Rotor size and tire choice for 29er

[mtbDobby]

Hi all,

 

I have bought a new wheelset(ztr arch with hope hubs - thanks cwc) and now need to add rotor and tires.

 

I have phoned 4 different bike stores and each one tells me a different story regarding the rotor size on a 29er.

 

To put things into perspective - I bought a Giant ANthem 29er which comes with a 185mm Rotor - 2 of the shops say keep that size, and 2 say the Rotor will warp, its just a matter of time...so who do I believe? I am 85kg's if that has any bearing and doing epic.

 

My next question is what set of tires should I use for the epic, racing ralph / rocket ron...is it worth the price? or should I go with mountain/ race king or the bontrager team edition?

[Velouria]

I put a 160mm rotor on my 29er, and it seems to work ok. I did it for no other reason than CRC had a special on Elixir brakes and they only came in a 160mm option. I have a 185mm rotor on my 26" bike. It's 4 years old and as straight as the day that I bought it.

 

I weigh 78kgs, and I think there is a noticeable difference in the stopping power between the 2 bikes.

 

Concerning tyres - I would be a little hesitant going with the Race Kings on the 29er for the Epic. Something with a bit more side wall protection. I've ridden Epic on Cross Marks, Geax Mezcals and Mountain Kings (all 26" UST) and never had an issue...

[AndreZA]

For cross country you don't need anything more then a 160mm on the front and 140mm on the back. But on 29" wheels it looks out of proportion. So I would go 180 front and 160 back. I don't know why they say it will warp. The downhill people use 203mm discs and it does not warp and they use their brakes alot more then XC riders do.

[Wyatt Earp]

I am 85kg's if that has any bearing and doing epic.

 

Here is the key, weight is the important factor.

I use 160 F 140 R on my geared bike.

I use 160/160 on my rigid simply because I need to brake a little more aggressively through very rocky terrain.

[levett]

I use 160mm Rotors (Front and Back) on both my 26er and 29er (I weight 115Kg) and never found them wanting for my riding.

 

I guess it's partly what you're used to (If you've had 185mm, you'll notice the extra 'pressure' needed) and what type of riding you do.

[Capricorn]

The downhill people use 203mm discs and it does not warp and they use their brakes alot more then XC riders do.

 

i sincerely doubt it..

 

Shouldn't be based on what looks good or not: y'all have weight considerations balanced against required stopping force requirements. I sincerely doubt 29ers roll that much faster that you have to upgrade your rotor size.

 

Levitt: what extra pressure you referring to? clamping force remains the same, the brake rotor thicknesses hardly vary between different rotor sizes, so dunno why a change in rotor size would result in 'extra pressure'. All you should feel if everything is setup properly, is increased stopping capability

Edited February 3, 2011 by Capricorn

[Thumper]

@ Dangle

Boet I can like to be soooo fast I run A 203 up front with a 180 at the back.

[Top Fuel]

On my 29er I have 160mm front and back. I weigh 85kgs and the thing stops on a dime. That is Hope Tech 2 brakes. I use Specialized Controll tyres and they are super good. Not any issues. I have the exact same wheels as you from the exact same place. Red hubs. They are flippin cool. Bling bling.

Not sure if this is common standard but at the Epic last year I used double the amount of Stans for the race as I thought better to be safe than sorry. I am no weight weenie so the extra 20mls wasn't an issue for me. Was lucky and didn't have any tyre issues.

Edited February 3, 2011 by Top Fuel

[Wyatt Earp]

@ Dangle

Boet I can like to be soooo fast I run A 203 up front with a 180 at the back.

With that maer gat

[crasher]

I have run a 160 and a 180 and a 203 on the front at different times the stopping power is the same the pressure needed to stop is the same I dont notice any difference.

 

The big difference is the bigger you go the cooler the brakes run so they take much longer to fade only really becomes an issue on long descents or when having to constantly brake hard.(Dangles post is good example)

 

Pad life and disc life/wear is also suppossed to be better with the bigger disc's but I dont have personal experience to confirm this due to too much chopping and changing.

[Capricorn]

crasher: While I do agree with the extra cooling due to the larger surface area, that is NOT the primary reason for a larger disc, the physics says your perception is insensitive to the physical reality at play wrt braking power. The moment arm at the contact point for the same brake force is longer, thus the stopping torque MUST be bigger. In simple terms

T = brake force applied x (distance of contact point from center of the axle, or roughly radius of the brake disc).

 

Unless of course you saying Archimedes was wrong when he said he could move the earth if he had a long enough lever

[King_Crispy]

If anyone has 185 brake rotors then they have a VERY limited edition. NONE of the major manufacturers make that size. Shimano, Avid, Hope, Ashima and Formula only make a 180...

[Johan Bornman]

There is some truth in the story that the larger discs will warp. All discs warp, just ride through a puddle when they're hot and tsssst - they're out of true.

 

With a larger disc, the run-out is magnified. I'll explain. Imagine a 140mm disc that is warped so that the run-out is 1mm at the outer edge. Now, imagine a 183mm disc (yup, some are nominally 183mm), with a 140mm one superimposed on it. The superimposed 140mm disc has a run-out of 1mm. Up to the 140mm mark, the 183mm disc is also 1mm out of true. But at the 183mm mark, where the pads touch, that same disc is actually 1.3mm out of true. For a 203mm disc it is 1.5mm All this for the same amount of "warpage". *

 

In the world of very small clearances, this is a big difference in the noise-factor.

 

The dealers are of course right. It is just a matter of time before these discs warp. They all warp, but that doesn't make them dangerous or ineffective, just irritating.

 

* Brand new word. Pronounced not as one would a page of war, but as warpidge. It means the state of warpedness. I may be more elegant to just say run-out, but that doesn't sound scientific enough.

Edited February 5, 2011 by Johan Bornman

[Gravity Sux]

crasher: While I do agree with the extra cooling due to the larger surface area, that is NOT the primary reason for a larger disc, the physics says your perception is insensitive to the physical reality at play wrt braking power. The moment arm at the contact point for the same brake force is longer, thus the stopping torque MUST be bigger. In simple terms

T = brake force applied x (distance of contact point from center of the axle, or roughly radius of the brake disc).

 

Unless of course you saying Archimedes was wrong when he said he could move the earth if he had a long enough lever

 

agree,

 

also, for same braking power on 29er as 26er you need rotor to be 11.5% larger because of the larger wheel (longer moment arm), therefore 185mm on 29er is approximately equivalent to 160mm on 26er. But then braking of course braking is a little more complex than just rotor size. It involves choice of pads, technique etc. If your brakes start fading on the way down the Groenlandberg you may have made the wrong choices

[Wyatt Earp]

:lol: :lol:

Logged in here and read some of the most entertaining BS I have seen in ages.

"just ride through a puddle when they're hot"

:lol:

Edited February 5, 2011 by Dangle

[gummibear]

also, for same braking power on 29er as 26er you need rotor to be 11.5% larger because of the larger wheel (longer moment arm), therefore 185mm on 29er is approximately equivalent to 160mm on 26er.

 

That is what i thought to.Surely 29ers get hotter?