My LBS relaced my rear 808 with a PT Pro+, 20H. I’m neither heavy (157 lbs) nor powerful but I get a lot of deflection when I pedal…the rear brake pads have to be set pretty far from the rim or they rub with each pedal stroke. Just pressing with my thumbs, my rear rim deflects more easily than my front, which has the standard Zipp hub, 16H, as it came from the factory.
Is my experience abnormal? Is this a case of a weak wheelbuilding job?
Most likely…sppoke tension needs to be checked with gauge to be sure. See zipp and powertap websites for proper spoke tension.
Low spoke tension will make the wheel feel whippy as the amount of deflection increases non-linearly when the non-drive side spokes are unloaded. The non-drive spokes have less tension because they are located farther from rim center than the drive spokes. The PT hub if anything increases stiffness because of the larger flange. That being said, I ride a 28H rear with a 68mm carbon rim because even a properly tensioned 20H rear wheel is not stiff enough for me, and I tend to break more spokes.
-P
Low spoke tension will make the wheel feel whippy as the amount of deflection increases non-linearly when the non-drive side spokes are unloaded.
This guy would disagree with you. I think I’m going with him on this one. I think this is one of those long-held myths that intuitively makes sense, but in actuality just isn’t true.
Having ridden a 20 hole 808, I was able to get the brake pads rubbing under hard acceleration. I ditched it and got a 24 hole wheel.
This guy would disagree with you. I think I’m going with him on this one. I think this is one of those long-held myths that intuitively makes sense, but in actuality just isn’t true.
Having ridden a 20 hole 808, I was able to get the brake pads rubbing under hard acceleration. I ditched it and got a 24 hole wheel.
I’ve got the 24 spoke 808 with the PT and it’s rock solid. Of course I’m no Tom Boonen either.
JJ
I’m not sure what you mean, the graph of spoke tension vs. wheel deflection proves my point. On the far right as tension is decreased, the deflection increases non-linearly. The true statement is that spoke tension has no affect on wheel deflection assuming the spokes are not unloaded to zero tension. Once this happens, the wheel will feel whippy. This does not often occur in front wheels often as the tension is even on both sides and the bracing angle is larger. But on a rear it can and does happen if the tension is not sufficient. This is the reason wheelbuilders take the rear drive tension as high as they can given the limits of the rim, nipple, and spoke. It is also the reason for things like radial lacing on drive spokes that has become popular. The tangential component of the crossed spoke slightly increases tension in the non-drive side and the radial lacing on the drive side slightly increases stiffness.
I get what you’re saying, and in a wheel with a bunch of miles on it, I have seen spokes loosen to the point of almost being ‘slack’. I’ve never seen anything like this though on a wheel with under a few thousand miles, say a wheel used for TT’s/tri or race-only.
The general thought though is that ‘higher tension equals a stiffer wheel’, which doesn’t seem to be true (sorry if that’s not what you meant). It does seem that number of spokes, spoke gauge and bracing angle are what really matter.
Your right, shouldn’t be an issue in a properly tensioned wheel, but from his description it sounds like there is too much deflection. A bad build with uneven tension in a low spoke count wheel can cause the nipples to loosen during riding pretty quick.
20 spokes?
Yeah sure,
I’would also blame the guy who actually agreed to built that wheel.
Post of the day!
20 spokes…
Ive got a 24 spoke FP 80 and it’s solid. I still cringe when I hit an unexpected pothole.
I am 157 also, and wouldn’t ride a 20 spoke, probably not even in a race.
" I think this is one of those long-held myths that intuitively makes sense, but in actuality just isn’t true."
Agreed that it’s somewhat intuitive to think that if you “tighten” something, it will resist movement better. However, here is a thought experiment (are you out there Greg-X?): take two identical spokes, affix them so they are hanging vertically. Hang a five pound weight from bottom of one and on one and ten pounds on the second. Now if you were to add an additional ten pounds weight to each, would you expect the one pre-loaded with ten pounds to stretch less than the one pre-loaded with five pounds? Any engineers out there that can explain this one?
Stupid ? But is it just out of dish
No, the wheel is true when not under load.
Ijust asked because I have a friend who had a powertap Aeluos wheel that looked fine while not ridding(though a little off center) but only rubbed the chainstay under load. Sent the wheel back…Now fine. Thanks for the response though