Wheel stiffness, or side-to-side flex

[54ny77]

Dug out a set of Mavic Cosmic Carbone SL wheels from the hoard stash, the kind with carbon fairing and aluminum rim. Probably 10 or so years old. Boy do they roll nicely when at speed on flats and downhill, I forgot what they were like. It's a pretty wide section rim.

Was on ride today and when grinding up hills I was flexing the wheel into the brake calipers repeatedly. Can't remember that happening before. It's certainly not my prodigious power output.... In any event, upon coming home I checked various rear wheels as mounted in frames. Some have more wiggle room than others.

Knowing largely nada about wheels, what's the scoop with the side to side variance?

[GregL]

Dug out a set of Mavic Cosmic Carbone SL wheels from the hoard stash, the kind with carbon fairing and aluminum rim. Probably 10 or so years old. Boy do they roll nicely when at speed on flats and downhill, I forgot what they were like. It's a pretty wide section rim.

Was on ride today and when grinding up hills I was flexing the wheel into the brake calipers repeatedly. Can't remember that happening before. It's certainly not my prodigious power output.... In any event, upon coming home I checked various rear wheels as mounted in frames. Some have more wiggle room than others.

Knowing largely nada about wheels, what's the scoop with the side to side variance?
There really shouldn’t be any lateral flex to speak of. I raced Carbone SLs for years, including standing climbs up double-digit grades. I never felt they were flexy under my 175lbs. If the wheel is true and all spokes are evenly tensioned, I’m stumped.

Greg

[hockeybike]

Might want to check spoke tension. Had a really annoying stans a340 that couldn’t hold even tension, I’d get some wooosh-whooshing noises from flexing the wheel on climbs. Solution: get a new wheel. In your case, probably just need to have the spokes looked at.

[Mark McM]

Lateral motion at the rim could be either from (elastic) flex in the wheel, or free-play in the bearings. When you wiggle the wheel from side to side, do you feel an initial free movement with a distinct "stop" where it takes significantly more force to deflect the rim further? If so, then the suspect would be free-play in the bearings. Depending on the year and model, there may be different procedures to adjust the bearings.

If instead the force to deflect the rim gradually and continuously increases as the rim is deflected, then it may just be that the design of this wheel is makes is flexier than others.

As far as spoke tension affecting stiffness, tension by itself does not affect wheel stiffness, as long as none of the spokes completely losses tension. As long as the spokes don't completely detension when flexing the wheel, tightening the spokes won't make the wheel any stiffer.

[reuben]

Might want to check spoke tension.

That was my first thought as well.

[jimoots]

This is a nice article to explain the why - I know I have only ever had brake rub wheels that are using very stiff carbon rims.

https://www.slowtwitch.com/Tech/Debunking_Wheel_Stiffness_3449.html

In your case it may be spoke tension (as identified by others) or simply the wheel doesn’t have enough spokes

[john903]

Thank You for the link to the article. I have this similar issue and have always wondered why. My Ambrosio Nemesis wheels don't rub the brakes, but my Campy Hyperons do rub the brakes climbing the same hill.

[54ny77]

This article was great. Is one aspect of it saying wheels can be so laterally stiff that they stay in plane while the rear triangle moves?

Out of curiosity, I just took a few wheel sets out for a quick test up the street (which has a pretty good rise and is about a quarter mile long), and went up at slightly quicker than normal pace, rocking bike from side to side, sometimes in an exaggerated motion.

Used the same bike, a Parlee Z5 with a 39 inner tooth small ring. Brake pad distance to rim (using carbon-specific pads or regular Dura Ace rubber) was set using the quick release/adjust cam at about 2 mm +/- from brake surface on rims. All wheels have 11-28 cassettes.

In order of most rub to least:

1) New Reynolds Aero 46 carbon w/Reynolds hubs and aero spokes: lotsa rub. Had to open up the quick release on brake caliper all the way, interestingly. No hub bearing play, rock solid. All spokes tight.

2) Mavic Cosmic Carbone SL (~10 yrs. old), Mavic hubs and aero spokes: good amount of rub depending on lean/effort. No hub bearing play, also rock solid. Checked spokes and found a couple of looser ones.

3) Edge 45 carbon (these are old wheels, ~10 yrs. old), Powertap+ rear wheel (which I think is DT internals) and aero spokes. Rubbed only a few times. No hub bearing play, and actually feels like it needs some attention. All spokes tight.

4) DT RR400-something (I forget, took the stickers off; am pretty sure they were the lightest alu. rim DT made at the time), about 10 yrs. old, with DT 190 ceramic hubs and aero spokes. Very light wheels, no brake rub. Zero bearing play. All spokes tight.

5) HB Son TB14 w/Ultegra hubs and round stainless spokes. No rub. Zero bearing play. All spokes tight.

Moral of story: no idea. How 'bout just set the brake calipers wider and haul ass when using go-fast carbon rims? Incidentally, the new Reynolds wheels I mentioned, purchased from a fine fellow forum member, are RIDICULOUSLY fast. I've effortlessly hit speeds of mid-high 30's on sections of a typical rolling/flat route that have never been achieved using the usual box section rims. Once up and running, man these things just wanna GO. This feature has fanned the flame of interest in deep section carbon wheels, and I'm now addicted. Apart from brake rub!

Thanks for all the food for thought.

This is a nice article to explain the why - I know I have only ever had brake rub wheels that are using very stiff carbon rims.

https://www.slowtwitch.com/Tech/Debunking_Wheel_Stiffness_3449.html

In your case it may be spoke tension (as identified by others) or simply the wheel doesn’t have enough spokes

[Peter P.]

I was going to suggest it being bladed spokes, but I see the slowtwitch article covers that, calling them "aero" spokes.

Reduce the lateral cross-section of the spokes and you'll reduce lateral stiffness.

[robt57]

Get it straight y'all. Stiffer rims/wheels rub brake blocks before flexy ones. Stiffer stays in plane better and there-fore across the fulcrum [axle] will move more, not less.

If your wheels did not rub and now do, my money is on your tires being pumped up higher than when they did not rub, your brakes where adjusted for narrow rims and now have wider rims, or maybe new pads got put on... Or you put on a tire with a less flexy sidewall.

My 2 cents, and worth every penny. ;)

[Mark McM]

This article was great. Is one aspect of it saying wheels can be so laterally stiff that they stay in plane while the rear triangle moves?

The author doesn't say that, he is quoting Zipp who are claiming that. But maybe he misunderstood what Zipp was saying, because the rear triangle has no significant amount of flex (with respect to the rear axle). Although we call it a "triangle", the rear of the frame is actually a tetrahedron (which is made of up of 4 triangles). A tetrahedron is the stiffest 3 dimensional structure in engineering.

Moral of story: no idea. How 'bout just set the brake calipers wider and haul ass when using go-fast carbon rims?

Even before carbon rims, it was common for those using lightweight (and therefore flexy) aluminum rim wheels to open up the rear brake QR lever before a climb, to prevent brake rub.

Reducing rear wheel brake rub was one of several advantages of Campagnolo's 'Differential' brakes, which used a single pivot brake on the rear and dual pivot brake on the front. Single pivot brakes have more clearance between pad and rim, and less likely to rub when the wheel flexes.

[Mark McM]

Get it straight y'all. Stiffer rims/wheels rub brake blocks before flexy ones. Stiffer stays in plane better and there-fore across the fulcrum [axle] will move more, not less.

It's a combination of the rim/spoke stiffness ratio, and the wheel total stiffness, not just the rim stiffness. If you were to use a typical deep and stiff carbon rim with 72 thick spokes, the brakes will not rub, because the wheel total stiffness is higher. Wheels with very light and shallow (and therefore flexy) rims also are more likely to rub, but they actually rub against the opposite brake pad, due to the buckling action of the wheel structure, as shown in this diagram:

https://www.wheelfanatyk.com/wp-content/uploads/2018/05/wheelcollapse.png

[54ny77]

oh man i thought i controlled the experiment by using the same bike...and now you're saying use the same tires and pressure to get to bottom of it? (yeah, you're prob. right though!)

i'm just gonna go with the answer being such massive power output as cause for wheel flex.....

....as little old ladies with battery-powered e- bikes blow past me...



If your wheels did not rub and now do, my money is on your tires being pumped up higher than when they did not rub, your brakes where adjusted for narrow rims and now have wider rims, or maybe new pads got put on... Or you put on a tire with a less flexy sidewall.

My 2 cents, and worth every penny. ;)

[robt57]

....as little old ladies with battery-powered e- bikes blow past me...


They pass me without motors.... ;O

[robt57]

It's a combination of the rim/spoke stiffness ratio, and the wheel total stiffness, not just the rim stiffness.

"Stiffer rims/wheels" I said in one place... ;)

[54ny77]

robt57 (or others): i tried your suggestion with low profile brake pads, 23c tires, and tire pressure at 90psi (vs. my usual 100 when using 25c tires). when i romp on the pedals, i'm still flexing the rear. front, less so (but still a little). and i tried this while on two bikes. same result.

again, there's zero play on the hub (bearing). wheels spin nice & true.

got any other ideas? are these just insanely "stiff" wheels, holding a vertical plane while the rest of the bike flexes?

Get it straight y'all. Stiffer rims/wheels rub brake blocks before flexy ones. Stiffer stays in plane better and there-fore across the fulcrum [axle] will move more, not less.

If your wheels did not rub and now do, my money is on your tires being pumped up higher than when they did not rub, your brakes where adjusted for narrow rims and now have wider rims, or maybe new pads got put on... Or you put on a tire with a less flexy sidewall.

My 2 cents, and worth every penny. ;)

[Mark McM]

robt57 (or others): i tried your suggestion with low profile brake pads, 23c tires, and tire pressure at 90psi (vs. my usual 100 when using 25c tires). when i romp on the pedals, i'm still flexing the rear. front, less so (but still a little). and i tried this while on two bikes. same result.

again, there's zero play on the hub (bearing). wheels spin nice & true.

got any other ideas? are these just insanely "stiff" wheels, holding a vertical plane while the rest of the bike flexes?

It is highly unlikely that the wheel is stiffer than the frame's rear triangle. In fact, I'd bet a lot of money against it. As indicated in the Slow Twitch article, a more likely scenario is that the rim is stiffer than the spokes. I think that you probably only have 3 ways to address the situation:


Change the wheels for stiffer wheels. Or, you rebuild the wheels with either a less stiff rim or stiffer spokes

Change the rear brake to one that has more pad clearance. Rear wheel flex is one of the reasons Campagnolo made their Differential brakes - the single pivot rear caliper has more pad clearance, so the pads are less likely rub when the wheel flexes.

Adopt a different climbing style, which generates lower lateral forces on the wheel (i.e, don't lean the bike back and forth as much).