29er braking performance

[Cassie]

Only one hole to poke in this - at a given speed, the 29" wheel will be turning slower.

 

The leverage aspect is correct, it's just not the whole picture. If the wheels were rotating at the same angular velocity, your explanation would hold.

 

Conservation of energy is the simplest way to explain this - K = Mv, and for a rider + bike at a given speed and assuming the 2 bikes weigh the same K will be constant. Therefore the force required to bring the two systems to a stop is the same regardless of wheel size.

 

I could reduce this to a set of equations, but I have far too many bikes to get done before the weekend...

 

Isn't the rotor distance from the hub fixed (i.e equal) for both 26 & 29?? then the "theory" should move more towards the weight / momentum side??

[Captain Fastbastard Mayhem]

Isn't the rotor distance from the hub fixed (i.e equal) for both 26 & 29?? then the "theory" should move more towards the weight / momentum side??

 

Nah. We're assuming weight (and therefore momentum) to be the same.

 

It's angular momentum / velocity and the size of the lever impacting how much braking force goes through the rotors over a given distance.

[ArrieS]

have you checked alignment of pads to disc...both in the direction of rotor movement and vertically. If not aligned you lose some braking power in warping the rotor to fit between the misaligned pads. The Shimano mounting system cannot compensate for mounting posts that are not exactly parallel to the disc plane. The cone washers on some Avid brakes do allow proper alignment but they also move and require more regular adjustment in my experience. Have not tried the cone washers on Shimano brakes yet.

Park Tools makes an alignment tool that can be used to grind the posts parallel.....dunno if any lbs uses this. Also it is made for alu. Would not want to try grinding a carbon frame.

 

Rear brakes also more difficult to bleed due to high/low points in the hose route and longer hose. Try taking caliper and hose off the bike and hang vertically to bleed.

 

Give us more detail:

1 is the brake lever spongy?

2 does it gradually collapse to the bar when holding at constant effort?

3 Is the lever stiff but the rotor still slides thru easily?

4 is the lever travel and free-stroke correctly adjusted?

5 have u checked disc thickness the same at inner and outer edge of the active braking area?

 

I haven't checked the disc/pad alignment, will do so tonight. With regards to the questions:

 

1. The lever does not feel spongy, although they do not feel as firm as those on my old bike. Possibly the reach adjustment resulting in the "strange" feel. Before being bled, they felt seriously spongy.

 

2. No collapsing under constant pressure.

 

3. Yes, that is the problem. When relative force (enough to stop my old bike dead) is applied, I can still push the bike forwards. Feels as it the amount of bite is less than it should be.

 

4. It appears to be, the lever travel appears to be normal and not excessive. Can't depress to the handlebar for instance.

 

5. Have not checked that either, will do so.

[Captain Fastbastard Mayhem]

Shimano brakes are awesome. Check for contamination on pads and/or rotors. 160mm rotors will stop just fine. SLX & XT brakes are bullet proof.

 

Maybe if you ride dirt paths and pavement, yes. Or weigh less than tinkerbell.

 

For anyone other than a superlight XCO / marathon racer, 160mm discs should not feature on any sort of bike whatsoever.

[Captain Fastbastard Mayhem]

I haven't checked the disc/pad alignment, will do so tonight. With regards to the questions:

 

1. The lever does not feel spongy, although they do not feel as firm as those on my old bike. Possibly the reach adjustment resulting in the "strange" feel. Before being bled, they felt seriously spongy.

 

2. No collapsing under constant pressure.

 

3. Yes, that is the problem. When relative force (enough to stop my old bike dead) is applied, I can still push the bike forwards. Feels as it the amount of bite is less than it should be.

 

4. It appears to be, the lever travel appears to be normal and not excessive. Can't depress to the handlebar for instance.

 

5. Have not checked that either, will do so.

 

Okay - then it sounds as if the pads and / or rotor are either contaminated, or not bedded in properly.

[DJ84]

I have read most the posts and agree with the calculations and understand them all although i am not an engineer.

Where i am getting at is if you are capable of locking the wheels up with either wheel size and the same size rotor, weather or not you need to apply more force. The brakes and the rotors are capable of providing the same stopping force/time/distance.

So it is not necessary to upgrade your rotor size unless your brakes are not performing correctly.

[dirtypot]

Droo. I am an engineer. Robrider has it correct whether he is an engineer or not....sound reasoning and more succintly put than I can do it.

 

Ground friction and some of the other arguments are side issues. Force applied by brake pads is converted to a perpendicular frictional force that decelerates the bike and rider. Work done in slowing down = friction force x distance that the rotor moves through the pads. The bigger wheel turns slower at same speed so it needs a bigger rotor in order to move the same distance through the pads and absorb the same kinetic energy in the same overall distance travelled over the ground.

 

This is exactly what I was meaning in my first post on page one. The 29er wheel carries more distance per rotation, so less disc moves through the pads on the 29er traveling at the same speed as the 26er

 

I think that we're all finally on the same page!

Doesn't seem to help the OP though....

[Cassie]

I haven't checked the disc/pad alignment, will do so tonight. With regards to the questions:

 

1. The lever does not feel spongy, although they do not feel as firm as those on my old bike. Possibly the reach adjustment resulting in the "strange" feel. Before being bled, they felt seriously spongy.

 

2. No collapsing under constant pressure.

 

3. Yes, that is the problem. When relative force (enough to stop my old bike dead) is applied, I can still push the bike forwards. Feels as it the amount of bite is less than it should be.

 

4. It appears to be, the lever travel appears to be normal and not excessive. Can't depress to the handlebar for instance.

 

5. Have not checked that either, will do so.

 

This is basically my exact problem as well...same brake system XLS...same back brake issue...

{remember my post on squealing Breakes...??}

 

Problem still persists ...I aslo cannot lock out may back wheel...no matte how hard I pull on lever...

 

 

PS: i'm reading some good advice / actions here...

[Robrider]

Ok, I am not a geek, *snip

 

 

 

yes... yes, you are.

 

But you are also right

[Cassie]

Riiiiight...

 

I think the only thing that has been confirmed by this thread is that anyone who's still involved is a massive nerd

 

AND that Brian Fantna was in REALLY borning seminar!! you should ask for your money back, Brian!! love your work though...

[Cassie]

I have read most the posts and agree with the calculations and understand them all although i am not an engineer.

Where i am getting at is if you are capable of locking the wheels up with either wheel size and the same size rotor, weather or not you need to apply more force. The brakes and the rotors are capable of providing the same stopping force/time/distance.

So it is not necessary to upgrade your rotor size unless your brakes are not performing correctly.

 

This is interestingly correct...IF you are able to lock up your back wheel...why should you go for anything larger?? (as increased costs)

 

The issue comes in in NOT being able to lock you wheel whilst braking...

[Robrider]

Eldron - I understand your reasoning, but here's mine: If the two systems are the same mass and travelling at the same speed they have the same E_k and as such require the same amount of power to be dissipated by the brake to bring them to a stop at the same rate. Assuming they have the same brake on the same size rotor, the force exerted must be the same since there are no material differences between the two systems.

 

Robrider - if you're an engineer, show me the numbers and I'll believe you.

 

I think you mixing up a few concepts.

 

Force, power and energy are all different.

 

Mechanical leverage is used often, and its a trade off.

 

For example, with a lever you can lift a heavier object. That doesn't mean you suddenly applying more energy. Energy is the same, because you are moving the heavier object a smaller distance (Ep = mxgxh, mass increases but h decrease, energy is conserved)

 

Other examples of leverage are pulley systems, hydraulics, gearing on a bicycle.

 

So in this case, 29er brake force must be higher because the disk is travelling less of a distance (wheel rotation is slower at the same bicycle speed). Energy is still conserved.

[Eldron]

No matter which way around you muddle the equations the difference is around 10% (wheek size, extra energy to brake, extra brake rotor size) so there is that answer.

 

@ArrieS - if you're Jhb side then bring your brakes to me and I'll sort them out. I'm in the East Rand.

[Captain Fastbastard Mayhem]

This is interestingly correct...IF you are able to lock up your back wheel...why should you go for anything larger?? (as increased costs)

 

The issue comes in in NOT being able to lock you wheel whilst braking...

 

there are plenty of reasons - a larger disc will allow you to have better modulation of the braking forces, which means more control. It also aids heat dissipation, which means that your brakes will perform better, for longer, as they won't get as hot - no fade, better pad life (not as much heat) and a better overall experience.

[Headshot]

Okay - then it sounds as if the pads and / or rotor are either contaminated, or not bedded in properly.

 

Agree, its not the big wheels at all, but a brake problem. Change the pads and clean the rotor with alcohol to remove any slippery stuff.

[Captain Fastbastard Mayhem]

Agree, its not the big wheels at all, but a brake problem. Change the pads and clean the rotor with alcohol to remove any slippery stuff.

 

the isopropyl kind, not the kind you drink... Just in case someone's wondering!

 

Oh - and burn it off after dousing it. Removes all trace.