Front wheel flex only when climbing

[jmoore]

I have a set of Pacenti SL23 wheels purchased directly from Kirk maybe 4-5 years ago. Recently I have noticed I am getting some flex when I’m out of the saddle climbing. This doesn’t happen any other time. And it has never happened before. Nothing else has changed on the bike.

What should I look for to find this issue? Spoke tension is my first thought. What else?

Im 6’5” and 240-ish. Wheels are 28hole, 3x

[Blown Reek]

What pressure are you running up front?

[jmoore]

What pressure are you running up front?

80 front
85 rear

Same as always

[Mark McM]

Presumably the front wheel flex is manifested as front brake rubbing? You've checked that the wheel is straight and true, and that the brakes are centered?

It's possible that low tension can cause wheel flex, but if the spokes have lost enough tension to cause rubbing, there's usually other symptoms (such as loss of wheel true, or ticking sounds as the spokes slacken and re-tighten)

Some other things to check are tension and adjustment of the front quick release, and also whether any play has developed in the bearings.

[93KgBike]

How much did you weigh 4-5 years ago?

240lbs is heavy enough to feel the fork spring and to flex the bars - simultaneously. Could that be some of it?

Could it be the sidewalls at that pressure?

Rebuilding it with radial spoke pattern will stiffen it.

[Mark McM]

Rebuilding it with radial spoke pattern will stiffen it.

... but only a little bit. When Keith Bontrager measured the lateral stiffness of a radial wheel (laced heads out) vs. a 3x wheel (same rim, hub and spokes), he found that the lateral stiffness only increased by about 5% - which is about the same as the difference in the length of the spokes (radially laced spokes are shorter, and shorter spokes are stiffer). However, he found that lacing the radial spokes heads in increased the lateral stiffness by about 10%. The extra stiffness came from the increased spoke bracing angle.

On a more practical level, it is usually not recommended to re-lace a hub radially after it has already been built with crossed lacing. However, increasing the spoke thickness will also increase wheel stiffness. Going from the thinnest spokes to the thickest spokes will have a larger difference than changing the lacing pattern.

[berserk87]

It could be attributed to a difference in riding style. For example, if you are favoring an injury (I have creaky knees myself) to your lower body, you could be weighting the bars more heavily when climbing to take pressure off of your legs.

I've been guilty of this. I am not a lightweight but I'm not 240 and if I lean on the bars too much I can create some rub on the pads.

If all other variables are the same, this might be something to consider.

I don't know what tire width you are riding. When I ride 25's I pump them up to 95 to 100 PSI. I have a history of flats with pressures below that. Has your tire width changed?

[GScot]

I had a Reynolds front wheel do exactly that once, went from no rub on climbs to climbing means brake rub and no other changes I could identify. Weighed about 180 at the time. Bearings also had a bit of noise so I replaced them and the problem went away.

[ryker]

Rebuilding it with radial spoke pattern will stiffen it.

While a radial pattern would be stiffer, I strongly advise against rebuilding a 3x-built hub to radial. Get a new hub if you wish to pursue this course.

Edit: sorry this advice was already dispensed! Furthermore I'd add a recommendation to check with your hub mfg. before radially lacing any 28h hub.

[thermalattorney]

Could be loose bearings. I had an AC Micro 58 loosen up, but only enough to where it could be noticed on climbs and up on the work stand.

[Marcovelo]

I am not familiar with these rims, however, if they are carbon there is a very strange phenomenon I experienced (and found documented in literature on wheel performance) according to which the stiffer the rim the more it transmits deflection across itself, enough that with a lot of lateral movement of the bike/wheel in violent sprinting, for example, the rim can hit the brake pads. Here's the important part: not because the rim/wheel is "soft" but because it is "too" stiff.

I really don't have an intuitive understanding of this effect, but I definitely experienced it with a new set of Zipp wheels.

[fmradio516]

Curious what size tire youre running. I am around 220 and am afraid to go below 90 psi in either tire.

[Mark McM]

I am not familiar with these rims, however, if they are carbon there is a very strange phenomenon I experienced (and found documented in literature on wheel performance) according to which the stiffer the rim the more it transmits deflection across itself, enough that with a lot of lateral movement of the bike/wheel in violent sprinting, for example, the rim can hit the brake pads. Here's the important part: not because the rim/wheel is "soft" but because it is "too" stiff.

I really don't have an intuitive understanding of this effect, but I definitely experienced it with a new set of Zipp wheels.

See the answer to question 4 in Damon Rinard's wheel stiffness test (https://www.sheldonbrown.com/rinard/wheel_index.html).


The basics are these:

If the rim was very, very stiff, then the deflection under a lateral load would be for the rim to "rotate" around the rim - if the bottom of the rim was pushed to the left, the top of the rim would move to the right:

https://www.slowtwitch.com/articles/images/1/65611-largest_Wheel_Deflection_Graphic_by_Depth_1.png


If the rim has very little stiffness, than a lateral load would act to deflect the rim in a buckling mode. A classic example of the deflection under buckling is a failed wheel that ends up with the rim in a "taco" or "potato chip" shape. In the buckling mode, the rim deflects in a "saddle" shape, so if you deflect the bottom of the rim to the left, the top of the rim also deflects to the left:

http://chuck.kichline.com/bikes/bits/bikewheel.jpg


Of course, most rims ave neither zero nor infinite stiffness, and the deflection will be a combination of these modes.

[Ken Robb]

See the answer to question 4 in Damon Rinard's wheel stiffness test (https://www.sheldonbrown.com/rinard/wheel_index.html).


The basics are these:

If the rim was very, very stiff, then the deflection under a lateral load would be for the rim to "rotate" around the rim - if the bottom of the rim was pushed to the left, the top of the rim would move to the right:

https://www.slowtwitch.com/articles/images/1/65611-largest_Wheel_Deflection_Graphic_by_Depth_1.png


If the rim has very little stiffness, than a lateral load would act to deflect the rim in a buckling mode. A classic example of the deflection under buckling is a failed wheel that ends up with the rim in a "taco" or "potato chip" shape. In the buckling mode, the rim deflects in a "saddle" shape, so if you deflect the bottom of the rim to the left, the top of the rim also deflects to the left:

http://chuck.kichline.com/bikes/bits/bikewheel.jpg


Of course, most rims ave neither zero nor infinite stiffness, and the deflection will be a combination of these modes.

When I read that Damon was doing these tests at San Diego State U. I loaned him my Rolf wheels and watched him run some tests in his campus lab/workshop. Hey, I even got a thank you at the end of his report. I enjoyed the opportunity of seeing how he did the testing.

[Dave B]

I had a set of Easton deep dish carbon tubulars that were great except fro when I was climbing out of the saddle. The saw-like motion of my handlebars moving back and forth caused the front wheel to flex and rub.

I don't know if that would apply to you, but possible riding style at certain moments?

[Marcovelo]

Hi Ken,

If I understand your post, that is what believe I was experiencing. Very odd, at least from an intuitive point of view. But, as so often, expert knowledge deviates dramatically from intuition.

Thanks for the clarification.

-marco

[jmoore]

Update: I checked the spoke tension and they are off enough to notice when they were plucked. I plan on pulling them and evening up the tension tonight. My ride today was fine but no standing climbing.

[robt57]

Update: I checked the spoke tension and they are off enough to notice when they were plucked. I plan on pulling them and evening up the tension tonight. My ride today was fine but no standing climbing.


If it is a V1 SL23 Rim, check very close for rim bed cracks at the spoke holes. Are you getting any errant noise or creaks?

Less likely on a front than a rear, but if over stressed or stress relieved after being brought to tension during build V1 rims need close look for this.

Is it all the original parts/build? If V2 Rim kindly ignore all I have spouted.


BTW, I am on the 3rd V1 SL23 'rear' 28h Rim. The front in my case has been fine. Each time I replaced the cracked rear SL25 V1 I used a little less tension and added washers.

I did not have to stress relieve having used the same spokes as orig build [with tension ga used] I did however tension and them stress relieve first ime with my bear mitts. But the second rim I did not expect to crack, nor even faster than the first time. 3rd one bee good for a while now.

[jmoore]

If it is a V1 SL23 Rim, check very close for rim bed cracks at the spoke holes. Are you getting any errant noise or creaks?

Less likely on a front than a rear, but if over stressed or stress relieved after being brought to tension during build V1 rims need close look for this.

Is it all the original parts/build? If V2 Rim kindly ignore all I have spouted.


BTW, I am on the 3rd V1 SL23 'rear' 28h Rim. The front in my case has been fine. Each time I replaced the cracked rear SL25 V1 I used a little less tension and added washers.

I did not have to stress relieve having used the same spokes as orig build [with tension ga used] I did however tension and them stress relieve first ime with my bear mitts. But the second rim I did not expect to crack, nor even faster than the first time. 3rd one bee good for a while now.It's the v1 as far as I know. I will check them both carefully. I do get an odd noise from the front (I think) but I've had it for a long time. Never a problem, just annoying.

Sent from my LG-H931 using Tapatalk

[robt57]

It's the v1 as far as I know. I will check them both carefully. I do get an odd noise from the front (I think) but I've had it for a long time. Never a problem, just annoying.

Sent from my LG-H931 using Tapatalk

If it has the wider brake track it is V1. It may be bigger issue than you think. Suggest you clean it and look very close by spokes for cracks.

I suppose it is possible in my case my rears were extruded first off a new die and my front off a well worn die leaving more material. And your front/rear opposite. I never weighed them before building. This was enough 'for me' to stop buying the brand. I do love the brake performance the type of machined brake track seems to afford though.