Wheel Flex

[Rude80]

Hi all

what are your experiences with wheel flex specifically with carbon rims?

Ive just bought some new carbon wheels 44mm 28H Rear (19mm inner) running with 25mm Conti GP4ks2 and noticed i didnt have a great deal of chainstay clearance so put some chalk down on the chainstays with some elec tape to see if id get tyre rub on the first ride. Bike is rated to take 25mm tyres. clearance is about 2-3mm each side.

My ride included some steep sections where the rim would most likely flex when out of saddle - and sure enough i got a few chalk marks on the side of the tyre. Nothing major but def contact between tyre and frame.

I am surprised to get this amount of flex given the 28h config out back on 44mm rims.

I weigh 93kgs - might not get the same issue if i was 5kgs lighter?

Any good 25mm alternatives to the GP's that might not measure as wide (apart from 23mm GP's)?

CHeers

[usernameforme]

Would not be surprised if the 25mm conti tyre on a 19mm rim measures 28mm, so the 23mm conti GP would measure 25mm (but I think you've already established that).

You probably won't notice much of a difference dropping 5kg. I'd suggest checking your technique out of the saddle - try and keep everything going forward rather than moving everything side to side, if that makes sense.

Spoke count doesn't have as large effect on stiffness as hub flange geometry and spoke choice. Perhaps you've built the rims onto a DT swiss or similar hub - the narrow flanges don't give a super stiff wheel. Another culprit could be flexy spokes, e.g. CX-Ray/Aerolight/Lazer etc. spokes.

[Rude80]

The wheels are Boyd's. Super wide flanges - quite unique hubs actually. Rely on pressure from closed QR to keep hubs in place. Definitely no excessive play in hub - although i thought i could try some DA style internal cam skewers which might impact lateral stiffness?

cx-ray spokes.

Wont be the end of the world if I run 23's on the back with 25's on front.

The bike is a Trek Domane RSL Koppengberg - was a bit suprised to see less clearance at chainstay than my Focus Cayo given that these were raced with 25's by Trek sponsored teams. Although the tubs they would use would no doubt measure less than 28mm

[usernameforme]

Don't think your skewers will help much. But I'd say the rim and hubs will be plenty stiff so it could just be flexy spokes/poor technique.

[Warin]

it could just be flexy spokes

Could be lack of tension in the spokes.

[Thoglette]

it could just be flexy spokes

Could be lack of tension in the spokes.

Could be frame flex or tyre flex. Or tyres that are bigger than advertised. How wide are the tyres once instaleld and pumped up?

2mm is not much room.

[Warin]

it could just be flexy spokes

Could be lack of tension in the spokes.

Could be frame flex or tyre flex.

I'd have thought frame flex would show up with the previous wheel set.. or am I reading too much into the 1st post?

[usernameforme]

^also thought frame flex would have shown up with the previous wheels. I'm pretty confident though that the spokes will be adequately tensioned - Boyd are a fairly reputable brand. CX-Rays are stiff enough for most people but I'd recommend Race or DT competition spokes for heavier riders.

@Thoglette, it's well established that continental tyres measure wider than advertised. 25mm tyres should measure around 27-29mm on wide rims. The 23mm tyres are closer to 25.

[Duck!]

It's spoke stretch, with a tiny bit of frame flex.

Carbon rims are inherently a lot stiffer than aluminium, which reveales deficiencies in other components. The greater ability of aluminium rims to flex means that only the sector between the load point - the hub and the ground - deflects at any given moment, so the rest of the wheel remains true to the frame. Carbon rims stay straighter in their own plane, but frame flex and spoke stretch conspire to move the hub and attached frame within the rim, leading to the perception of wheel flex.

CX Rays (Sapim in general) are comparitively stretchy spokes, which I find does affect the overall stability of the wheels. The stretch means they are harder to build up to a given static tension (around 120kgf is typical for carbon rims, vs 90-100kgf for aluminium), plus under dynamic tension they'll stretch even more, resulting in a relatively "floppy" wheel. I find DT Aerolite or Pillar spokes to better, more stable options in any given build.

[Thoglette]

@Thoglette, it's well established that continental tyres measure wider than advertised. 25mm tyres should measure around 27-29mm on wide rims. The 23mm tyres are closer to 25.

Except their ETRO 630 product, which seems to be smaller - the 32s are closer to 28!

[usernameforme]

^was not aware of that, sorry I was reffering specifically to their 622 tyres.

[Rude80]

Thanks for the replys - a few interesting topics covered!

The bike is new and have never had wheels this wide in the frame so have no comparison. The alu wheels i use are Mavic OP's 32h with 23mm tyres so definitely no risk of contact with frame.

My selection of frame/wheel combo was to give me a sturdy/stiff ride as oppose to super light which is why ive gone the Domane Kopp with the 28/24 hole carbon wheel build (at 93kegs im no weight weenie). So am surprised im getting this much frame/wheel flex with a sturdy setup. But i guess 2-3mm movement from flex isnt unusual by the sounds of things..

Im looking at running the new tubeless Schwalbe Pro Ones which apparently measure as wide as the GP4000's so am considering running 23's on the back and 25's on the front

[uart]

Im looking at running the new tubeless Schwalbe Pro Ones which apparently measure as wide as the GP4000's so am considering running 23's on the back and 25's on the front

Last time I measured my 700c x 25 continental tyres, they were almost exactly 26.5 mm width (that's on about a 14.5 to 15 mm internal width rim). They are definitely a little wider than most of the other 25c tyres I've used.

[Rude80]

rode this morning and concentrated on technique - deliberately trying to move the bike forward as oppose to swinging the bike side to side when putting power down up hills.

There was no frame rub

Technique definitely a contributor... going to keep an eye on it for the moment. Very interesting...

[Warin]

rode this morning and concentrated on technique - deliberately trying to move the bike forward as oppose to swinging the bike side to side when putting power down up hills.

There was no frame rub

Place the bike at ~45 degrees ... crank pedal down .. then put your foot on the pedal and push inwards (towards the other side of the bike) on the pedal. This should reveal where the flex is .. if your very observant.

[usernameforme]

Glad to hear that technique appears to have solved the issue. According to my coach you'll be faster if you can keep everything going forward too! This is pretty contentious, but if it protects your frame, and it doesn't make you slower maybe it's worth investing some time into developing a straight and smooth pedalling style.

[MelodyWheels]

lateral wheel flex (brake/frame rub) far far more common on carbon rims that alloy wheels, especially with lightweight spokes.

With an alloy rim, the rim is flexible under heavy lateral loads, however this flex is absorbed in the rim and the spokes below the axle, between the tyre on the road and the hub. Think of it like fish's tail oscillating right to left while the body remains straight.

With a carbon rim, the rims are much much much stiffer. When the wheel is laterally loaded it tends to flex, however this time the hub acts like the fulcrum point with the spokes flexing and the rims oscillates like a see-saw with the hub in the middle. There's a great diagram by NOX Composites explaining the issue here.


The material stiffness of carbon rims, plus that modern rims tend to be a lot wider mean that brake rub has become more common issue. With a carbon wheels, the spokes are the weakest component in the wheel system. Sapim CX Ray spokes are stiff in the torsionally direction under pedaliing load, however they are very flexible laterally like a filleting knife. The cross-sectional area of a CX Ray is the same as a Sapim Laser. Its important to recognise that increasing spoke tension has absolutely no effect on the lateral stiffness of the wheel past a certain threshold. See this TEST. If your mechanic thinks that ramping up the spoke tension will solve the issue, go to another mechanic.

The only way to reduce wheel flex/brake rub on carbon rims is to build wheels with thicker/stiffer spokes, or build them more spokes. Hub design can also have an affect on lateral stiffness but thats another issue.

These are probably two of the best links on wheel stiffness I know about.

Wheel Stiffness, Greg Kopecky
http://www.slowtwitch.com/Tech/Debunkin ... _3449.html

Damion Rinard “Wheel Stiffness Test”
http://www.sheldonbrown.com/rinard/wheel_index.html