Picking the right rotor size

[Jonathan M]

You mean like solid axles JM?

Thats one solution, but if QRs and rear of leg mounted discs were an issue, some design engineer with a lawyer looking over his shoulder would have changed the design and come up with some bollox why a front mounted caliper was "better" - lets face it, mountain bikers wouldn't buy something for being safer, they'd want a performance benefit from it.

 

[GrasB]

Of course, solid axles while are more suitable for disc application are mainly there because they're so much tougher compared to normal axles. I know people who have said they have had disc brakes 'pop' QR mechs. I tell them I had QRs 'pop' my self... just when they start to go on about how bad discs are I then tell them the bike uses V-brakes (which it did at the time)

 

[rogerzilla]

I've read the sensational reports concerning the forces exerted by the disk brake and they all failed to take into account one very important factor. The weight of the bike and rider pushing the fork into the ground under braking, which renders the scaremongering largely irrelevant.

You need to do the maths to understand. The weight on the front wheel is rarely more than the weight of the rider and bike combined, the exception being if you've just run into something. The force created by the caliper is larger than this - for a given deceleration it's greater with a smaller disc - and there is a net ejection force. With a steel fork the lawyer lips can take this sort of abuse, but maybe not indefinitely on an aluminium or magnesium fork leg.

The manufacturers (Marzocchi being an exception) refuse to admit the problem because it would probably result in the biggest recall of bikes in history, as well as requiring a new International Standard.

Jobst Brandt worked in the original automotive disc brakes for Porsche, so knows a bit about the subject:

http://yarchive.net/bike/disk_brake_qr.html

I have the QR lever on the left and it's tucked under the caliper in such a way that it cannot loosen. It's also a steel fork with lawyer lips and the skewer is one of those old Shimano jobbies, well-oiled and done up tight. It will hold, I'm sure.

 

[Mr Pig]

You need to do the maths to understand.

Not really. I need to belt down a hill, pull on the brakes and find my front wheel stay attached to the bike to understand.

 

[Panter]

To be frank I think that the type and quality of calliper, pad and lever are going to make a much bigger difference to the feel and effectiveness of the brake than the size of the rotor. I think that the biggest reason for people wanting big rotors of the 'willy' factor.

But given the same pad compound, same brakes etc and two different sized rotors, the larger rotor excerts more braking force than the smaller one.
I don't run a 185 on the front for pose value, I do it to stop effectively and I'd run a 203 if I could to further reduce the lever pressure needed.
But, as I say, it's personal preference.....

 

[GilesM]

To be frank I think that the type and quality of calliper, pad and lever are going to make a much bigger difference to the feel and effectiveness of the brake than the size of the rotor. I think that the biggest reason for people wanting big rotors of the 'willy' factor.

With the same caliper, pad etc, a larger rotor will help, whether it helps at sub superstar DH speeds I am not sure, but the bling factor makes it worth it, same with floating rotors.

 

[Mr Pig]

I know! Why don't we make the brake disk really huge, for ultimate maximum braking power? As big as possible. In fact, we could make it really enormous and save weight by combining two functions in one part by making the braking surface part of the wheel rim!

I'm a genius. Money will flow. Of course we'd then need some kind of brake caliper that could fit around the outside of the edge of the wheel and tyre. Any ideas?...

Muppets :0)

 

[Panter]

As I say, personnal preference.

You stick to your V brakes idea and I'll stick to my "oversize" rotors.......

 

[PpPete]

I know! Why don't we make the brake disk really huge, for ultimate maximum braking power? As big as possible. In fact, we could make it really enormous and save weight by combining two functions in one part by making the braking surface part of the wheel rim!

I'm a genius. Money will flow. Of course we'd then need some kind of brake caliper that could fit around the outside of the edge of the wheel and tyre. Any ideas?...

Muppets :0)

Ah but you'd need a mechanism for keeping such a big disc flat....

 

[GilesM]

Ah but you'd need a mechanism for keeping such a big disc flat....

And a rigid caliper and non stretch cable that doesn't flex under the huge forces that can be applied to a disk via a hydraulic disc brake.

 

[Mr Pig]

You stick to your V brakes idea and I'll stick to my "oversize" rotors.......

I haven't got 'V' brakes.

 

[Panter]

Didn't say you had.....

 

[Jonathan M]

Of course, solid axles while are more suitable for disc application are mainly there because they're so much tougher compared to normal axles. I know people who have said they have had disc brakes 'pop' QR mechs. I tell them I had QRs 'pop' my self... just when they start to go on about how bad discs are I then tell them the bike uses V-brakes (which it did at the time)

Doesn't this contradict what Rogerzilla has originally said then? If you can pop a QR on a V braked bike, others pop a QR on a disc braked bike, then the commonality is the QR and the user surely, not the braking system?

 

[GrasB]

I'd agree there, JM. I've just done a test, I have theft proof 'secure' torx clamping QR, so I can use my digital torque wrench to record peek clamping pressure. This morning I did the front wheel up to a torque of 31.8Nm (5 clamps in a row with the same peek torque), after 75 miles the clamping torque is 31.2Nm. If that's 75 miles difference then what's say 500 or 1500 miles?

 

[Steve Austin]

I would say you need to tighten it up more in the first place