How much wobble in the wheel?

Rich8P said:

Hi,

Yesterday evening I went on a hill tour with some pretty steep decents. I found that the big starts to shake a little and feels unstable on some of the faster decents.

This could well just be my in experience and fear of fast downhills on a racing bike. I was overtaken by some really fast riders with thier hands on the bar near the stem and leaning right out over the front wheel so I know it is possible to go much faster. I'm usually covering the brakes at 60-65kph.

When braking lightly I noticed that the back wheel has a slight wobble caused by the rim being very slightly buckled. I put the wheel in a truing stand and measured but it is only 2mm maximum. Can this really be the cause of unstable behavior?

How perfect can you centre a wheel? I have off-the.shelf Mavic Askium wheels with system spokes.

Thanks

Rich

Rich, 2 mm max lateral runout isn't bad at all and shouldn't give you any problems. Most wheels can be trued closer that this, but they may not "hold" the true for long. Believe you're better off with even spoke tension rather than constantly tweaking spokes in an attempt to keep the wheel perfectly true.

If I read correctly, your back wheel starts to shake (or bounce?) at 60 kph and up. Suggest you check tire pressures, roundness of the wheel/tire assembly, and wheel balance. If you're 80 kg or less, 100-105 psi (~7 bar) should be plenty.

For roundness, look at the radial runout of the wheel and tire. If you see more than 2-3 mm of "hop" at the rim, that could be trued out. Or perhaps the tire is out of round significantly. If so, you might try remounting it in a different position to reduce the hop, or replacing the tire.

To check balance, put the bike in a stand, select a big gear, and spin the rear wheel up to 50-60 kph using the cranks. If the saddle is bouncing vertically more than a cm, suggest you add balance weight to smooth it out. Rims are often heavy at the splice (where they are pinned together) which is across from the valve stem hole. If that's the case, you can add a second nut on the stem, or twist wire solder around the spokes on either side of the stem.

Sorry if I got into too much detail, but we have some 80+kph descents here and I like the feeling that both tires are stuck down on the road. The smooth ride is nice as well; makes high speeds more relaxing and fun 🙂

dhk2 said:

Rich, 2 mm max lateral runout isn't bad at all and shouldn't give you any problems. Most wheels can be trued closer that this, but they may not "hold" the true for long. Believe you're better off with even spoke tension rather than constantly tweaking spokes in an attempt to keep the wheel perfectly true.

I guess I need to get a tension-meter to make sure that the tension is even. I'll look into that. At the moment I've never touched the spokes since I purchased the bike except to have one replaced at the LBS after I picked up some debris.

dhk2 said:

If I read correctly, your back wheel starts to shake (or bounce?) at 60 kph and up. Suggest you check tire pressures, roundness of the wheel/tire assembly, and wheel balance. If you're 80 kg or less, 100-105 psi (~7 bar) should be plenty.

Now that you mention it, I'll have to check again by riding it. I got the feeling that the shake is lateral.

The tip with the tyre pressures is a good one. I am 97kg and ride 10 bar (just inflated to the maximum without considering it). Based on your comment here I've read a bit more and think I'll try a little less. I looked on a few FAQ sites and people aren't recomending a pressure. Do you have a rule of thumb?

dhk2 said:

For roundness, look at the radial runout of the wheel and tire. If you see more than 2-3 mm of "hop" at the rim, that could be trued out. Or perhaps the tire is out of round significantly. If so, you might try remounting it in a different position to reduce the hop, or replacing the tire.

Again I'll check this on the bike as soon as I get back from travells.

dhk2 said:

To check balance, put the bike in a stand, select a big gear, and spin the rear wheel up to 50-60 kph using the cranks. If the saddle is bouncing vertically more than a cm, suggest you add balance weight to smooth it out. Rims are often heavy at the splice (where they are pinned together) which is across from the valve stem hole. If that's the case, you can add a second nut on the stem, or twist wire solder around the spokes on either side of the stem.

What kind of stand to you mean? I do have a maintainance stand but I don't think I could get the bike to 'hop' with the stand (I'm very new to this).

dhk2 said:

Sorry if I got into too much detail, but we have some 80+kph descents here and I like the feeling that both tires are stuck down on the road. The smooth ride is nice as well; makes high speeds more relaxing and fun 🙂

Thanks for taking the time to write the detail, great advice.

I see others doing 80+kph with thier hands by the stem or out on the tri-bars. Perhaps much of this is in the mind of the rider but I'd like to get that stable and I don't see them wobbling.

Thanks again.

1mm out-of-true is already considered poor quality for wheelsets. 2mm is a lot. Also wheel balance is also crucial.

So I put the bike in my repair stand, moved the wireless tacho to the back wheel and wound it up to 60 kph. Other than a getting a sweat on and feeling like an america's cup winchman, I didn't notice any movement. Perhaps the stand holds the bike too firm.

I then put the wheel in the stand that I use to centralise my MTB wheels. I assumed that the heaviest part would go to the lowest point. Sure enough: value at the bottom every time. I can't spin the wheel fast in this stand because it is no longer attached to a crank.

Autobile rims are thicker oposite the valve to balance the valve weight but I don't think my cycle rims are. Do you guys balance your rims including the valve weight? Am I looking at this wrong?

Is there a better way? Can an LBS do wheel balancing?

Rich8P said:

So I put the bike in my repair stand, moved the wireless tacho to the back wheel and wound it up to 60 kph. Other than a getting a sweat on and feeling like an america's cup winchman, I didn't notice any movement. Perhaps the stand holds the bike too firm.

I then put the wheel in the stand that I use to centralise my MTB wheels. I assumed that the heaviest part would go to the lowest point. Sure enough: value at the bottom every time. I can't spin the wheel fast in this stand because it is no longer attached to a crank.

Autobile rims are thicker oposite the valve to balance the valve weight but I don't think my cycle rims are. Do you guys balance your rims including the valve weight? Am I looking at this wrong?

Is there a better way? Can an LBS do wheel balancing?

The fact that the wheel settled to the valve at the bottom does indicate the wheel is not perfectly balanced, but if the bike didn't start shaking in the stand at 60 kph, believe your wheel is not out of balance significantly.

I don't know of anyone else who worries about wheel balance. I noticed the wheels on my current bike were far out of balance, so decided to balance them. The wheels are Easton (Velomax) Circuits. They are not a welded rim, but are pinned together with steel pins opposite the valve stem holes, making that point heaviest.

Didn't use any machine, other than spinning up the rear wheel, and adding weight slowly until all the hopping disappeared. Then, when to the front, and duplicated the weight as a baseline. Adjusted the front weight using many spin/stop trials until it was stopping in random places, and wouldn't begin to turn regardless of what position I released the wheel.

10 bar is too much pressure, in my opinion, and could well be the source of your problem. Assume you're using 23mm section tires. Most race tires, eg Conti GP 4000 and Michelin ProRace2, have lower max ratings. ProRace2 has a "recommended range" of 6-8 Bar on the sidewall, while the GP4000s show a "maximum rating" of 120 psi. I'm 83 kg, and use 7 bar front, 7.5 rear. At your weight, I'd try 7.5 front, 8 bar rear. Even better would be to run 25 mm tires at 7/7.5 bar, if you can find them.