Racing Brake Pad / Brake system discussion/questions
01-15-2015, 03:33 AM
#676
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Now worries.... I had to read it a couple of times myself to remember the point. absolutely, there is more engine braking force from the engine in the lower gears to the tires. the point was that the engine braking force goes down with RPM. almost linear. I measured coast down and engine braking, on the dynojet 248e dyno. (20ftlbs down to 10ftlbs at the engine for coast down, but as measured at the wheels, and 180-100ft-lbs during compression for RPM range from 6500 to 4500)
This means through a 4.5:1 3th gear it would be near 800ft-lbs , or 400ft-lbs at each tire. (or 400lbs of force at the road at each tire).
This means through a 4.5:1 3th gear it would be near 800ft-lbs , or 400ft-lbs at each tire. (or 400lbs of force at the road at each tire).
again, remember, under 1g braking, there can be near a 75% of the vehicle weight up on the front wheels, which leaves only 25% of the overall weight to be split between the rear two tires.
I measured rear braking force on jack stands , finding what gear I could go WOT and use the brakes to get constant RPM. since I know the torque of the engine at that RPM, I can deduce that that's the braking force at the wheels. (then divide by the gear ratio to get ft-lbs of force divided by 2 for each wheel...... and since the radius of the tire is near 12", that's the force at the rear tire acting on the pavement as well)
I measured rear braking force on jack stands , finding what gear I could go WOT and use the brakes to get constant RPM. since I know the torque of the engine at that RPM, I can deduce that that's the braking force at the wheels. (then divide by the gear ratio to get ft-lbs of force divided by 2 for each wheel...... and since the radius of the tire is near 12", that's the force at the rear tire acting on the pavement as well)
I also did an experiment where I totally removed the front brakes, and did an engine braking decel in 1st gear vs using just the rear brakes and compared the distance to 10mph from about 45mph. (6000rpm to 1300rpm) it was very close even with a very light bias valve. I even put in a higher rated bias valve and saw a slight difference in the stopping distance.
on the track at the runnoffs, that higher rating bias valve, made terrible tail wagging on trail braking turns, (turn 3 at laguna) that was quickly fixed by going back to the stock valve setting.
So, my comment regarding lifing off the throttle did point to 4th gear being 3.2 to 4:1 and that would mulply the compression braking by 4. the "50ft-lbs" was what the "even if" comment, showing that even a lower compression engine could produce this kind of force, (4x the engine braking force, or near 200ft-lbs) and that might be at turn in. but on my engine at near 200ft-lbs going down to 100ftlbs at turn in, there is a lot of engine braking force that equates to near 800ftlbs going down to 400ftlbs just off throttle and half of that at turn in when the engine RPMs are 75% of redline.
on the track at the runnoffs, that higher rating bias valve, made terrible tail wagging on trail braking turns, (turn 3 at laguna) that was quickly fixed by going back to the stock valve setting.
So, my comment regarding lifing off the throttle did point to 4th gear being 3.2 to 4:1 and that would mulply the compression braking by 4. the "50ft-lbs" was what the "even if" comment, showing that even a lower compression engine could produce this kind of force, (4x the engine braking force, or near 200ft-lbs) and that might be at turn in. but on my engine at near 200ft-lbs going down to 100ftlbs at turn in, there is a lot of engine braking force that equates to near 800ftlbs going down to 400ftlbs just off throttle and half of that at turn in when the engine RPMs are 75% of redline.
I'm not sure how the ABS adds heat if it is reducing braking force. That should be adding less heat, since the brakes are doing less work.
01-15-2015, 04:24 AM
#677
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see my inserts >>>>>>>>>>>>>>>>>>
Wow, if this is right, you get almost as much engine braking as I can with the engine brakes on my diesel pusher.
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I think the diesel pusher has a little more. Yes, its right, it came right off the dyno and you can do a rough estimation on the street too, with KE change vs time.
I really don't see how that measure of brake force can be valid. I can overpower the engine in 1st gear with the brakes -- unless I overheat them first. And if that was not the case, this would be an unstable test for most of the power curve and thus very difficult to find the equilibrium.
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you forgot one very important element.... IM TESTING ONLY the rear brakes! fronts are on the ground, not moving when its on the jack stands.
then I can do a road test as well, with no front brakes ( as I disconnected them totally). so, yes, you are right if the fronts are involved, if not as I tested, the rears are only able to produce 3rd gear, max torque values. and through the gear box that's 400ft-lbs , times 4.5:1 ratio , or near 1800ft-lbs. that's 900ft-lbs per wheel, max
So my problem with the use of engine braking is that coming to a corner with big speed loss, you have to use multiple gears as you approach. To prevent lockup, you need the brake bias set for 2nd gear engine braking -- and you have less in 3rd and even less in 4th gear. That means that rear brake + engine braking is at the rear tire threshold in 2nd gear and you have excess grip that you are not using at higher speeds because you are in 3rd and 4th. So you are leaving a good few percentage of your potential braking on the table.
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Not necessarily. you set it for a balance at 3rd gear for example, and in 2nd, any excess will be fought by the engine. in 4th, you might leave a little on the table, but we are talking such little rear force capability in the rear, I don't think its an issue especially, since you are not going to be in 4th for as much time as 2nd and 3rd combined.
I'm not sure how the ABS adds heat if it is reducing braking force. That should be adding less heat, since the brakes are doing less work.
>>>>>>>>>>>>>>>>>>>>>>>>>
This is a guess, but my logic there is the clamping force is greater, even though the ABS is cycling the force at some specific frequency. I have to think about that one for sec.....
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I think the diesel pusher has a little more. Yes, its right, it came right off the dyno and you can do a rough estimation on the street too, with KE change vs time.
I really don't see how that measure of brake force can be valid. I can overpower the engine in 1st gear with the brakes -- unless I overheat them first. And if that was not the case, this would be an unstable test for most of the power curve and thus very difficult to find the equilibrium.
>>>>>>>>>>>>>>>>>>>>>>>
you forgot one very important element.... IM TESTING ONLY the rear brakes! fronts are on the ground, not moving when its on the jack stands.
then I can do a road test as well, with no front brakes ( as I disconnected them totally). so, yes, you are right if the fronts are involved, if not as I tested, the rears are only able to produce 3rd gear, max torque values. and through the gear box that's 400ft-lbs , times 4.5:1 ratio , or near 1800ft-lbs. that's 900ft-lbs per wheel, max
So my problem with the use of engine braking is that coming to a corner with big speed loss, you have to use multiple gears as you approach. To prevent lockup, you need the brake bias set for 2nd gear engine braking -- and you have less in 3rd and even less in 4th gear. That means that rear brake + engine braking is at the rear tire threshold in 2nd gear and you have excess grip that you are not using at higher speeds because you are in 3rd and 4th. So you are leaving a good few percentage of your potential braking on the table.
>>>>>>>>>>>>>>>
Not necessarily. you set it for a balance at 3rd gear for example, and in 2nd, any excess will be fought by the engine. in 4th, you might leave a little on the table, but we are talking such little rear force capability in the rear, I don't think its an issue especially, since you are not going to be in 4th for as much time as 2nd and 3rd combined.
I'm not sure how the ABS adds heat if it is reducing braking force. That should be adding less heat, since the brakes are doing less work.
>>>>>>>>>>>>>>>>>>>>>>>>>
This is a guess, but my logic there is the clamping force is greater, even though the ABS is cycling the force at some specific frequency. I have to think about that one for sec.....
01-16-2015, 11:19 PM
#678
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Not necessarily. you set it for a balance at 3rd gear for example, and in 2nd, any excess will be fought by the engine. in 4th, you might leave a little on the table, but we are talking such little rear force capability in the rear, I don't think its an issue especially, since you are not going to be in 4th for as much time as 2nd and 3rd combined.
You also said that your measurements show a straight line decrease as the RPM falls. This means you are not at maximum braking at the rear for more than a few moments.
I hear you say that the rears do little compared to the fronts... but overall the rears do 25% of the braking. So you are easily leaving 15% of your braking potential on the table over the course of a braking zone. It's different in F1 where they can harvest energy and electronically top-up the braking force to give the exact driver requirement. For all except a few ms, you are using less than the available... let's assume you are making all brake zones 15% longer than they need to be. That sounds like a lot to me. Especially if competition is close.
Perhaps I'm missing something important.
01-19-2015, 03:23 PM
#680
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So I don't understand how the engine would "fight" the excess. Isn't this just a situation that the wheels are turning the engine against a vacuum and the 400ft-lbs at the flywheel is the force required to spin the engine with the throttle closed at 6000rpm. If the wheels can't, through the gearbox, reach the 400ft-lbs, then the wheels are sliding.
You also said that your measurements show a straight line decrease as the RPM falls. This means you are not at maximum braking at the rear for more than a few moments.
I hear you say that the rears do little compared to the fronts... but overall the rears do 25% of the braking. So you are easily leaving 15% of your braking potential on the table over the course of a braking zone. It's different in F1 where they can harvest energy and electronically top-up the braking force to give the exact driver requirement. For all except a few ms, you are using less than the available... let's assume you are making all brake zones 15% longer than they need to be. That sounds like a lot to me. Especially if competition is close.
Perhaps I'm missing something important.
You also said that your measurements show a straight line decrease as the RPM falls. This means you are not at maximum braking at the rear for more than a few moments.
I hear you say that the rears do little compared to the fronts... but overall the rears do 25% of the braking. So you are easily leaving 15% of your braking potential on the table over the course of a braking zone. It's different in F1 where they can harvest energy and electronically top-up the braking force to give the exact driver requirement. For all except a few ms, you are using less than the available... let's assume you are making all brake zones 15% longer than they need to be. That sounds like a lot to me. Especially if competition is close.
Perhaps I'm missing something important.
400ft-bs value was the torque that the engine is generating at WOT, and if the rear brakes can hold the engine at 4500rpm (peak torque) then the force that the rear brakes has is times the gear ratio of 4.5:1 (3rd gear) , so over 1800ft-lbs at the tires, or 900ft-lbs at each wheel. (that was a result of test)
yes, 25% (average at threshold braking) of the braking is coming from the rears. if Engine braking( even with a low HP / compression engine) can put out 150ft-lbs at the flywheel, that's also x 4.5:1 or 340ftlbs each wheel. that's near the max that you can ever have on the wheel if 300lbs is sitting on it during a 1 g slow down. so any other forces, (i.e. actual rear brakes) will just try and lock up the rear tires during a slow down, especially when there is a trail brake situation involved. (one wheel might have little or no weight on it and lock up very easily via brakes, but via the engine, be limited to a slip %. (i.e. RPM of the engine vs gear ratio vs vehicle speed)
you are never under the braking potential of the rear.... only over it and how you manage that is how you are driving... if for example you have too much bias in the rear. you still have the braking force during decel if you are using the gears..... if that braking force from the engine is matching what the rear tires are capable of creating as force, then any additional braking force will allow rear tires to slip. one might not notice it much during straight line braking , but in a trail brake, its will allow one of the lighter loaded wheels to lock and create great instability. a higher locking LSD will fight this, or even a locker, then this is where the engine decel force is fighting with the rear braking forces. (in other words, you cant lock the rear tires, unless you stall the engine if you have a differential that is a "locker", so examine the forces in the rear of the car)
yes, the engine braking forces go down linear with speed , but they are not 0 before a downshift. they are about half. so one might start out at 150ftbls to 200ftlbs of engine braking at the flywheel, and end up at 75 to 100ft-lbs pre downshift. (that's at the flywheel, so you neeed to multiply this by the gear ratio and divide by 2 for each wheel force) which varies the braking force tremendously in the rear upon each downshift. its why you see a lot of "pros" lock up the rears , like Patrick Dempsy did at laguna last year, as he approached the corkscrew and had a downshift spin. It's not just the downshift forces, it's a combination of the declining forces in the approach gear, and then a 4x increase in engine braking upon a downshift, which might exceed the rear tires capability to grip.
also remember, the potential for rear braking is greatly depending on weight distribution . front engine cars vs 911s have 2x the difference in how much rear brakes they can use and near 15-20% less force available in the front (which saves the fronts actually from over heat, fade, etc)
