Picking the right rotor size

[irontam]

I dug out my old Specialized Rockhopper (2003) for a trip down to Glentress last week. Had a most excellent time and am now considering upgrading the V-brakes to discs.

My question is; how do I pick the rotor size? Is it decided by the caliper mounting points on the frame? Are they standard on Rockhoppers?

Thanks for any help!

 

[rogerzilla]

You can have any size you want given the right caliper and adaptor. Generally DHers use 8" discs and XCers use the smallest ones, but the front disc is often a size up from the rear one to balance the brakes a bit better (cars are like this too). Bigger discs add weight but require less lever force and resist heat build-up better. Smaller discs have the opposite characteristics but are also more likely to be true - it's quite hard to make a 205mm diameter, 2mm thick disc perfectly flat.

 

[plank]

Theres not much point getting a 8" rotor, like rog said its only worthwhile for DH. 6inch would do you fine, these look good
http://www.chainreactioncycles.com/Models.aspx?ModelID=22185

you need to check the bike and wheel for mounts!

 

[Jonathan M]

I run a 185mm on the front and a 160mm on the rear of my Cove. Why? Well the frame has IS mounts and the Formula B4 brakes mount to IS with their "standard" size disc, on the forks (Manitou) I needed to use an adaptor for IS caliper to post mount fork, and this nicely spaced the caliper out to 185mm.

As Rogerzilla said a bigger disc will dissapate heat better, and in motorsport terms bigger discs give better braking as well - something to do with the physics of the pads acting upon an area further away from centre of rotation or somesuch. It does work though!

 

[psmiffy]

How do you convert from V to disc I thought you needed fittting onthe frame and forks so that the caliper is correctly aligned

 

[rogerzilla]

Unless the fork already has disc mounts, you don't convert - you buy a new fork. The fork must be designed for the loading of a hub brake and, in any case, brazing on disc mounts is very difficult and costs more than a new fork.

Brazing a rear disc mount may be worthwhile if you have an expensive steel frame, but I'd still rather have something designed for the job.

 

[Panter]

I run 185 front and 160 rear too, it's a very good combination.

Check the spec of your forks though, they will have a maximum rotor size that you mustn't exceed otherwise you'll put too much strain on the mounts.

If I could I'd have a 203mm front and 185 rear....

 

[GilesM]

I run 185 front and 160 rear too, it's a very good combination.

Check the spec of your forks though, they will have a maximum rotor size that you mustn't exceed otherwise you'll put too much strain on the mounts.

I run the same 185 front, 160 rear on my full sus, and 160 front and 140 rear on my hardtail, this works well for me, very important that you follow Panter's advice about the max rotor size. You'll also need disc compatable hubs.

 

[irontam]

Thanks everyone.

185/160 looks quite good... I'm going to have to save the pennies up

 

[Panter]

I'm not on commission, honestly (I've posted this at least three times in the last couple of weeks ) but Wiggle have, or had, the Avid Juicy Fives on sale for half price.
I run them on both MTB's, they're very good, especially at that price.

 

[rogerzilla]

I run 185 front and 160 rear too, it's a very good combination.

Check the spec of your forks though, they will have a maximum rotor size that you mustn't exceed otherwise you'll put too much strain on the mounts.

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.

 

[Mr Pig]

I have two pairs of wheels with 160 disks on all four. One advantage is that you can swap rotors around if need be.

I've done heavy, sustained braking on big hills and I don't see why you would need better bakes, they coped perfectly well.

 

[Panter]

Mine are probably trying to stop a fair bit more weight

I also prefer less finger pressure, personal preference I suppose.
I also swap wheels around, it's nice being ably to take the swampthings off the ht and put them on the full susser if needed as I run 185mm fronts on both

 

[GrasB]

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 )

 

[Mr Pig]

Mine are probably trying to stop a fair bit more weight

Doubt it, I weigh about 15 stone!