Rear flexing when sprinting

Spooked

Hi guys,

I'm not sure where to start looking to resolve the issue.

On Saturday as part of my training I did some sprint intervals. I had to back off however on each sprint due to flex at the rear of the bike. It feels like the rear wheel is going over ice and kind of fish tailing in feel.

I didn't know if it may be due to bad technique or a problem with the bike and wondered what you guys thought.

When I sprint I'm literally standing up and pedalling as hard as I can utilising my upper body for as much power as possible.

My bike is a Ribble gran Fondo. Fulcrum racing 5 wheels, schwalbe one tyres, Planet X ti skewers and I weight in at 11.5 stone.

I didn't know if the quick releases may not be up to the job or the wheel is flexing. It may be normal and perhaps I need to mtfu and get used to it. I normally mountain bike so haven't experienced it before.

It was on a dry surface. Could it be the wheels slipping? Is it normal?

Hope this isn't too vague. I just don't know what to expect.

Cheers,

iron-clover

You say the rear end was flexing- did you hear the brakes rub on the rim? Normally if the frame/ wheels are flexing badly then you'll hear it before you feel it (in my case anyway).

What you describe sounds much more like a grip problem. When you stand out of the saddle you take some weight off of the rear wheel (and put a little more on the front) and then when you put the extra force in your tyre might skip/ slide much more.

Spooked

I run my brake blocks pretty wide. But thinking about it more I think your probably right. So is it just a case that I need to learn to put more weight over the rear as I sprint off?

ugo.santalucia

Spooked wrote:

So is it just a case that I need to learn to put more weight over the rear as I sprint off?

Or get a rear wheel which has more than 20 spokes.

The only low count rear wheels which are stiff are those made with over engineered aluminium or carbon spokes (Fulcrum Zero, Mavic SLR and R-SYS, Campagnolo Shamal etc... ). For normal tiny steel spokes you need more of them

iron-clover

Spooked wrote:

I run my brake blocks pretty wide. But thinking about it more I think your probably right. So is it just a case that I need to learn to put more weight over the rear as I sprint off?

I think so- if it is the rear wheel skipping rather than flexing then lower tyre pressures might help with grip, but other than that yes, technique is the answer. When you stand up the center of gravity also changes quite a lot so the handling can take some getting used to (it took me about a year and a half to get used to riding out of the saddle on my road bike- although it is at least one size too small which doesn't help).

Tighten the brake blocks and listen for bad rubbing- if it is the back end flexing then the stiffer wheels will help, but I have a feeling it's more of a control issue.

BrandonA

My winter bike came with Fulcum Racing 6s. They buckled a hell of a lot when you stood up and threw down the power. The reason I knew this is because the spoke magnet would start to hit the Garmin speed/cadence sensor. My solution was to buy a decent pair of wheels, having done this the rear of the bike feels good.

Spooked

I've been out again, tightened the brake blocks and it is the wheel that's flexing. I didn't realise my speed sensor is also hitting the armature of the sensor thingy so it looks like I will have to save up for some new wheels and use these for the turbo and winter use.

Thanks for the replies. Really appreciate it.

debeli

Odd thread.... I hope the OP had solved his predicament.

If you are losing traction, you will feel it very clearly - your foot will slam suddenly to the bottom of the stroke and you will experience differential acceleration - as in the mismatch in a car between revs and road speed when the clutch is going.

If the wheel is flexing, it will be barely perceptible, but there will be a sort of Rrrr dugga-dugga drag between brake and rim at the point of greatest Ooomph in the pedal stroke.

Neither circumstance is easily mistaken for the other.

I wouldn't necessarily bother to change the wheels. There is a smoothly-smoothly way of adding speed which doesn't involve a sort of Tashkent Terror bodily explosion through the cranks. Wheels have become a lot stiffer in the past decade or two, so what may be seen by the cycling press as a 'more flexible wheel' is in truth a good deal stiffer and better able to transmit power than the things the pro-peloton were riding a few years ago.

You can buy the stiffer wheels (or just a stiffer rear), but that may just be allowing you to ride less smoothly or transition to sprint less ably.

In the current hype-inflated bicycle marketplace (and encouraged by the advertising-fed cycling press) we are looking for problems where none exist and solutions through retail where none are necessary.

Do by all means buy new wheels and blame the old ones for flexing, but do so while accepting that you may be paying handsomely for imagining an issue and spending your way to an imagined solution.

It is just a bicycle. This will become apparent in due course. I may be wrong, but to be honest I never am.

simon_masterson

Debeli wrote:

Odd thread.... I hope the OP had solved his predicament.

If you are losing traction, you will feel it very clearly - your foot will slam suddenly to the bottom of the stroke and you will experience differential acceleration - as in the mismatch in a car between revs and road speed when the clutch is going.

If the wheel is flexing, it will be barely perceptible, but there will be a sort of Rrrr dugga-dugga drag between brake and rim at the point of greatest Ooomph in the pedal stroke.

Neither circumstance is easily mistaken for the other.

I wouldn't necessarily bother to change the wheels. There is a smoothly-smoothly way of adding speed which doesn't involve a sort of Tashkent Terror bodily explosion through the cranks. Wheels have become a lot stiffer in the past decade or two, so what may be seen by the cycling press as a 'more flexible wheel' is in truth a good deal stiffer and better able to transmit power than the things the pro-peloton were riding a few years ago.

You can buy the stiffer wheels (or just a stiffer rear), but that may just be allowing you to ride less smoothly or transition to sprint less ably.

In the current hype-inflated bicycle marketplace (and encouraged by the advertising-fed cycling press) we are looking for problems where none exist and solutions through retail where none are necessary.

Do by all means buy new wheels and blame the old ones for flexing, but do so while accepting that you may be paying handsomely for imagining an issue and spending your way to an imagined solution.

It is just a bicycle. This will become apparent in due course. I may be wrong, but to be honest I never am.

Wise post. I tend to refer to Mike Burrows on this subject. 'Blind' testing is a bit of a tricky one with cycling, but I remain to be convinced that most people (if anyone) can really detect a difference in stiffness between radial and cross/different patterns, for example. They only need to declare that the latest wheelset is "11% stiffer", or some other marketing driven BS, and the punters will all line up. I seem to recall that someone posted here recently that he was buying new wheels because cross spoke wheels apparently can't be stiff enough...

MountainMonster

Oh the joys!

roux_guy

Just a thought but maybe try replacing the rear skewer with a steel one, at least for a couple of rides? Ti is a lot springier than steel under tension.

ugo.santalucia

Debeli wrote:

Odd thread.... I hope the OP had solved his predicament.

If you are losing traction, you will feel it very clearly - your foot will slam suddenly to the bottom of the stroke and you will experience differential acceleration - as in the mismatch in a car between revs and road speed when the clutch is going.

If the wheel is flexing, it will be barely perceptible, but there will be a sort of Rrrr dugga-dugga drag between brake and rim at the point of greatest Ooomph in the pedal stroke.

Neither circumstance is easily mistaken for the other.

I wouldn't necessarily bother to change the wheels. There is a smoothly-smoothly way of adding speed which doesn't involve a sort of Tashkent Terror bodily explosion through the cranks. Wheels have become a lot stiffer in the past decade or two, so what may be seen by the cycling press as a 'more flexible wheel' is in truth a good deal stiffer and better able to transmit power than the things the pro-peloton were riding a few years ago.

You can buy the stiffer wheels (or just a stiffer rear), but that may just be allowing you to ride less smoothly or transition to sprint less ably.

In the current hype-inflated bicycle marketplace (and encouraged by the advertising-fed cycling press) we are looking for problems where none exist and solutions through retail where none are necessary.

Do by all means buy new wheels and blame the old ones for flexing, but do so while accepting that you may be paying handsomely for imagining an issue and spending your way to an imagined solution.

It is just a bicycle. This will become apparent in due course. I may be wrong, but to be honest I never am.

It's all very true of course and stiffness per se is not necessarily the Holy Grail, but the problem is another. If the OP uses the wheel for hard sprinting and the wheel flexes, having only 20 spokes they will fatigue very rapidly and fail. They are not designed to flex in that way. Is that a problem? Well, it depends how far you are from home when it happens, I suppose. The deflection of a 19 spoked wheel is beyond the width of an open brake caliper, so the wheel cannot rotate any more

simon_masterson

Roux_guy wrote:

Just a thought but maybe try replacing the rear skewer with a steel one, at least for a couple of rides? Ti is a lot springier than steel under tension.

It is, but the skewer is not exactly high up the list of factors that influence the stiffness of a bicycle wheel.

itboffin

ugo.santalucia wrote:

Spooked wrote:

So is it just a case that I need to learn to put more weight over the rear as I sprint off?

Or get a rear wheel which has more than 20 spokes.

The only low count rear wheels which are stiff are those made with over engineered aluminium or carbon spokes (Fulcrum Zero, Mavic SLR and R-SYS, Campagnolo Shamal etc... ). For normal tiny steel spokes you need more of them

+1 this

Anonymous

^ Funny, as I have just started using some Racing 3's with Schwalbe One's (tubeless) and have noticed some odd rear end movement that am yet to nail down.
I have not done any out and out sprinting, but weigh a bit more than the OP.
Have mainly used Zero's and R-Sys prior to these.

How do people find the Schwalbe One's in the wet?
I possibly have the pressure a bit high (110psi 23mm) at the moment for tubeless.

Spooked

I've been doing a little more tinkering with regards to everyone's advice which has been brilliant.

I first changed the skewer over and I couldn't detect any real difference so it can't be the skewer.

I then changed the tyre back to my rubino pros and I still had the same problem.

Changed the wheel with the Schwalbe to my old Fixie wheel and the wobble has gone.

Put the fulcrum back in the frame and I can move the wheel laterally about an inch either way. Im not exactly muscly either so I reckon it's got to be the wheel.

Contemplating getting them re tensioned as they are about 5 years old and have never been looked at since the day I bought them. I have also noticed that the rims are fairly concaved now so it is going to be a case of new wheels at some point anyhow.

Really appreciate the help. It gives me an idea of what to look for in the next set of wheels.

I'm now off for a lay down. All this sprinting has taken its toll

simon_masterson

Get some nice handbuilt wheels with 32 good quality butted spokes, good quality hubs, and good quality rims. Be happy.

Buckie2k5

Spooked wrote:

I

Put the fulcrum back in the frame and I can move the wheel laterally about an inch either way. Im not exactly muscly either so I reckon it's got to be the wheel.

Wheel either needs the preload nut turned in and tightened or the bearings need replaced. I would bet the preload nut has worked lose. Happened to my fulcrums a few times.

on-yer-bike

Racing 5s that are 5 years old and have been used regularly are due for the skip. Even if you replace the bearings the rims must be nearly shot.

ai_1

Spooked wrote:

....Put the fulcrum back in the frame and I can move the wheel laterally about an inch either way. Im not exactly muscly either so I reckon it's got to be the wheel...

Well that's the problem found by the sounds of it! That's WAY too much play.
You can probably tell by how it moves whether it's flex in the wheel or play in the bearing. I'd assume flex would require progressive increase in pressure to produce deflection whereas the bearing would probably allow a certain amount of play with little or no resistance, perhaps rattle.
I don't know the wheels but if the bearings are adjustable cone type, you may just need to tighten them up.

Spooked

I've bought a wheel jig going to give it a whirl myself. God help me.

Will post up in the general buying bit with regards to recommended rims, Campag compatible hubs and best spoke and nipple recommendations.

Really looking forward to it

Once again thank you so much for the help. Cool forum with brilliant helpful people on board cheers!

bigmat

ugo.santalucia wrote:

Debeli wrote:

Odd thread.... I hope the OP had solved his predicament.

If you are losing traction, you will feel it very clearly - your foot will slam suddenly to the bottom of the stroke and you will experience differential acceleration - as in the mismatch in a car between revs and road speed when the clutch is going.

If the wheel is flexing, it will be barely perceptible, but there will be a sort of Rrrr dugga-dugga drag between brake and rim at the point of greatest Ooomph in the pedal stroke.

Neither circumstance is easily mistaken for the other.

I wouldn't necessarily bother to change the wheels. There is a smoothly-smoothly way of adding speed which doesn't involve a sort of Tashkent Terror bodily explosion through the cranks. Wheels have become a lot stiffer in the past decade or two, so what may be seen by the cycling press as a 'more flexible wheel' is in truth a good deal stiffer and better able to transmit power than the things the pro-peloton were riding a few years ago.

You can buy the stiffer wheels (or just a stiffer rear), but that may just be allowing you to ride less smoothly or transition to sprint less ably.

In the current hype-inflated bicycle marketplace (and encouraged by the advertising-fed cycling press) we are looking for problems where none exist and solutions through retail where none are necessary.

Do by all means buy new wheels and blame the old ones for flexing, but do so while accepting that you may be paying handsomely for imagining an issue and spending your way to an imagined solution.

It is just a bicycle. This will become apparent in due course. I may be wrong, but to be honest I never am.

It's all very true of course and stiffness per se is not necessarily the Holy Grail, but the problem is another. If the OP uses the wheel for hard sprinting and the wheel flexes, having only 20 spokes they will fatigue very rapidly and fail. They are not designed to flex in that way. Is that a problem? Well, it depends how far you are from home when it happens, I suppose. The deflection of a 19 spoked wheel is beyond the width of an open brake caliper, so the wheel cannot rotate any more

This is probably true - I have ridden a 21 spoke wheel (you know the one!) and that was touch and go...

keef66

"The deflection of a 19 spoked wheel is beyond the width of an open brake caliper, so the wheel cannot rotate any more"

The brake caliper wasn't the problem when I suddenly acquired a 19 spoke rear wheel; the tyre was jammed firmly against the chainstay. Couldn't even push the flipping thing along as I searched for a better mobile phone signal in order to summon my support vehicle. Luckily only 8 miles from home, so wife not inconvenienced too much...