Rotor size and tire choice for 29er

[LazyTrailRider]

Surely 29ers get hotter?

 

Hmmmm. Why?

[ichnusa]

The beauty of 29ers is that they maintain their momentum because of their bigger "lever arm". This is part of their downfall as well as they feel sluggish on the get-go and need more effort to build up the momentum. Therefor, i think it probably makes sense that they need bigger discs to curb that momentum. Having said that... whatever... just put a 183 mm up front and a 160 mm at the back (i do that on my 26ers as well) and ride!

 

They get hotter because they have more rotational momentum that needs to be stopped...

Edited February 6, 2011 by ichnusa

[gummibear]

Hmmmm. Why?

 

Higher speeds and bigger diameter wheels.

[Aggies]

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.

 

I have the same bike and I will stick with the current brakes. The Elixer CR with 180 front and 160 rear is just brilliant. Had them bleeded at Epic Sport Shop and I have never used brakes that is so good. I am running 2.4 Mountain King Front and 2.2 Race King Rear and have no issues. For the Epic I would suggest the Maxxis Crossmark UST that must now be available. The Conti's does not have any protection.

[Brian Fantana]

Only thing I agree with on this thread is that 29ers are hotter!!

[LazyTrailRider]

...Higher speeds...

 

Haha, that's what the marketing monkeys would like us to believe!

 

(Unfortunately though, the last time I was bombing down Tokai and checked, all the 29ers were going slower than me )

[Montblanc]

Does Crossmark do a 29'er UST?

[Wyatt Earp]

Does Crossmark do a 29'er UST?

No, Maxxis does a tyre called the Crossmark, the 29'er tubeless is fantastic, I installed them on Thursday and have done around 130 k's with them from Thursday to Saturday and rode this morning again on my other bike with the SmallBlock 8's.

The Crossmark's are fast , they roll effortlessly

[Montblanc]

Done 2 Epic's with Maxxis Crossmark UST on my 26'er. This time round will be on 29'er and not to sure what will be the best tyre to use. What is the sidewall like?

[Johan Bornman]

The beauty of 29ers is that they maintain their momentum because of their bigger "lever arm". This is part of their downfall as well as they feel sluggish on the get-go and need more effort to build up the momentum. Therefor, i think it probably makes sense that they need bigger discs to curb that momentum. Having said that... whatever... just put a 183 mm up front and a 160 mm at the back (i do that on my 26ers as well) and ride!

 

They get hotter because they have more rotational momentum that needs to be stopped...

 

Momentum is not a function of wheel size. Momentum = velocity (speed) times mass. Thus, for a 90kg 29er and rider, the momentum at a given speed is the same as for a 90kg BMX and rider. Further, to slow the two down from a given speed to a given speed, requires the same energy or rather, produces the same energy.

[greatwhite]

Momentum is not a function of wheel size. Momentum = velocity (speed) times mass. Thus, for a 90kg 29er and rider, the momentum at a given speed is the same as for a 90kg BMX and rider. Further, to slow the two down from a given speed to a given speed, requires the same energy or rather, produces the same energy.

 

You have discussed linear momentum only - a 29er, will have larger angular momentum due to the product of its larger diameter and the additional mass associated with such (in basic tech speak: mass and radius of gyration are both larger). In a nut shell assuming in all cases the rider + bike = say 90kg, the guy with the larger wheels will need to disapate more energy to slow him down.

 

To answer the original question: To my mind, the main reason for larger discs on a 29er is the fact that a 29er creates about 10% more torque than a 26er that the rotors must overcome during breaking. This can be overcome 2 ways:

1. Make the rotor 10% larger

2. Apply 10% more force to the calipers on the same size disc. (This measn max breaking power is reduced by 10%)

 

Personally I go for option 2. I had Ø160 XTR front and back. I found the 160 front to be inadequate on my 26" and changed to 180. It would be 10% worse on a 29er. I reckon a Ø185 front will be fine and if/when it warpes, grin and bear it, but don't change to smaller front disc. (That and the fact that such a large wheel looks lame with such a dinky disc on it - makes me think of these guys with golf 1/citi golfs that put 17" wheels on their car - the disc looks like a littre buscuit lost in the centre)

[WorcesterWheelers]

Some comments from personal experience while dealing with some customers with the questions you pose.

 

The lighter riders:

- have had very few issues with 180 front 160 rear rotor setup on their 29ers.

- the vast majority of them are still happily using Conti Race King 29er tyres without any problems.

 

One particular rider we've helped is 100kg of muscle (yes there are some large "Boerseuns" that also ride their bikes.)

- Upgraded to AVID solid sweep rotors, 180 front & rear

- He had a few problems with side walls on the Continental tyres.

- We fitted a set of GEAX Saguaro tyres before the Attakwas for his, and he's been smiling ever since.

 

P.S. A handy tip for all the 29er riders coming to our valley in March for the EPIC....

Same abovementioned rider usually rides XX groupset, and he's certainly strong enough the pedal the gears, even up our annual Jonaskop hillclimb (14.6 km climb with 1125m ascent!!)

- For Epic he's recently decided to change his gearing for Epic to 3 x 10.

[Flea]

Did Epic last year on a 29er - 180 up front, 160 rear. I am 100kg, and was happy.

Used Mountain King 2.4 up front with Race King 2.2 on rear - no problems

BTW changes the MK for a Race King 2.2 for the Desert Dash and immediately got a side wall puncture from a rock....

Would check the weather before going for the bigger tyres, as the 2.4 does not have too much clearance. For Joberg2C, I will be changing to MKing 2.2 (bought one then thought it too skinny, so immediately put the 2.4 on, so I have a brand new tyre to use...)

Will try to fix the new RK to fit onto the back (try to fix the sidewall) as the existing is getting worn.

 

If there is a proper UST, I would go for that. By the way, I had American Classic Rims, and the width of these would affect the tyre? Narrow rim, less sidewall exposed?