29er braking performance

[JXV]

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.

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.

[droo]

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.

 

Whether he was an engineer or not had nothing to do with whether or not he was right, more to do with the ability to do the maths. Engineering dynamics is beyond the understanding of many, including some engineers...

 

I'll accept your reasoning, I'm just trying to reconcile it with conservation of energy, as per the post you quoted. There must be a factor I'm missing...

[Shebeen]

I'm waiting for someone to throw in a third order integral. This can't be a linear solution

[JXV]

I'm waiting for someone to throw in a third order integral. This can't be a linear solution

He he he.......that would be for when you factor in the effect of spoke stress and tyre deformation.....and gradual accumulation of heat in the pads....way beyond me.

[porqui]

It's really simple it works the same as levers.

1st lever is from road contact point to centre

2nd lever is from pad contact point on rotor to centre

When braking there is opposing forces on levers

Short lever applies force to long lever

On 29er long lever is longer in relation to short lever than on a 26er.

Increase length on short lever (180 or 203 rotor) to restore original ratio of 26er

 

[porqui]

Remember that nut you could not get loose - put a pipe on the spanner and strip the moer

[ArrieS]

Thanks for all the feedback guys, but I'm still unsure as to how I'm going to solve the problem.

 

I've checked the brake lines, genuine Shimano. Running 180mm at the front and working as anticipated. 160mm at the back, where the problem is. Lever does not feel spongy but doesn't feel crisp. Any idea if new discs will sort it out? Considering upgrading to 180mm at the back, desperate for some stopping power!

[porqui]

BBleeeed!!

[JXV]

Thanks for all the feedback guys, but I'm still unsure as to how I'm going to solve the problem.

 

I've checked the brake lines, genuine Shimano. Running 180mm at the front and working as anticipated. 160mm at the back, where the problem is. Lever does not feel spongy but doesn't feel crisp. Any idea if new discs will sort it out? Considering upgrading to 180mm at the back, desperate for some stopping power!

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?

Edited September 4, 2014 by JXV

[Eldron]

Ok, I am not a geek, but I was sitting in a very boring seminar, and put pen to paper.

 

My arrows are the wrong way round but the values won't change.

 

Same energy required to stop the two systems, but different brake forces are required to achieve that.

 

This is for ideal friction free environments, and it does not take the larger contact area of the larger diameter tyre into account.

 

 

By jove I think you've cracked it!

 

Only 1 issue though - a 29"/700C wheel has a BSD of 622mm not 700mm which makes d1 363.34mm which makes Fbrake 252.76N for the 29er wheel.

 

I thought something was wrong when the 29" wheel required 37% more braking.

 

Using the right diameter the 29" wheel needs 9% more braking.

 

Excellent - now I can sleep tonight

[Captain Fastbastard Mayhem]

So, literally, go up a rotor size.

[Brian Fantana]

 

By jove I think you've cracked it!

 

Only 1 issue though - a 29"/700C wheel has a BSD of 622mm not 700mm which makes d1 363.34mm which makes Fbrake 252.76N for the 29er wheel.

 

I thought something was wrong when the 29" wheel required 37% more braking.

 

Using the right diameter the 29" wheel needs 9% more braking.

 

Excellent - now I can sleep tonight

 

Cool, I mistakenly thought the 700 equates to 700mm diameter.

 

In my defence, I don't frequent wheel size threads that often, so not really clued up on diameters. Only knew 26" is 559 because I still ride one, haha.

[Pieter1]

I'm telling you, get a gnarly tire.

[Eldron]

So, literally, go up a rotor size.

 

Thanks to the Brain Fontana - yup.

 

160 to 180 or 180 to 200 is close enough to the 9% additional braking required.

[Superfli]

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

[Captain Fastbastard Mayhem]

 

 

Thanks to the Brain Fontana - yup.

 

160 to 180 or 180 to 200 is close enough to the 9% additional braking required.

 

It's a simple concept really. Because a niner wheel goes through fewer rotations over a given distance. If you kept the rotor size the same you wouldn't be able to put as much braking force (in joules) through the rotor as it has also gone through fewer rotations, meaning the work done on the niner would be less, and you stop slower (over a longer distance)

 

The only way to counter that is to reduce overall weight by that 10% (including rider) Or increase the rotor size so that the same amount of rotor passes through the calipers on both bikes.

 

 

 

But. To the OP. Sounds as if they need a bleed