front wheel flexes whilst climbing? cause

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martinn
Posts: 307
Joined: 1 Dec 2012, 8:20pm

front wheel flexes whilst climbing? cause

Postby martinn » 1 Oct 2017, 9:41pm

Hi all,

I have a SP dynamo hub with a 32 hole H-plus son rim (Handbuilt wheel from a reputable supplier). I weigh about 80Kg. The forks are carbon

When I climb,and get out of the saddle and put some effort in, I can hear and feel the wheel rubbing against the brake pads, this happens on both sides, as the bike swings one way and then the other. The only way to stop this is to move the pads far away from the rim, so I have about 4-5 mm of space each side, which does reduce my braking efficiency somewhat.

So I am thinking either its the wheel and my weight combination, and there is flex in the actual wheel itself.
Its the skewer (I am using a Mavic one from an older wheel set)
Its the forks flexing somehow, which is a worry considering I didn't think carbon flexed much, before it snapped :shock:

What is the most likely cause, ie what would you try fixing first...

Many thanks
Martin

pwa
Posts: 10070
Joined: 2 Oct 2011, 8:55pm

Re: front wheel flexes whilst climbing? cause

Postby pwa » 1 Oct 2017, 10:01pm

4-5mm clearance isn't excessive, so maybe you don't have such a problem.

Not familiar with SP dynamo hubs, but do they have adjustable bearings? Any looseness there?

Other than that, insufficient spoke tension?

Brucey
Posts: 35201
Joined: 4 Jan 2012, 6:25pm

Re: front wheel flexes whilst climbing? cause

Postby Brucey » 1 Oct 2017, 10:57pm

SP hubs have narrow spaced flanges. Some wheelbuilders won't build them or won't build them with certain rims. You don't say which exact rim model you have; I'd say if it is an Archetype and the spoke tension is reasonable then the wheel ought to be OK, but if it is a TB14 maybe not; I'd be leery of building that rim onto that hub.

On another forum a user has recently reported that his front wheel (SP hubbed, 'reputable mail order wheelbuilder') just collapsed whilst he was riding, cause unknown, trip to hospital required. It is not possible to say that the wheel collapsed because of the narrow spaced flanges but they certainly wouldn't have helped.

My suggestion is that you borrow a different front wheel and try that. That will isolate if it is the wheel that is flexing or the fork. 4-5mm deflection is enormous and worrying; it is in the league of wheels with half as many spokes as that one.

Unless it is really very loose (daft question; you do know how to tighten it correctly, right?) the QR skewer is unlikely to cause a problem.

If the wheel, have an experienced wheelbuilder check the tension in the wheel; it could be too loose or too high. "Too high" is a much lower tension than with a hub with wider spaced flanges, and/or a stiffer rim (if it is a TB14), because the wheel is that much closer to Euler buckling from the start.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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NATURAL ANKLING
Posts: 10477
Joined: 24 Oct 2012, 10:43pm
Location: English Riviera

Re: front wheel flexes whilst climbing? cause

Postby NATURAL ANKLING » 1 Oct 2017, 11:04pm

Hi,
1-1.5mm would be ok but anymore and I would say that there is a problem with wheel / axel.
Lateral flex is less likely on forks.
I have a similar problem with steel forks, at about 1.5 mm clearance rim to block, this is normal but depends on wheel rigidity.

Is front wheel dished / asymmetric spokes ? This will not help.
Are spokes tight?
Is there any play you can see /feel in the front wheel?
If You Don't Try You Don't Do.....Don't Do You Don't Get...I'm Still Trying....Well Very..
You'll Find Me At The Top Of A Hill...............Somewhere...After Dark..

martinn
Posts: 307
Joined: 1 Dec 2012, 8:20pm

Re: front wheel flexes whilst climbing? cause

Postby martinn » 2 Oct 2017, 6:25am

Ok,

Rim is archetype, spokes all seem to be the same tension, no loose spokes identified.
Skewer is tight,
The original wheel was a mavic with 18 spokes, and that was the same. I have another handbuilt wheel, which was fine, (28 hole).
I was suspecting that it might be the hub, as it does look very narrow.
So assuming the underlying cause is the narrow flange width, and I want a dynamo wheel, hat options are there?
Son dynamo, would that build a less flexible wheel?
Different rim?
Thro axel hub, ( I have a different bike that I am planning to have a dynamo wheel for)
Anything else?
Any thoughts on f I can do anything to fix what I currently have??

Many thanks

Martin

Brucey
Posts: 35201
Joined: 4 Jan 2012, 6:25pm

Re: front wheel flexes whilst climbing? cause

Postby Brucey » 2 Oct 2017, 9:52am

is the spoke tension high enough? Even with a narrow flange hub that seems like a lot of movement for a wheel built with that rim.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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NATURAL ANKLING
Posts: 10477
Joined: 24 Oct 2012, 10:43pm
Location: English Riviera

Re: front wheel flexes whilst climbing? cause

Postby NATURAL ANKLING » 2 Oct 2017, 12:31pm

Hi,
Is wheel visible moving or is brake touching tyre :?:
If You Don't Try You Don't Do.....Don't Do You Don't Get...I'm Still Trying....Well Very..
You'll Find Me At The Top Of A Hill...............Somewhere...After Dark..

le.voyageur
Posts: 63
Joined: 10 Jan 2017, 8:30am

Re: front wheel flexes whilst climbing? cause

Postby le.voyageur » 2 Oct 2017, 2:23pm

can you push the rim over whilst the wheel is still i.e. not riding it?

martinn
Posts: 307
Joined: 1 Dec 2012, 8:20pm

Re: front wheel flexes whilst climbing? cause

Postby martinn » 3 Oct 2017, 5:59pm

I can push the rim over to touch the brakes with my hands, relatively easily, but I can do that with another wheel that does not have this problem, BUT the wheel that does have this problem is much easier to move

Brucey
Posts: 35201
Joined: 4 Jan 2012, 6:25pm

Re: front wheel flexes whilst climbing? cause

Postby Brucey » 3 Oct 2017, 10:18pm

vs an otherwise similar wheel but with flanges twice as far apart, the SP wheel could be about half as stiff, laterally.

If in doubt, check the spoke tensions are sensible for that rim/hub combination. The wheel will also move around more if

a) the wheel is built too slack or
b) the wheel is built too tight (and is on the cusp of Euler buckling)

If I knew the tension limit for b) with that rim and hub, I'd say what it is. If you are in a type b situation the wheel is in real danger of spontaneous collapse.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Samuel D
Posts: 2802
Joined: 8 Mar 2015, 11:05pm
Location: Paris

Re: front wheel flexes whilst climbing? cause

Postby Samuel D » 3 Oct 2017, 10:41pm

Why would higher spoke tension make the wheel stiffer? The spokes pull from both sides and obey Hooke’s law, no?

JakobW
Posts: 271
Joined: 9 Jun 2014, 1:26pm
Location: The glorious West Midlands

Re: front wheel flexes whilst climbing? cause

Postby JakobW » 3 Oct 2017, 11:28pm

With higher spoke tensions a given external load makes for a proportionally smaller deflection, no? At least that's what I get when I try and visualise the vector triangles.

Brucey
Posts: 35201
Joined: 4 Jan 2012, 6:25pm

Re: front wheel flexes whilst climbing? cause

Postby Brucey » 4 Oct 2017, 12:07am

the simplest example/explanation I can give is this; under certain conditions of load, some of the spokes go completely slack and cease to contribute usefully to the stiffness of the wheel any more. The greater the spoke tension (up to a point) the greater the external load that is required to cause slackness.

A trivial example of this happening is under high torque, some of the 'pushing' spokes may go completely slack. If (say) all the pushing spokes go slack, at that point the wheel is presumably about half as stiff laterally (as well as in torsion) as if all the spokes still had some tension in them.

The situation is complicated by the stiffness properties of the rim and the fact that in most wheels, the spokes are crossed and braced; this means that spokes that lose load, lose less than they would otherwise and also create tension changes in other spokes that are in different locations, but happen to be braced against the slackened spoke.

BTW I've had to partially rethink my idea of fatigue in spokes; on the one hand it might usually be worse in the spokes which have higher tension in them. On the other hand the spokes that see the largest tension variations can have the worst fatigue damage and spokes that go completely slack may also be flexed in bending, which may be extremely harmful. It turns out (according to some FEA models) that the pushing spokes on the NDS may see the largest tension variations and they are also the ones that are most likely to momentarily go completely slack.

cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

martinn
Posts: 307
Joined: 1 Dec 2012, 8:20pm

Re: front wheel flexes whilst climbing? cause

Postby martinn » 4 Oct 2017, 11:27am

Hi Brucey,

If I have interpreted your post correctly, the current wheel set up is less stiff than an equivalent one which had a wider hub/ flange.

But the flexing may be due to the tension being either too much or not enough?
so how do you tell what tension it should be? I don't have any tension meters and I am tone deaf in case you can hear a tone difference.

Out of interest Brucey would you build a wheel set up as mine for an individual weighing 80Kg? I ask as it seems to be that if you are over 80-85Kg your considered a heavy rider and need "more robust" solutions?

Many Thanks

Martin

mig
Posts: 2032
Joined: 19 Oct 2011, 9:39pm

Re: front wheel flexes whilst climbing? cause

Postby mig » 4 Oct 2017, 12:52pm

what is the spoke pattern on this wheel?