Braking down a very long steep hill
Discussion
A certain place I go to every few months requires driving down a very long steep hill from the top of a mountain to almost sea level.
The hill is about 0.5 miles long, during which the altitude decreases from 280 metres to 75 metres (205m descent). This is around 25% on average, but is around 35% in some places.
The road is tarmaced and straight, the hairpins are wide, and 30mph would be a reasonable speed to travel down.
How would you go about controlling your speed, from and AD and brake preservation point of view? Engine braking is useless (at least in a 1.6 petrol), you would easily hit the limiter in any gear.
1. Maintain a constant pressure on the pedal to keep it around 30.
2. Let it creep up to 35-40 then brake to bring it down to 30, repeat.
The hill is about 0.5 miles long, during which the altitude decreases from 280 metres to 75 metres (205m descent). This is around 25% on average, but is around 35% in some places.
The road is tarmaced and straight, the hairpins are wide, and 30mph would be a reasonable speed to travel down.
How would you go about controlling your speed, from and AD and brake preservation point of view? Engine braking is useless (at least in a 1.6 petrol), you would easily hit the limiter in any gear.
1. Maintain a constant pressure on the pedal to keep it around 30.
2. Let it creep up to 35-40 then brake to bring it down to 30, repeat.
The only reason I can think that you would want to do anything other than "1" is if your brakes are overheating (although I'm not sure if "2" would prevent that). If your brakes are OK, then there's no problem.
I'm not sure what you mean when you say engine braking is useless. If you're hitting the limiter in top gear, you must be going a lot faster than 30mph. You might not be able to use engine braking alone to prevent the car from going too fast, but you should be able to use it to some degree. If the engine's revving too fast at 30mph in, say 2nd, then use 3rd (I'm guessing these are the relevant ratios in your car at that speed) and you'll have at least some retarding effect from the engine.
I'm not sure what you mean when you say engine braking is useless. If you're hitting the limiter in top gear, you must be going a lot faster than 30mph. You might not be able to use engine braking alone to prevent the car from going too fast, but you should be able to use it to some degree. If the engine's revving too fast at 30mph in, say 2nd, then use 3rd (I'm guessing these are the relevant ratios in your car at that speed) and you'll have at least some retarding effect from the engine.
The brakes are fine, I was just wondering which one would be best for keeping the temperature down.
Of course I kept it in gear for some effect, but I didn't think about your last point that you would have more engine braking in a higher gear if the lower one would cause the limiter to be hit.
Of course I kept it in gear for some effect, but I didn't think about your last point that you would have more engine braking in a higher gear if the lower one would cause the limiter to be hit.
robwales said:
Of course I kept it in gear for some effect, but I didn't think about your last point that you would have more engine braking in a higher gear if the lower one would cause the limiter to be hit.
I'm not sure that is true. I expect that the way to preserve the brakes as well as possible is to select 2nd and apply enough braking to maintain 30. In spite of the cooling between applications of the brakes, I think they will have less work to do if you keep the speed down. But I don't think that a normal car should have a problem coping with this length of descent.In that situation I select a lower gear and use engine braking. I then use the footbrake as necessary - personally I prefer to brake for the hairpins, go around them, and then allow speed to build up whilst off the brakes and then brake again - giving them some cooling time.
I expect this is completely unnecessary but it is what I prefer to do.
Oh...and get drilled disk brakes...
Fade free even after repeated heavy braking...
I expect this is completely unnecessary but it is what I prefer to do.
Oh...and get drilled disk brakes...
Fade free even after repeated heavy braking...
GreenV8S said:
Get grooved discs if your brakes are going spongy due to overheating pads, never drilled. Drilled discs are the work of the devil.
Don said:
In that situation I select a lower gear and use engine braking. I then use the footbrake as necessary - personally I prefer to brake for the hairpins, go around them, and then allow speed to build up whilst off the brakes and then brake again - giving them some cooling time.
I expect this is completely unnecessary but it is what I prefer to do.
Oh...and get drilled disk brakes... Fade free even after repeated heavy braking...
I don't know about the relative merits of drilled discs versus grooved ones (as Peter mentions later) as I've never had anything other than plain ones, but I agree with the braking technique you use and it's what I do when descending severe hills. It may not be necessary with modern braking systems in proper mechanical condition, but it is a sensible precaution.I expect this is completely unnecessary but it is what I prefer to do.
Oh...and get drilled disk brakes...
Fade free even after repeated heavy braking...Best wishes all,
Dave.
GreenV8S said:
Don said:
Oh...and get drilled disk brakes... Fade free even after repeated heavy braking...
Fade free even after repeated heavy braking...Get grooved discs if your brakes are going spongy due to overheating pads, never drilled. Drilled discs are the work of the devil.
So. Why do you prefer grooved disks? Very interested.
Sorry about the digression
Porsche, I believe, cast the holes into the disk. Aftermarket "drilled" disks for other cars are just that, drilled. This then leaves stress raisers (not sure if that's the correct term) into the disk so excess heat can cause cracking.
The sponginess is due to heat causing gasses to be emitted from the pads (vapourising of the resin?) that then forms a barrier between disk and pad that you have to push through to get the brakes to work. Holes or grooves in the disk will whisk the gases away giving immediate disk-pad contact on brake application.
The sponginess is due to heat causing gasses to be emitted from the pads (vapourising of the resin?) that then forms a barrier between disk and pad that you have to push through to get the brakes to work. Holes or grooves in the disk will whisk the gases away giving immediate disk-pad contact on brake application.
LexSport said:
Porsche, I believe, cast the holes into the disk. Aftermarket "drilled" disks for other cars are just that, drilled. This then leaves stress raisers (not sure if that's the correct term) into the disk so excess heat can cause cracking.
The sponginess is due to heat causing gasses to be emitted from the pads (vapourising of the resin?) that then forms a barrier between disk and pad that you have to push through to get the brakes to work. Holes or grooves in the disk will whisk the gases away giving immediate disk-pad contact on brake application.
If the manufacturers of drilled discs were to de-burr and slightly countersink (or better still, radius) the holes, might this be sufficient to overcome the stress-raising problem?The sponginess is due to heat causing gasses to be emitted from the pads (vapourising of the resin?) that then forms a barrier between disk and pad that you have to push through to get the brakes to work. Holes or grooves in the disk will whisk the gases away giving immediate disk-pad contact on brake application.
What you say about the cause of sponginess is interesting, and I didn't know it was due to a change in the nature of the pad material. I always thought of it as being caused by overheating of the brake fluid, creating vapour in the hydraulic system.
Best wishes all,
Dave.
p1esk said:
If the manufacturers of drilled discs were to de-burr and slightly countersink (or better still, radius) the holes, might this be sufficient to overcome the stress-raising problem?
What you say about the cause of sponginess is interesting, and I didn't know it was due to a change in the nature of the pad material. I always thought of it as being caused by overheating of the brake fluid, creating vapour in the hydraulic system.
Both do happen, although it's normaly highly contaminated (with water) hydraulic fluids that exhibit the vapour phenomenom. More often than not it's the pad matrix going over temperature and a) losing friction and b) losing pad modulus (i.e. they compress more with the hydraulic pressure applied) that's to blame.What you say about the cause of sponginess is interesting, and I didn't know it was due to a change in the nature of the pad material. I always thought of it as being caused by overheating of the brake fluid, creating vapour in the hydraulic system.
StressedDave said:
p1esk said:
If the manufacturers of drilled discs were to de-burr and slightly countersink (or better still, radius) the holes, might this be sufficient to overcome the stress-raising problem?
What you say about the cause of sponginess is interesting, and I didn't know it was due to a change in the nature of the pad material. I always thought of it as being caused by overheating of the brake fluid, creating vapour in the hydraulic system.
Both do happen, although it's normaly highly contaminated (with water) hydraulic fluids that exhibit the vapour phenomenom. More often than not it's the pad matrix going over temperature and a) losing friction and b) losing pad modulus (i.e. they compress more with the hydraulic pressure applied) that's to blame.What you say about the cause of sponginess is interesting, and I didn't know it was due to a change in the nature of the pad material. I always thought of it as being caused by overheating of the brake fluid, creating vapour in the hydraulic system.
When I started driving we never used to hear about brake fluid being changed on a routine basis the way we do now. Given that it so readily absorbes moisture from the atmosphere, perhaps reducing the risk of vapour creation is the reason for changing it.
With regard to loss of frictional properties of the pad material, I've usually understood this to be a temporary effect and that the pads recover as they cool so they are not permanently damaged. Is the loss of pad modulus also temporary?
Best wishes all,
Dave.
StressedDave said:
The old advice used to be every 24 months, although I understand many servicing outfits actually test the fluid and decide rather than automatically doing it.
Yes, I've encountered that when having the old Pug. serviced. I expect they've got a device that checks the water content, and maybe other qualities.BTW, am I right in saying that we didn't used to renew brake fluid at all on a routine basis - at any kind of intervals. Admittedly I'm thinking back to the 1950s/1960s, but I don't recall it being mentioned then.
Best wishes all,
Dave.
StressedDave said:
Pad modulus does tend to return unless the over-temperature condition has changed the matrix structure. In that case you've permanently damaged both. I fitted some pads (once!) that did just that.
...translates as...They'll be fine unless they actually catch fire...
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