Bigger rotor on a carbon-forked cyclo-cross bike?

jack_kelly

Hi there,

I'm about to buy a Genesis Vapour 2012 disc cyclo-cross bike. It has Avid BB7 mechanical disc brakes, 160mm rotors and a carbon fork. In my test rides, I've been a little disappointed by the performance of the front brake. My mountain bike has mechanical disc brakes and a 180mm rotor on the front wheel, which can reliably throw me over the handle bars if I so desire. I know the Vapour's brakes need to be worn in a little bit but I'm worried that the Vapour will never stop as fast as my MTB. And, as far as I'm concerned, it's essential that a commuting bike can stop fast!

So my question is: can I safely fit a 180 or 200mm rotor to the front wheel of the Vapour? In particular, I'm worried that the carbon fork and/or the relatively light front wheel won't be able to cope with the forces. (And yes, I know from my MTB that you have to ensure the front wheel is bolted on very securely or the brake will attempt to rip the wheel from its mount!) Maybe I should consider one of Genesis' steel-forked bikes if I really want to put a large rotor on the front wheel?

Greer_

It is unlikely that the brakes have bedded in. As with most disc brakes, they wont stop you one second, and you'll be on your face the next. It just needs a bit of time. For fitting a bigger rotor, I imagine 180mm would be okay but it depends on the spec of the fork.

jack_kelly

Cool, thank you for the quick reply.

I have contacted Genesis to ask what maximum rotor diameter their carbon fork can handle (it's not mentioned in their specs)

I assume that if I do buy the bike, and if I do discover that the brakes are not powerful enough, and if I find that the fork can't handle a rotor larger than 160mm then I can always swap the fork for one which can handle a larger rotor. Is that correct?

Greer_

Yes or else you could fit new, more powerful brakes (hydro). BB7's are meant to be one of the best, if not the best mechanical disc brakes out there.

jack_kelly

My dream bike would be a bike with drop handle bars and hydro disc brakes! Unfortunately, as far as I can tell, hydro brake levers for drop handle bars are very, very rare (and hence very, very expensive) at the moment. Maybe that'll change in a few year's time.

MichaelW

I use 160mm discs and they stop me pronto.
Do the usual bedding in: clean off any contamination from the disc, find a big hill and generate some heat in the pads.

jack_kelly

Thanks for the reply.

I suppose I should expand on exactly what my aim is:



When riding on the hoods (as shown in picture above) I want to be able to brake the front wheel hard enough that I can do a "nose wheely". I don't have much experience with drop handle bars but, as far as I can tell, my fingers cannot exert much pressure on the brake levers when riding on the hoods (not as much pressure as I can exert when riding in the drops); so I need a really efficient front brake to be able to convert the relatively modest pressure exerted by the brake lever into sufficiently rapid deceleration to do a nose wheely.

(I want to be able to brake as quickly as possible because I'll be commuting through London and, frankly, I'd like to minimise the chance of being killed! 10 years of commuting on my mountain bike (with 180mm front brake rotor) has made be pretty confident that excellent brakes are essential. I know some bike purists disagree. But, frankly, I'd rather jog into work than ride on a bike with sub-standard brakes.)

@MichaelW are you using drop handlebars and, if so, can you stop pronto when riding on the hoods?

bails87

The thing with a road bike is that the front wheel will probably lose traction a lot earlier than a MTB with a fat tyre.

So having a load of 'grabby' power (which is, relatively speaking, what you'll get with a bigger rotor) is probably a bad thing because you'll grab the brake, lock up the front wheel and probably crash.

FWIW, I've got a boardman CX with BB5s running on Shimano 105 STI levers and they're fine. Not as powerful as the Formula K18s on my MTB, but they don't need to be. If they were then I'd probably crash due to losing the front wheel.

rolf_f

Post this on the MTB forum. They'll tell you plenty of reasons why there is no way that your Genesis needs more than a 160mm rotor.

FWIW, though I have no idea why I would want to, I can do a nose wheelie on my road bikes with caliper brakes. You should lose some braking efficiency when riding on the hoods but really not much. If you are going fast enough for it to matter, you'd be better off in the drops anyway or using interruptor levers.

jack_kelly

Rolf F wrote:

Post this on the MTB forum. They'll tell you plenty of reasons why there is no way that your Genesis needs more than a 160mm rotor.

I do see your points but I beg to differ.

On my MTB, I couldn't do a nose wheely on the stock 160mm rotor. I can easily do a nose wheely on my 180mm rotor. Maybe the stock brakes were rubbish. But this experience on my MTB is what has made me enthusiastic about 180mm front rotors.

Rolf F wrote:

FWIW, though I have no idea why I would want to, I can do a nose wheelie on my road bikes with caliper brakes. You should lose some braking efficiency when riding on the hoods but really not much. If you are going fast enough for it to matter, you'd be better off in the drops anyway or using interruptor levers.

One of the situations I'm most afraid of is this: you're (carefully) making your way through stationary traffic (this happens a lot when commuting in London!). So you're on the hoods, not the drops. You're passing an HGV (and hence can't see round the HGV). Just as you pass the HGV, a pedestrian steps out in front of you. You have a fraction of a second to stop and literally about a meter of stopping distance. Yes, you're only doing about 10-15MPH but that's still gonna cause a lot of pain to both you and the pedestrian if you collide. In this situation on my MTB, I can easily do a controlled emergency stop which brings the bike to a stop as fast as the tyre traction will allow. On the Vapour I test rode, the limiting factor (by some margin) in the braking speed was the brakes, not the tyre traction (even when riding in the drops).

(And yes, before anyone mentions it, if the traffic is stationary then I go nice and slowly and of often use by bell when I pass a large vehicle which I can't see around).

Another emergency stop situation at higher speed (which I've seen happen to another cyclist): you're overtaking a car just as the car driver decides to pull a u-turn. Again, you have a fraction of a second to decelerate as rapidly as possible. Again, because you're passing a vehicle you're unlikely to be going flat-out and so will probably be on the hoods.

rolf_f

If you are in an obviously potentially dangerous situation, just go to the drops. Or get interruptors.

I often commute on an old tourer with pre-aero brakes. Those have the pivot in a lower position so braking from the tops is much less efficient. I just switch to the drops any time I need the brakes.

I'd assume the Vapour is much lighter than your MTB so really shouldn't need big discs. 200mm are going to be pretty laughable but even 180 seems overkill to me. But really, the MTB folk probably know most about when you need big discs.

nicklouse

bails87 wrote:

The thing with a road bike is that the front wheel will probably lose traction a lot earlier than a MTB with a fat tyre.

but a larger diameter wheel has a longer foot print.

160 is fine for any road use.

I would put it down to poor set up or lack of bedding in.

Style over speed

I've got a lynksey cx with disc bb7 road brakes and sram red levers, and initially I was fairly disappointed with the power of the brakes… however Nicklouse is totally right, with bedding in of the pads and readjustment the braking is good… lightness, modulation and obviously wet weather conditions are where these brakes score over stopping power alone.

I'd add that I don't think the BB7 bedding instructions are right and also that really 160mm is the minimum these bikes need, I'd be pretty concerned about a bike with 140mm rotors!