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Rythmic Thumping On Smooth Pavement

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Rythmic Thumping On Smooth Pavement

Old 09-16-13, 08:20 PM
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Rythmic Thumping On Smooth Pavement

I have a Cannondale T700 touring bike and I recently changed out the 38c tires for Marathon Plus 28's to help speed my commute along. On one particularly smooth section of road I noticed a rhythmic thump, thump, thump. I can feel it all through the bike, though it seems less pronounced if I remove my hands from the bars. If I stand on the pedals I feel it strongly, so I assume it is a front end issue. At any rate, I reseated the tires and brought the wheels in for truing. After truing I could almost say the problem was more pronounced. I'm 180lbs and I have been running them at 100psi and even 90 a couple times, but I don't notice a change with pressure. I began to suspect the headset or hub, but I can't detect anything obvious, such as play when I hold the breaks and apply force to the bars. Two bike shops have failed to find anything, telling me the problem seems very subtle. Maybe I'm the princess and the pea or maybe I am just lucky to have a couple miles of asphalt that is smooth as a babies bum, but the thump is anything but subtle to me. Any ideas?
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Old 09-16-13, 08:35 PM
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Is the frequency synchronous with wheel rotation or crank rotation? Does it occur both when you are pedaling or not or just when pedaling? Any chance the tube is kinked inside the tire? Check for a tight chain link. Are the hubs smooth and free of play?
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Old 09-16-13, 08:42 PM
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Odds are it's a tire problem.

Start by spinning the the wheel with your finger as an indicator gauge at the crown of the tire. Use the front brake as a support, and see if you can find high or low spots. If the tire rises and falls, but the rim is true. It's either a seating problem, or the tire itself may have a local bulge.

Often I've found thumping to be an early warning of a tire about to blow out due to wall weakness in one area.
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Old 09-16-13, 08:43 PM
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Originally Posted by HillRider View Post
Is the frequency synchronous with wheel rotation or crank rotation? Does it occur both when you are pedaling or not or just when pedaling? Any chance the tube is kinked inside the tire? Check for a tight chain link. Are the hubs smooth and free of play?
It occurs without pedaling and it keeps the same frequency despite changes in speed (less frequently than wheel rotation.) Tube is not kinked as I have tried two different tubes, two different tires and even grabbed another front wheel from a different bike and still feel it the same.
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Old 09-16-13, 08:46 PM
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Originally Posted by FBinNY View Post
Odds are it's a simple tire seating problem.

Start by confirming that your problem is in fact related to the tire being eccentric. Spin the front wheel with your thumb ot something braced against the front brake bolt or fork crown as close to the tire as possible. Look for the gap to change as the wheel spins. Assuming you do discover that the tire is eccentric, then you need to see if the rim is also, or if it's just the tire.

Spin the wheel slowly, watching the tire's molded reference line just beyond the rim's edge. That will confirm whether the rim is too low in one area, or high and about to jump off the rim. Let out enough air that the tire is just about limp, and maneuver it around the rim to seat it more evenly. Then reinflate. If the tire is out of the rim in one area, there's a decent chance that you have some of the tube trapped between the bead and rim.
Even a different wheel gives the same problem. I had a very reliable shop true the wheel and reseat the tire and tube.
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Old 09-16-13, 09:05 PM
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Originally Posted by WoodsterSS View Post
Even a different wheel gives the same problem. I had a very reliable shop true the wheel and reseat the tire and tube.
You talk about the wheel, which I suppose means the front wheel. Have you run the same diagnostics on the rear wheel?
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Old 09-16-13, 09:20 PM
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Originally Posted by FBinNY View Post
You talk about the wheel, which I suppose means the front wheel. Have you run the same diagnostics on the rear wheel?
Well, I had the year wheel trued and inspected at the same time as the front. But, I have not tried a different rear wheel.
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Old 09-16-13, 09:24 PM
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Originally Posted by WoodsterSS View Post
Well, I had the year wheel trued and inspected at the same time as the front. But, I have not tried a different rear wheel.
Don't switch the wheels, Spin them with an improvised feeler gauge looking for any sign of "hop" or radial runout. Check both the tire at the crown and the rim by watching as passes the brake shoes.

BTW- even a fairly true wheel or tire can make a noticeable bump as you ride if the hop is local as opposed to broad based. Compare the difference between a speed bump and speed hump to get what I'm referencing.

Don't rely on anyone to say if a wheel is true. You have eyes and ears and you can do it yourself. Use a piece of plastic or credit card as a feeler gauge. Brace the gauge against the frame of brake, and spin the wheel and slowly advance it toward the tire until it first makes contact. Wheels are never perfect so it'll hit at the highest spot first making a rubbing sound. If the rubbing is in short pulses, or long and continuous with a short interval you know you have the kind of local hop you'll feel.
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Old 09-17-13, 02:31 AM
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Also it can be a seating problem. Check the sidewall markings and ridges to be equidistant from the rim edge.

I had a similar problem: installed new tyre and a hump was felt. Check runout it was fairly true, but spotted the center tread was twisting a bit. At that point the tire was not seated and was just about in the well in that area. So the sidewall markings were not equidistant but the lettering was about to contact the rim. Also there are some factory casting lines that run near the bead and can look for them in relation to the rim.

To avoid this problem I now inflate to 8-10bar (max for the tyre), and then lower the value to whatever i like (6-7bar, on 25C). It makes the popping sound while seating, like for a car tyre .. as the rim and tyre are somewhat a tight fit.
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Old 09-17-13, 08:17 AM
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Originally Posted by WoodsterSS View Post
It occurs without pedaling and it keeps the same frequency despite changes in speed (less frequently than wheel rotation.)
This is what makes the problem so odd. It's not related to the crank/chain and seems independent of wheel speed.
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Old 09-17-13, 08:35 AM
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This makes no sense to me at all. When coasting the only rhythmic components are wheel rotation on the road and in relation to the cassette/freewheel. The frequency in both cases changes with speed. I cannot believe that you have a problem on the bike that does not change with speed. The only rhythmic pattern I can think of that would not correlate exactly to speed would be your pulse, and that does not seem to match either. I am wondering if it is only one particular section of road, rather than any smooth section - because that means it could be that section of road. Nobody else would notice the problem because they are not riding the same road.
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Old 09-17-13, 08:45 AM
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Originally Posted by cny-bikeman View Post
I am wondering if it is only one particular section of road, rather than any smooth section - because that means it could be that section of road. Nobody else would notice the problem because they are not riding the same road.
I've ridden on roads that give a rhythmic thumping because of the expansion joints or periodic waves in the pavement but, of course, these are definitely speed related.
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Old 09-17-13, 09:02 AM
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Originally Posted by HillRider View Post
This is what makes the problem so odd. It's not related to the crank/chain and seems independent of wheel speed.
That statement throws me off too. I can't think of anything that would cause such a phenomenon.
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Old 09-17-13, 09:45 AM
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I know this sounds odd, but since it isn't bike or pedal speed related, I'm down to the odd.

Is it at all possible that you're feeling your own pulse or heartbeat? If not, consider having a friend join you on a ride to that good section of pavement and switch bikes.

f
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Old 09-18-13, 04:13 PM
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Originally Posted by FBinNY View Post
I know this sounds odd, but since it isn't bike or pedal speed related, I'm down to the odd.

Is it at all possible that you're feeling your own pulse or heartbeat? If not, consider having a friend join you on a ride to that good section of pavement and switch bikes.

f
I do have a an incredible ticker that would make Chuck Norris envious, but I think that, no, it isn't my heart. The one shop guy who rode it could feel it as well, but said it was very subtle, which I would agree with in reference to the rough road he tested it on. The thing is, I have six bikes and lots of experience riding, changing flats, changing tires, etc and this is the first time I have encountered this problem. The only thing that is new here are the type of tires: Marathon Plus. When I first installed them (changing from 35's to these 28's) I felt the problem. I inspected everything and found a small bump on the rear tire. It was bad enough that Schwalbe sent me a new tire. But, the problem did not go away, making me think the other tire is still good. I am still thinking it is the either the wheel or the tire. I have swapped out both front and rear wheels with my other bikes (23c's though) and I think the problem completely goes away. Could Marathon QC really be that bad? I thought Schwalbe was a well respected brand. I seems to make more sense to blame the wheel.
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Old 09-18-13, 05:42 PM
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Well, too bad about the pulse, which I also mentioned earlier. I did not know when the mechanic said it was subtle that you meant he actually felt it, but that means it's the bike rather than you. The problem here is your insistance that the frequency is constant, independent of speed - there's nothing in the mechanics of a wheel that would allow such a phenomenum - at best it would be something random, such as water inside the tube. It does sound like the wheels are more likely the culprit, but did you change tubes when you changed tires?
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Old 09-18-13, 09:38 PM
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Old 09-19-13, 06:56 AM
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Originally Posted by cny-bikeman View Post
Well, too bad about the pulse, which I also mentioned earlier. I did not know when the mechanic said it was subtle that you meant he actually felt it, but that means it's the bike rather than you. The problem here is your insistance that the frequency is constant, independent of speed - there's nothing in the mechanics of a wheel that would allow such a phenomenum - at best it would be something random, such as water inside the tube. It does sound like the wheels are more likely the culprit, but did you change tubes when you changed tires?
Yes changed the tube and made sure it didn't bind. I have tried reducing the pressure to 80 psi vs 110 as well. I have another Marathon Plus that I am going to try on the front (as I said I have switched out the one on back) as a last ditch attempt to solve this. If it is the tire, then, I received two brand new, bum Marathons, which seems so unlikely. I will pay more attention to the variation with speed, perhaps it does, but my speed variations when I notice the effect have been too small to detect.
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