the rate at which the vehicle slows down (ideally it should be speeding up half the travel, and slowing down the rest).
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Re: High Speed Braking

Postby HandsFreeCookieDunk » April 27th, 2016, 6:58 pm

windu34 wrote:I have been having some trouble with braking my vehicle. My run time is about 1.5s and I'm using a brushless setup. I have been having some trouble with the vehicle "fishtailing" and curving when the vehicle starts to decelerate. The vehicle brakes using the motor which is located in the rear. We have tried adding weight above the rear wheels, increasing the coefficient of friction between the wheels and the floor, as well as lowering the brake power, but all have not had a significant impact on the problem described. The vehicle travels straight during the acceleration period, it is only near the end of the braking that the vehicle experiences this effect. Any suggestions? Thanks


Someone else might have to correct me if I'm wrong, but I believe that fishtailing will only occur in rear wheel drive vehicles, so maybe you could turn your vehicle around and reverse the direction of the motor so it becomes front wheel drive.

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Re: High Speed Braking

Postby windu34 » April 27th, 2016, 7:09 pm

HandsFreeCookieDunk wrote:
windu34 wrote:I have been having some trouble with braking my vehicle. My run time is about 1.5s and I'm using a brushless setup. I have been having some trouble with the vehicle "fishtailing" and curving when the vehicle starts to decelerate. The vehicle brakes using the motor which is located in the rear. We have tried adding weight above the rear wheels, increasing the coefficient of friction between the wheels and the floor, as well as lowering the brake power, but all have not had a significant impact on the problem described. The vehicle travels straight during the acceleration period, it is only near the end of the braking that the vehicle experiences this effect. Any suggestions? Thanks


Someone else might have to correct me if I'm wrong, but I believe that fishtailing will only occur in rear wheel drive vehicles, so maybe you could turn your vehicle around and reverse the direction of the motor so it becomes front wheel drive.

I believe you are correct, but Id really rather not change my vehicle seeing as how nationals is rapidly approaching. I was actually considering a parachute-like apparatus controlled by a servo that could help slow the vehicle, but if there is something simpler to implement, Id rather take that route.
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Re: High Speed Braking

Postby windu34 » April 27th, 2016, 7:10 pm

Bazinga+ wrote:
windu34 wrote:I have been having some trouble with braking my vehicle. My run time is about 1.5s and I'm using a brushless setup. I have been having some trouble with the vehicle "fishtailing" and curving when the vehicle starts to decelerate. The vehicle brakes using the motor which is located in the rear. We have tried adding weight above the rear wheels, increasing the coefficient of friction between the wheels and the floor, as well as lowering the brake power, but all have not had a significant impact on the problem described. The vehicle travels straight during the acceleration period, it is only near the end of the braking that the vehicle experiences this effect. Any suggestions? Thanks

You need the same amount of distance to accelerate as to deccelerate, and if these problems are only present during decceleration, then you just need to decrease the rate at which the vehicle slows down (ideally it should be speeding up half the travel, and slowing down the rest).

Do I though? I feel like there should be a way to rapidly brake using a brake curve or changing the way/rate I apply the brake.
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Re: High Speed Braking

Postby Chris_L » April 27th, 2016, 7:16 pm

windu34 wrote:
Bazinga+ wrote:
windu34 wrote:I have been having some trouble with braking my vehicle. My run time is about 1.5s and I'm using a brushless setup. I have been having some trouble with the vehicle "fishtailing" and curving when the vehicle starts to decelerate. The vehicle brakes using the motor which is located in the rear. We have tried adding weight above the rear wheels, increasing the coefficient of friction between the wheels and the floor, as well as lowering the brake power, but all have not had a significant impact on the problem described. The vehicle travels straight during the acceleration period, it is only near the end of the braking that the vehicle experiences this effect. Any suggestions? Thanks

You need the same amount of distance to accelerate as to deccelerate, and if these problems are only present during decceleration, then you just need to decrease the rate at which the vehicle slows down (ideally it should be speeding up half the travel, and slowing down the rest).

Do I though? I feel like there should be a way to rapidly brake using a brake curve or changing the way/rate I apply the brake.


Why don't you make the rear wheel axle longer, making the vehicle wider? And then add more weight to the very back? That could solve ur problem of the back brake fishtailing? Maybe I'm wrong though but that's my intuition.
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Re: High Speed Braking

Postby Bazinga+ » April 27th, 2016, 7:19 pm

windu34 wrote:
Bazinga+ wrote:
windu34 wrote:I have been having some trouble with braking my vehicle. My run time is about 1.5s and I'm using a brushless setup. I have been having some trouble with the vehicle "fishtailing" and curving when the vehicle starts to decelerate. The vehicle brakes using the motor which is located in the rear. We have tried adding weight above the rear wheels, increasing the coefficient of friction between the wheels and the floor, as well as lowering the brake power, but all have not had a significant impact on the problem described. The vehicle travels straight during the acceleration period, it is only near the end of the braking that the vehicle experiences this effect. Any suggestions? Thanks

You need the same amount of distance to accelerate as to deccelerate, and if these problems are only present during decceleration, then you just need to decrease the rate at which the vehicle slows down (ideally it should be speeding up half the travel, and slowing down the rest).

Do I though? I feel like there should be a way to rapidly brake using a brake curve or changing the way/rate I apply the brake.

Nope. The maximum decelerating force is equal to the friction coefficient times the weight of the car pushing down in the wheels. It is physically impossible to decelerate more rapidly than that using the wheels. Not sure if you are still using a wingnut breaking system, but if you are, you will need to implement a spring break, so the car slows down continuously rather than immediately and completely. If you are using the motor to break, I advise to use timing to your advantage, and time it so the time it takes for the motor to get to full speed is the same as to get back to a complete stop. Other fixes won't( correction, shouldn't ) work since you are decelerating too fast.
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Re: High Speed Braking

Postby windu34 » April 27th, 2016, 7:33 pm

Bazinga+ wrote:Nope. The maximum decelerating force is equal to the friction coefficient times the weight of the car pushing down in the wheels. It is physically impossible to decelerate more rapidly than that using the wheels. Not sure if you are still using a wingnut breaking system, but if you are, you will need to implement a spring break, so the car slows down continuously rather than immediately and completely. If you are using the motor to break, I advise to use timing to your advantage, and time it so the time it takes for the motor to get to full speed is the same as to get back to a complete stop. Other fixes won't( correction, shouldn't ) work since you are decelerating too fast.

I am no longer using the wingnut braking system and am using the motor. If I were to do that, I would have to sacrifice my time score by at least .4 seconds which would result in 25% more points than I intend on scoring. There must be another way to do this that we have not thought of. Maybe a way to apply friction to the front axle during the braking time and keeping it friction-less during acceleration.
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Re: High Speed Braking

Postby windu34 » April 27th, 2016, 7:33 pm

Chris_L wrote:
Why don't you make the rear wheel axle longer, making the vehicle wider? And then add more weight to the very back? That could solve ur problem of the back brake fishtailing? Maybe I'm wrong though but that's my intuition.

How would making the rear wider help?
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Re: High Speed Braking

Postby Bazinga+ » April 27th, 2016, 7:54 pm

windu34 wrote:
Bazinga+ wrote:Nope. The maximum decelerating force is equal to the friction coefficient times the weight of the car pushing down in the wheels. It is physically impossible to decelerate more rapidly than that using the wheels. Not sure if you are still using a wingnut breaking system, but if you are, you will need to implement a spring break, so the car slows down continuously rather than immediately and completely. If you are using the motor to break, I advise to use timing to your advantage, and time it so the time it takes for the motor to get to full speed is the same as to get back to a complete stop. Other fixes won't( correction, shouldn't ) work since you are decelerating too fast.

I am no longer using the wingnut braking system and am using the motor. If I were to do that, I would have to sacrifice my time score by at least .4 seconds which would result in 25% more points than I intend on scoring. There must be another way to do this that we have not thought of. Maybe a way to apply friction to the front axle during the braking time and keeping it friction-less during acceleration.

It should be possible to fine tune your car so you have a solid break and don't sacrifice any time. Right now your car is more or less sliding to a stop, which is the maximum deceleration you can have. If you fine tune it so it degenerates just a bit less rapidly than it currently is you will have about the same time and improved accuracy. So just play around with the rate if breaking, and find the optimal balance between accuracy and speed.
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Re: High Speed Braking

Postby dcambrid » April 28th, 2016, 6:02 am

When you say motor braking are you just setting the speed to 0 or reversing the speed for a few ms? We saw something similar to what you describe, we used a loop to reduce the speed every 5ms/10ms down to zero and we noticed a slight turn to the left which looked like skidding. The only explanation I came up with was that our driving gear is on the left side of the axle and could be causing the left wheels to slow just slightly quicker than the right. From searching online I believe the term is Torque steering which you sometimes see in rear wheel drive vehicles. We also saw a similar curve the other direction when speeding up quickly. Now our vehicle is slower than yours (2.5/2.6 secs) and brushed so not sure if that helps any. In the end ours was very consistent so we just planned for it to happen and lined up our vehicle to compensate. If you are just suddenly going from top speed to 0 you will likely skid at the speed you are going. Have you tried a more gradual slow down? We found the controlled slow down to be the most consistent.

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Re: High Speed Braking

Postby windu34 » April 28th, 2016, 8:47 am

dcambrid wrote:When you say motor braking are you just setting the speed to 0 or reversing the speed for a few ms? We saw something similar to what you describe, we used a loop to reduce the speed every 5ms/10ms down to zero and we noticed a slight turn to the left which looked like skidding. The only explanation I came up with was that our driving gear is on the left side of the axle and could be causing the left wheels to slow just slightly quicker than the right. From searching online I believe the term is Torque steering which you sometimes see in rear wheel drive vehicles. We also saw a similar curve the other direction when speeding up quickly. Now our vehicle is slower than yours (2.5/2.6 secs) and brushed so not sure if that helps any. In the end ours was very consistent so we just planned for it to happen and lined up our vehicle to compensate. If you are just suddenly going from top speed to 0 you will likely skid at the speed you are going. Have you tried a more gradual slow down? We found the controlled slow down to be the most consistent.

Our vehicle travels backwards so we usually are accelerating right up to the 8.5m line and then brake. While it doesn't really matter how far our vehicle travels while braking, we don't know for sure that there will be an unobstructed path beyond the target point so we are trying to brake as quickly as possible and if needed, adjust the length of our acceleration if there is an obstruction. Our axle connecting the rear wheels is one solid piece (no differential) and the length/distances are equal. We are not skidding, I have adjusted the delay in the loop so that it does not skid as far as I can tell, but ill take another loop at it
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Re: High Speed Braking

Postby Bazinga+ » April 28th, 2016, 9:46 am

windu34 wrote:
dcambrid wrote:When you say motor braking are you just setting the speed to 0 or reversing the speed for a few ms? We saw something similar to what you describe, we used a loop to reduce the speed every 5ms/10ms down to zero and we noticed a slight turn to the left which looked like skidding. The only explanation I came up with was that our driving gear is on the left side of the axle and could be causing the left wheels to slow just slightly quicker than the right. From searching online I believe the term is Torque steering which you sometimes see in rear wheel drive vehicles. We also saw a similar curve the other direction when speeding up quickly. Now our vehicle is slower than yours (2.5/2.6 secs) and brushed so not sure if that helps any. In the end ours was very consistent so we just planned for it to happen and lined up our vehicle to compensate. If you are just suddenly going from top speed to 0 you will likely skid at the speed you are going. Have you tried a more gradual slow down? We found the controlled slow down to be the most consistent.

Our vehicle travels backwards so we usually are accelerating right up to the 8.5m line and then brake. While it doesn't really matter how far our vehicle travels while braking, we don't know for sure that there will be an unobstructed path beyond the target point so we are trying to brake as quickly as possible and if needed, adjust the length of our acceleration if there is an obstruction. Our axle connecting the rear wheels is one solid piece (no differential) and the length/distances are equal. We are not skidding, I have adjusted the delay in the loop so that it does not skid as far as I can tell, but ill take another loop at it

As I said before, if you start braking at 8.5 meters and you accelerate as fast as you can up to 8.5 meters, you will need exactly that distance to stop(so the best you can do is stop at 17 meters of you really are accelerating as fast as you can), and it seems like you are able to stop before 17 meters, so that means you could accelerate even faster. So I recommend accelerating faster for the first like 5 or 6 meters and decelerating right after.
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Re: High Speed Braking

Postby windu34 » April 28th, 2016, 7:22 pm

Bazinga+ wrote:As I said before, if you start braking at 8.5 meters and you accelerate as fast as you can up to 8.5 meters, you will need exactly that distance to stop(so the best you can do is stop at 17 meters of you really are accelerating as fast as you can), and it seems like you are able to stop before 17 meters, so that means you could accelerate even faster. So I recommend accelerating faster for the first like 5 or 6 meters and decelerating right after.

I figured it out today. I can brake in three meters without fishtailing coming right off a 1.5ish run
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