Choosing brake discs?

Peanut651

Strange and stupid question, but does the pattern of the holes cut into brake discs affect the braking performance? Forgive me, I've been out of action with my bikes for a number of years now due to an accident. Trying to get back into it :twisted:

For example, do discs with less holes have more braking power because there will be more pad in contact with the disc, or vice versa? More holes cool down quicker than ones with less? Or is it not really noticable on a mountain bike?

I need to buy some new discs because my old ones were a spline fitment and I now have wheels with 6-bolt hubs. I'm not interested in changing my Shaimano brakes because I find they still do the job very well even though they are 9 years old. They can still lock up the front wheel on tarmac with one finger

nicklouse

It makes very little difference at all.

Peanut651

Thought so on a mountain bike, just thought I'd ask. I've ordered some Hope floating saw ones now anyway

cooldad

But of all the choices out there, that's a bad, pointless one.

nicklouse

cooldad wrote:

But of all the choices out there, that's a bad, pointless one.

And they may catch your calipers.

Ouija

nicklouse wrote:

cooldad wrote:

But of all the choices out there, that's a bad, pointless one.

And they may catch your calipers.

Yep. Had that problem on the shorter (160mm) versions as the distance between the inner/outer ring connecting bolts and the brake surface were so short that they kept banging into the edge of the caliper rather than passing below it. Even with the rotor centralized in the caliper there wasn't enough clearance either side. Gets even worse on mechanical systems where the rotor is flexed slightly into a static pad. Kinda depends on how wide the hole in the caliper is for your rotor.

Peanut651

Ach well, if they don't fit or end up being crap I'll get some solid ones. My local bike shop said they should be ok. We'll soon see if they will be or if they were just trying to get a sale lol

cooldad

Pointlessly expensive with no benefit.

So for the bike shop, ka ching...

Peanut651

Trade price

The Rookie

Circa 35% off then?

Giraffoto

Getting back to the original question . . .

A rotor with fewer holes in the swept area will, all other things being equal, be able to absorb a bit more energy as the bike's motion is converted into heat. But it's marginal. A rotor with more holes will allow the air to circulate around it a bit more and provide a greater surface area/volume ratio for cooling - again, marginal - and will allow the pads to bite just fractionally (or marginally) quicker if there's muck on the rotors.

So, if you think your floating rotors look more than marginally better than non-floating ones, that's probably the biggest difference they'll make. I have very conventional one-piece ones, but I think the floating designs look cool.

The Rookie

Giraffoto wrote:

Getting back to the original question . . .

A rotor with fewer holes in the swept area will, all other things being equal, be able to absorb a bit more energy as the bike's motion is converted into heat. But it's marginal. A rotor with more holes will allow the air to circulate around it a bit more and provide a greater surface area/volume ratio for cooling - again, marginal - and will allow the pads to bite just fractionally (or marginally) quicker if there's muck on the rotors.

Erm, why is a disc with fewer holes be able to absorb more energy, there is no area in the calculation of friction, so I'd like to know why you think it's able to.

One weighs less and by dint of that heats up faster as it has a lower thermal inertia......simples.

jimothy78

The Rookie wrote:

Erm, why is a disc with fewer holes be able to absorb more energy?.... One weighs less and by dint of that heats up faster as it has a lower thermal inertia......simples.

Erm, haven't you just answered your own question? Or was it intended to be rhetorical?

A rotor with fewer/smaller holes has more material, so can absorb more energy for the same rise in temperature, or, to put it better for the purposes at hand; will experience a smaller increase in temperature to absorb the same amount of energy as the disc with less material.

EDIT - not that I think the above is important - I'd much rather choose my rotors on other performance factors, such as the ability to clear mud, etc.

The Rookie

That's not absorbing more energy, its having a lower rise in temperature for the same energy....you never mentioned temperature above and it implied you meant increased braking....

nicklouse

So who remembers the old Gustaf disc?

cooldad

With floating calipers?

nicklouse

yep single sided piston and a suicide rotor.

it either ripped your arms out of their sockets or did nothing.

cooldad

Never used them but I do quite like the HS22s on my old Explosif. 660mm rotors and fluoro yellow obviously.

Peanut651

Back off topic lol, Made a mistake earlier. I ordered the Hope mini/M4 saw SOLID rotors, not floating. I must have gotten mixed up in the shop because they gave me the option between the two.

jimothy78

The Rookie wrote:

That's not absorbing more energy, its having a lower rise in temperature for the same energy....you never mentioned temperature above and it implied you meant increased braking....

I did no such thing
the above was my first post in this thread.

Giraffoto

The Rookie wrote:

Giraffoto wrote:

Getting back to the original question . . .

A rotor with fewer holes in the swept area will, all other things being equal, be able to absorb a bit more energy as the bike's motion is converted into heat. But it's marginal. A rotor with more holes will allow the air to circulate around it a bit more and provide a greater surface area/volume ratio for cooling - again, marginal - and will allow the pads to bite just fractionally (or marginally) quicker if there's muck on the rotors.

Erm, why is a disc with fewer holes be able to absorb more energy, there is no area in the calculation of friction, so I'd like to know why you think it's able to.

One weighs less and by dint of that heats up faster as it has a lower thermal inertia......simples.

My preference is not to explain previous posts, but I'll go for it with this one. Fewer holes = more steel = more material. Which can absorb more energy. If your rotor has to reach a temperature x before its effectiveness starts to decline, it will take more braking to make a disc with more material reach temperature x. That is, more kinetic energy transformed to heat in the rotor. When I say energy, I mean energy.

I did say it was marginal.

The Rookie

jimothy78 wrote:

The Rookie wrote:

That's not absorbing more energy, its having a lower rise in temperature for the same energy....you never mentioned temperature above and it implied you meant increased braking....

I did no such thing
the above was my first post in this thread.

Sorry, confused your comment with the original, I was correcting the way it was written which did imply better braking even if it wasn't what it was meant to.

A lighter disc can of course, if playing semantics, absorb the same energy, it just gets hotter doing so, even if it ends up as vapour......

Giraffoto

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