Brake rub

[wooly]

If I get brake rub when I'm climbing out of the saddle does that mean my rear wheel spokes need to be tensioned? I just bought some Edge clinchers from the classifieds and love them. But just noticed today that I got some rub when out of the saddle. The wheels is true and it's one of Edge's standard builds (20 front 24 rear). Never had the problem before. Not too much of a heavyweight either - 175 lbs... I think...

Any thoughts?

[pitcrew]

Let's eliminate the easy things first.....

Is the wheel in the dropouts straight? Is the skewer clamped down tight enough?

[bkboom123]

Thats where you are wrong my friend....175 is gigantic in cycling. I learned this a couple years ago when I found out I was an XXL in cycling gear at 6' 173 pounds and a 31" waist.... go figure :rolleyes:

[wooly]

Affirmative - skewer tight, wheel on straight. With little pressure I can push the rim against the brake pad...

175 lbs = heavy? :crap:

[RPS]

All wheels flex some, so my first question would be how close do you have your brake pads adjusted to rim?

[regularguy412]

Bearings properly adjusted?

First think that came to my mind.

Mike in AR:beer:

[Louis]

With little pressure I can push the rim against the brake pad...


This is definitely not right and probably the cause of your problem. Wheels should not be sloppy side-side.

[RPS]

This is definitely not right and probably the cause of your problem. Wheels should not be sloppy side-side.
Define how much force is "little pressure". ;)

Regardless, without knowing how close the pads are to rim the amount of force is meaningless. If 0.1 mm it wouldn't take much force with any wheel. More data is needed IMO.

[Louis]

More data is needed IMO.

Maybe we should ask the OP for a hysteresis curve...

[wooly]

All wheels flex some, so my first question would be how close do you have your brake pads adjusted to rim?

1.5 to 2mm from the brake pads. I have the pad toed in a bit too. Thanks all.

[fogrider]

thats a lot of movement, I would take the wheel to your LBS!

[TimmyB]

Bearings properly adjusted?
If the wheel's true, in straight and the QR's tightened down properly, this would be my guess as to what the problem would be. That, or really low spoke tension.

EDIT: Also, I'm assuming you don't have this problem with other wheels? As in it's not a result of your frame flexing? Some of them older lightweight aluminum frames flex quite a lot.

[RPS]

1.5 to 2mm from the brake pads. I have the pad toed in a bit too. Thanks all.
That shouldn't be the problem then (which by the way happens to me often when I switch wheels and forget to readjust calipers for wider rims). I'd check the frame for damage as TimmyB suggested. I’d also do your “pushing” test in both directions to see if there is a significant difference which “may” indicate damage you can’t see (i.e. – cracks can pull apart easier than push together). Having said that for safety reasons, I think it’s highly unlikely the problem is due to your frame. It’s probably the wheels are not stiff enough for your weight and strength.

To answer your primary question below, adding additional spoke tension doesn’t do much for lateral stiffness unless spokes are presently being unloaded when you climb out of the saddle. I doubt that’s happening, but if you want to do a quick check push the rim over until it touches the brake pad and feel the spokes on the other side to ensure they are still in tension and not slack.

If I get brake rub when I'm climbing out of the saddle does that mean my rear wheel spokes need to be tensioned?

[firerescuefin]

I don't hold myself as engineer type, but could this be do to flex in the rear triangle. Wooly, how stiff is that Waterford? Would be curious to hear Old Potatoes take as he sells these frames and probably has seen this or similar combos in his shop.

[Mark McM]

I don't hold myself as engineer type, but could this be do to flex in the rear triangle.

In a nutshell, no. Or at least, not likely. The rear "triangle" is actually a tetrahedron, one of the stiffest 3 dimensional structures. Unless one of the members is broken, there is very little flex within the rear triangle itself.

On the other hand, there can be a substantial amount of lateral flex in a wheel, even at the top, away from the ground contact point. Here's a graph of the data from Damon Rinards Wheel Stiffness Test (http://www.sheldonbrown.com/rinard/wheel/index.htm) (see question 4: "How does the shape of the wheel change in response to a lateral load?").

http://www.sheldonbrown.com/rinard/wheel/saddle.gif

It is much more likely that brake rub is due to flex in the wheel, either due to a flexy axle, flexy rim, or even stretchy spokes. The decreased brake pad clearance due to modern high leverage brakes has only made this problem more likely.

[firerescuefin]

Thanks Mark!

[wooly]

Thanks all. The frame is in good shape - no cracks, etc. And I didn't have the same problem with the Dura Ace 7850 Tubeless wheels I came from. My guess is it's the bearings or the spokes. I plan on taking the bike to the shop this morning to get their thoughts. Bummer but hopefully its a simple fix.

[RPS]

In a nutshell, no. Or at least, not likely. The rear "triangle" is actually a tetrahedron, one of the stiffest 3 dimensional structures. Unless one of the members is broken, there is very little flex within the rear triangle itself.
Mark, I think that depending on the bike there can be significant flex in the rear triangle. The tubes may be arranged in a triangle but when loaded sideward they are placed in bending which can lead to significant deflection.

When the rider stands and pedals hard up a hill, chain tension (particularly with compact and triples and when running on a smaller sprocket further towards drive side) creates a moment that tries to flex the chainstays towards the drive side. Also, leaning the bike sideward to maintain balance while pedaling out of the saddle loads the chainstays to the side, also helping flex them. Since the brake caliper is normally attached up high and wouldn’t shift sideward as much, rim-to-pad clearance can be reduced. Some of this flex can be seen when standing behind a bike on a trainer. The BBKT shifts sideward both due to rider weight on pedals but also due to chain tension.

The other mode of loading that can affect clearance on many new bikes is due to curved seat stays intended to act as a minor form of passive suspension to reduce vibration and ride harshness. Since each seat stay acts as an independent spring for the most part, when the bike’s rear wheel is loaded sideward the two stays won’t compress equally, hence allowing the rear wheel to “roll” relative to the main triangle. This also reduces rim-to-pad clearance because the brake is usually attached much higher and thus doesn’t move as much.

I don’t know what he is riding or how much flex his frame has, but it’s important to remember that flex is cumulative. Although I agree the wheel probably contributes to more deflection, it doesn’t have to be all due to wheel or all frame. A bad combination can be the last straw.

[SPOKE]

If the bearing pre-load is ok have the shop check spoke tension. A simple cal or note to the manufacturer asking what the tension should be will give you some enlightening info.
I have a pair of Reynolds DV46C's that suffer the problem you describe. They were like that out of the box. Once everything checked out I just opened the brake calipers a bit using the quick release on the caliper.

[Black Dog]

This all sounds like the function of a low spoke count. Generally, saving a few grams on spokes leads to a wheel that flexes a lot. 24 rear spokes is not enough for a rider of his mass to keep the lateral flex down.

[lhuerta]

The OP just said that his brake shoes are ONLY 1.5-2mm away from the rim, plus toe-in...so I am assuming he measured the distance from the trailing edge of the shoe. This is an excessively close distance, not to mention that at his size he is probably getting 1.5-2mm of flex out of his rear triangle in high torque efforts. The solution ssems pretty simply...let some cable out of your brakes and get a bit more slack in your calipers.
Lou

[RPS]

This all sounds like the function of a low spoke count. Generally, saving a few grams on spokes leads to a wheel that flexes a lot. 24 rear spokes is not enough for a rider of his mass to keep the lateral flex down.
I generally agree; but much also depends on spoke size and rim stiffness. Remember the old and discontinued Shimano low-spoke-count tandem wheels? They can be made strong enough but I doubt there is much weight savings since rim has to be so much stronger and stiffer.

[wooly]

UPDATE - I took the wheels into my shop and the spokes were tensioned. And viola! All is good in the world. They checked the hubs and they were absolutely fine. Thanks for the help.

[ergott]

Delete

[ergott]

UPDATE - I took the wheels into my shop and the spokes were tensioned. And viola! All is good in the world. They checked the hubs and they were absolutely fine. Thanks for the help.


Wait, your spokes were not tensioned enough?

That would be a horrible build if that is what the cause was.

[wooly]

I'm going out on a limb here, but I bet you have a DT hub. Swap the spokes out for DT Competitions or Sapim Race and you will have the problem solved. I've said it many times before, DT rear hubs don't build into laterally stiff wheels. It's more noticeable at lower spoke counts.

King r45 hubs. Tension was off a little. That, plus the fact that my brake pads were realllllllly close to the rim was the cause to the rubbing. Like I said, the retensioning and pad adjustments made all the difference in the world. The wheels are solid as reported by many other Edge reviewers. (they were built by edge early last year).

[RPS]

King r45 hubs. Tension was off a little. That, plus the fact that my brake pads were realllllllly close to the rim was the cause to the rubbing. Like I said, the retensioning and pad adjustments made all the difference in the world. The wheels are solid as reported by many other Edge reviewers. (they were built by edge early last year).
When two changes are made at same time it's impossible to know what corrected the problem. The important thing is that your problem is fixed.

Having said that, only one change has much science behind it. :beer:

[ergott]

King r45 hubs. Tension was off a little. That, plus the fact that my brake pads were realllllllly close to the rim was the cause to the rubbing. Like I said, the retensioning and pad adjustments made all the difference in the world. The wheels are solid as reported by many other Edge reviewers. (they were built by edge early last year).

Good to hear it's fixed. King hub is better.

[wooly]

Now I've gotta lose some weight as mentioned earlier in the thread! :crap: