Picking the right rotor size

[Panter]

Doubt it, I weigh about 15 stone!

Mine are definately working harder then.........

 

[Mr Pig]

Mine are definately working harder then.........

I can't get up the hills but nobody can catch me on the way down! ;0)

 

[Jonathan M]

Yeah, because you can't already do a stoppie with a 160 disc

The manufacturers have very little clue about engineering. If they did, the caliper would be on the front of the fork blade, for a start.

Why?

 

[Jonathan M]

I'd go for 160mm as this is the smallest easy to get disc size for the front, if you find you're getting problems due to fade etc. then you can buy an adaptor & step up a rotor size. If you go to big you'll find the brakes snatchy & hard to modulate.

Jonathan M, in motorsport size matters but only when your brake setup is working properly! 6 pot callipers, 257x38mm wide set vented discs & HUGE pads were making a mockery of single pot 273mm & 288mm setups. Not to mention the 257mm setup could be squeezed under 13" rims rather than requiring 15" or 16" wheels (important cause single seaters run 13" wheels )

True. But a move up to a bigger disc using standard calipers and pads will still produce more braking "effort" and the only difference there is the disc diameter - done that in my mis-spent youth. Theres also vehicle weight & pad compound that comes into play, so possibly more variables to consider in the motorsport world when it comes to braking??

 

[GrasB]

True. But a move up to a bigger disc using standard calipers and pads will still produce more braking "effort" and the only difference there is the disc diameter - done that in my mis-spent youth. Theres also vehicle weight & pad compound that comes into play, so possibly more variables to consider in the motorsport world when it comes to braking??

Thing is though, can the system give you proper modulation of that extra effort? No use having loads of power if you have little control of that power. But as you say there are so many variables.

 

[Mr Pig]

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.

 

[rogerzilla]

Why?

Undo your QR completely. Turn the bike upside down, spin the front wheel and apply the disc brake. What happens?

 

[GrasB]

The wheel stops with a deadened plasticy twang & the QR skewer bounces around. I presume you're talking about the moment of torque trying to pull the wheel out of the fork mounts, thing is even on an unclamped QR skewer I've never had the wheel jump out on me due to braking.

 

[Mr Pig]

Undo your QR completely. Turn the bike upside down, spin the front wheel and apply the disc brake. What happens?

Unless you ride around with the QR undone I fail to see the relevance of this.

 

[GrasB]

He's trying to say that the moment of torque is trying to pull the axle out of the drop out, problem is this doesn't seem to work too well & in all honesty if you're riding on an open QR you're gonna have more problems with wheel wheel float than the wheel jumping out.

Oh, yeah there are sus forks, not yet road/cross forks, you can get which use motorbike style wheel clamping.

 

[rogerzilla]

People have had them loosen or come out altogether, although the risk is minimised with lawyer lips (finally they're useful for something) and an old-fashioned steel closed-cam skewer which can exert a decent clamping force.

If you work out the forces involved in a rapid stop, the ejection force is around the limit of what a QR can stand. Putting the caliper in front of the fork would mean that braking merely forced the axle harder into the dropout.

 

[Mr Pig]

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.

 

[Jonathan M]

People have had them loosen or come out altogether, although the risk is minimised with lawyer lips (finally they're useful for something) and an old-fashioned steel closed-cam skewer which can exert a decent clamping force.

If you work out the forces involved in a rapid stop, the ejection force is around the limit of what a QR can stand. Putting the caliper in front of the fork would mean that braking merely forced the axle harder into the dropout.

Fair enough on paper, but the accounts of people loosing disc braked wheels under heavy braking still all have one common factor that can't be ruled out; user error in failing to enure the QR is closed properly.

Surely if this was as big a concern in reality as it is on paper, bearing in mind the american origins of a lot of fork & disc manufacturers, there would have been a move towards yet another industry "standard"?

 

[GrasB]

You mean like solid axles JM?

 

[Steve Austin]

not this old load of cobblers again. Disc brakes on bikes are safe. very safe, and the braking forces exerted will not force a wheel to suddenly eject from the forks.
One of the worst urban myths ever.