Vented Brake Caliper Pistons

Haggis

Haggis

Hero member
Location
UK
For the very fast and the very fat...

For disc brakes that are running a bit hot, I like the simple idea of venting the void inside brake pistons. Either with some grooves like this
vented_piston.jpg

or a number of small holes drilled just inside the pad contact edge.

Easy to do. Paul.
 
LOL, what cock!  Anyway, fat and fast- who had you in mind for these essential mods..?
 
Reduced heat contact area says it all, Hag.  Also reduces heat dissipation.... :o

Might be OK for carbon discs and pads that need a certain amount of heat to work properly, but for steel discs and road pads??

piet
 
An unnecessary mod IMO...when the brakes are off and the pistons withdraw there should be a minute clearance between pad and piston which allows air to circulate anyway.
 
Reggie, it's not 'cock', it's a perfectly valid way of reducing the heat transfer path between pad and fluid, the temperature of which are critical for 'at the edge' performance. I never said anything about it being essential.

Piet/Greg. Of course it's not needed for normal road use. I put the idea up for general interest and also maybe there are one or two folk out there who are struggling with fluid temperature and can accept a little hotter at the pad/disc. In my mind I had somebody like Bob Dixon, who is running small calipers, of a fast bike, on track, endurance, riding in hot countries, generating repeated high pad-to-disc pressures. (Not saying you will cook your fluid Bob, just using you as an example of someone doing things at the upper performance edge of your machine).
 
I don't know where I come in the parameters of very fast or very fat Paul, probably somewhere in the middle! :D :D :D
As you know braking on the track is worlds apart from normal useage. Looking back to last weekend's meeting the brakes would be subjected to four full-on applications every 90 seconds or so and that treatment can go on for four hours [assuming ahem...the bike doesn't pack in].

The heat build-up on track is immense and like you, I'd been wondering what the effect would be on the smaller capacity callipers. As mentioned in a seperate thread, my current brakes work really well for me so I want to stick with the configuration we've got therefore anything which might potentially improve or maintain their performance is worth examining

I'm at the interesting stage in my bike's develoment where I can now start looking at the little things which can collectively add up to a creating a competitive bike. I never dismiss stuff like this until I've seen the science.

What's your opinion on the lihght alloy/teflon coated goldline pistons versus the OE steel ones. The effect on unsprung weight across three callipers is significant but I wonder what the effect on heat dissipation is?
 
Dot 5.1 fluid boils (assuming it's 'dry') at 500 degrees F.  I see rally cars with red hot glowing discs on the telly with the calipers shielded from airflow by the wheels, and their brakes still work OK it seems.
 
Don't forget, problems can occur even if you don't exceed the boiling point of the fluid .On my bike you have to be careful about the brake lever /master cylinder clearance. It has to be left fairly slack for track use because the heat build up in the system expands the fluid [presumably] which can take up the clearance forcing the pads against the disc and exacerbating the problem .

In practice at Rijeka last year the front brakes stuck on after only 4 laps even though there was no sign of the fluid boiling. I'd done quite a bit of testing on the road the previous week and not experienced any problems so my conclusion is heat build up in the discs/pads and subsequent tranference to the fluid can potentially be an issue.
 
G'day Bob. Expanding fluid shouldn't force the pads against the disk. When the brake lever is released, the hydraulic fluid can bleed back into the fluid reservoir through the smaller of the two holes hole in the master cylinder. If the fluid expands, I'd expect the only effect would be to raise the level in the reservoir. Maybe the wee hole in your master cylinder is blocked.

Cheers,
Cam
 
Hi Cam,

i've experienced the same issue as Bob describes, last year in our 1 hour event halfway through the rear caliper started to lock on and by the time the bike got to the pits and pulled up for the rider change the caliper was on fire and well and truly locked in place. Racing certainly tests everything out to the max.

Cheers Crispin
 
Boiling is a common thing on dirt bikes,easy to hold the rear on slightly and heat them up.I consciously move my foot away from the pedal when not using it.There was a test done by the late great John Robinson in Performance Bike Mag years ago on what brake fluid had the highest boiling point,I forget the results but the differences were a lot.It also helps to chance the fluid,its hydroscopic and the water in suspension lowers the boiling point as well.I have had the rear on a couple of bikes overheat and seizes on through lack of freeplay on the lever.I like a minimum of freeplay and a direct action but have adjusted it to close.
 
If cooling the caliper helped to a significant degree we'd still all be running them in front of the sliders wouldn't we?

Using thicker 10mm pads might help a little bit, as would leaving sufficient space in the master cylinder reservoir to allow for any fluid expansion.  And of course making sure the system is as fresh as possible as even around 3% water contamination will reduce the boiling point of the fluid by 25%.
 
Dellortoman said:
G'day Bob. Expanding fluid shouldn't force the pads against the disk. When the brake lever is released, the hydraulic fluid can bleed back into the fluid reservoir through the smaller of the two holes hole in the master cylinder. If the fluid expands, I'd expect the only effect would be to raise the level in the reservoir. Maybe the wee hole in your master cylinder is blocked.

Cheers,
Cam
Click to expand...

You're right of course Cam [as usual... :D :D], but the problem is obviously relatively common judging by Vince and Crispin's comments. Perhaps the heat build up is so rapid that the expansion overcomes the ability of the holes in the master cylinder to flow enough fluid. Who knows? I'll be having a look at the master cylinder at the weekend and will take a peek.
 
Maybe quick men on classic racers are pushing their antique braking systems further than the makers intended.  I don't recall many Brit or World Superbike racers (running steel discs) running out of brakes.
 
It's an interesting point Reggie but I wouldn't delude myself by thinking I'm somehow pushing the brakes more than Messrs Davies, Brettoni, Gallina et al once did.  One thing did occur to me though; modern tyre compounds, even at club level, are so superior to the stuff being raced on in the seventies that it's probably possible to brake later and deeper than our forefathers did [assuming you've got the stones to do it :D] and therefore the brakes could potentially be more stressed.
 
Tyre technology was going to be my next point  :D

Or maybe the brakes are hauling up a chunkier rider.  Oooh, hush my mouth :o
 
"I don't know where I come in the parameters of very fast or very fat Paul, probably somewhere in the middle! Grin Grin Grin"

Hey Bob, of course 'fast' applies to you. I know for sure, because I overheard it said in the pits at Mallory...
Bystander... "Who's that fast bastard on the V6"
Piero... "Oh, that's Bob Dixon"  :D :D :D

Anyway, I like the teflon coated Alu pistons a lot, huge weight saving as you say, which can only be a good thing for reducing momentum for the numerous movements that front suspension and steering have to accomplish.

Having said that Alu has 4/5/6/7 times the thermal conductivity of steel (depending on the alloys), so, in therory, will tranfer the pad heat to the fluid to a greater and faster extent. I guess the teflon coating has an insulating effect, but will be so thin as to make little difference I suspect.

The lower mass of the Alu piston will also allow them to be knocked-back [into the caliper] easier. As you know piston knock back [driven by tiny amounts of disc wobble] is essential to disc brakes, as it provides the running cleanace at the pad/disc when the brake is not applied. The low friction teflon coating is intended to help here. Lack of knock-back causes drag and thus heat build up.

Hag.
 
I ran the aluminium goldline pistons for years in my Monty and they always performed reliably with very good feel.
Never had an issue with fade and they did get used quite hard on some of the long downhill twisty bits over on the west coast.

I think Big Bob posted about the way then outperformed the anchors on his Pegaso by about the length of a petrol station forecourt.

Is Mr Dixon running ebc discs rather than cast iron on his race bike?

Had issues years ago on an FJ using EBC greenstuff pads apparently outwith their design parameters on EBC prolite discs.
Apparently the black stuff on the discs was melted pad material.
Nice man at Freeman automotive sent me some new discs and sintered pads.
You have to get a bit of heat into them to get them working smoothly but they were fantastic when hot.
Word of warning though, you get used to the stopping power in the dry then it rains and drops the brake temps and there is a
very noticeable drop in performance. Probably just as well as it would be easy to lock up with the hot brake performance.

Only drawback of the instant brake power was 2,500 miles out of a front BT45 with 5,000 out of the rear.
 

 
sonicboom500 said:
Is Mr Dixon running ebc discs rather than cast iron on his race bike?
Click to expand...
Indeed I am Sonic, I'm very happy with them although they weigh quite a bit.
 
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