How much power before your wheels start to flex?

Back by popular demand, the general all-things Road forum!

Moderator: robbosmans

Post Reply
BugsBunny7788
Posts: 67
Joined: Fri Jul 10, 2015 6:48 am

by BugsBunny7788

Assuming you have power meters, then for wheels you own, how much power do you put out before your wheels start to flex laterally?

My inputs:

2015 RSYS clinchers - approx 300W+
2015 BORA 50 tubs - no flex noticable.

by Weenie


Visit starbike.com Online Retailer for HighEnd cycling components
Great Prices ✓    Broad Selection ✓    Worldwide Delivery ✓

www.starbike.com



Marin
Posts: 4035
Joined: Wed Jan 22, 2014 11:48 am
Location: Vienna Austria

by Marin

I can do 1000W without my wheels touching the brake pads?

I'm sure the wheels do flex when I make 1W, but only by a small amount.

Nefarious86
Moderator
Posts: 3669
Joined: Sun May 25, 2014 4:57 am

by Nefarious86

Bike = SL4 + up to about 90kg rider.
CLX 60 have pushed 1500 through them no rub.
V3 808 Clincher anything over about 1100 pushes the brake track into the chainstay. I just have a sacraficial tape on the inside of the stays now lol.
Using Tapatalk

AZK
Posts: 130
Joined: Thu Jun 23, 2016 10:11 am

by AZK

Maybe its worth noting the force you push when flex starts. Power is not really a good representation.

Calculator link - http://www.wentec.com/unipower/calculat ... torque.asp

Torque causes flex. This is why all stiffness tests are expressed in mm/degree deflection per Nm

If you compare a sprint at 1000w @ 120rpm to grinding up a +15% climb at 500w @ 60rpm they will both cause the same amount of flex = 79.57Nm

Theoretically you could put 10 000w (@ 1200rpm 8) ) through a wheel set without causing any more flex than the above scenarios

Also if you were to express the Nm you could relate it to stiffness tests and estimate wheel/frame deflection

bm0p700f
in the industry
Posts: 5777
Joined: Sat May 12, 2012 7:25 pm
Location: Glermsford, Suffolk U.K
Contact:

by bm0p700f

All my wheels dont flex but i build them to be stiff. I climb at low cadence so torque out put high. One soluton to the flex issue is to use a single pivot rear brake as then you can have the pas further fro the rim. Dp rear brakes are quite pointless. Campagnolo know this. SP brake is lighter too.

Nefarious86
Moderator
Posts: 3669
Joined: Sun May 25, 2014 4:57 am

by Nefarious86

Brake rub isn't my issue unfortunately :(
Using Tapatalk

User avatar
wheelsONfire
Posts: 6283
Joined: Mon Jul 07, 2014 8:15 am
Location: NorthEU

by wheelsONfire

BugsBunny7788 wrote:Assuming you have power meters, then for wheels you own, how much power do you put out before your wheels start to flex laterally?

My inputs:

2015 RSYS clinchers - approx 300W+
2015 BORA 50 tubs - no flex noticable.


I have no idea! But what i would like to ask, isn't it so that more spokes prevent this issue?
Bikes:

Ax Lightness Vial EVO Race (2019.01.03)
Open *UP* (2016.04.14)
Paduano Racing Fidia (kind of shelved)


Ex bike; Vial EVO D, Vial EVO Ultra, Scott Foil, Paduano ti bike.

bm0p700f
in the industry
Posts: 5777
Joined: Sat May 12, 2012 7:25 pm
Location: Glermsford, Suffolk U.K
Contact:

by bm0p700f

Flex can nds spoke loosing tension and the wheel flops into the pads. Difficult to know. The test you dont do is push thectim with your fingers as that is not how wheels flex when riding.

Higher spoke tension more spokes, thicker spokes, higher bracing angles and a stiffer rim are all way to increase lateral wheel dtiffness but given 28h dt swiss rr415 rim built with lasers 2x on miche hubs dont flex into the pads when pushing out 600w at low cadance i have to wonder what the op is actually feeling.

wingguy
Posts: 4318
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

AZK wrote:If you compare a sprint at 1000w @ 120rpm to grinding up a +15% climb at 500w @ 60rpm they will both cause the same amount of flex = 79.57Nm


Is that the whole story, or does it also depend on what cassette cog you're in (how far from the hub shell the force is being applied)?

AZK
Posts: 130
Joined: Thu Jun 23, 2016 10:11 am

by AZK

wingguy wrote:
AZK wrote:If you compare a sprint at 1000w @ 120rpm to grinding up a +15% climb at 500w @ 60rpm they will both cause the same amount of flex = 79.57Nm


Is that the whole story, or does it also depend on what cassette cog you're in (how far from the hub shell the force is being applied)?


The Torque in the above calculation is at the BB spindle.

Yes the change to the wheel flex will vary due to chain angle (11t vs 28t) but the variability will be extremely minimal as the cassette cog/freehub is on a axle fixed in place and triangulated by the chain stays and seat stays.

Wheel flex occurs when the spoke tension goes out of balance and pulls the rim out of alignment. Provided the hub itself can provide enough spoke tension, I don't think the hub shell itself contributes much to flex.

In short Nm readings might not be 100% (maybe more like 95% :wink: ) translatable to mm of flex, but using watts is basically pointless.

wingguy
Posts: 4318
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

The Torque in the above calculation is at the BB spindle.

Yes the change to the wheel flex will vary due to chain angle (11t vs 28t) but the variability will be extremely minimal as the cassette cog/freehub is on a axle fixed in place and triangulated by the chain stays and seat stays.

But what I mean is for a given torque at the BB is the resulting torque at the hub the same in every ratio?

If you put the same tension on the chain the torque at the freehub would be different in different cogs...

AZK
Posts: 130
Joined: Thu Jun 23, 2016 10:11 am

by AZK

wingguy wrote:
The Torque in the above calculation is at the BB spindle.

Yes the change to the wheel flex will vary due to chain angle (11t vs 28t) but the variability will be extremely minimal as the cassette cog/freehub is on a axle fixed in place and triangulated by the chain stays and seat stays.

But what I mean is for a given torque at the BB is the resulting torque at the hub the same in every ratio?

If you put the same tension on the chain the torque at the freehub would be different in different cogs...

Ah I see what you're saying, yes the closer to a 1:1 ratio or even "overdrive" gear you get the more torque will be translated to the freehub.

Eg. 34/28 would reduce 100Nm to roughly to 82Nm. 53/11 would reduce it down to 21Nm.

I believe this is what you're implying? It's a very valid point, for more accurate calculations torque division/multiplication caused by gear selection should be considered.

It however only leans further in the favour of using torque instead of power as a comparative metric for wheel (or any bike part) flex.

User avatar
MattSoutherden
Posts: 1376
Joined: Fri Jul 10, 2009 7:22 pm
Location: London

by MattSoutherden

Also, isn't the amount of wheel flex dependent on how much force is being applied laterally to the bike? i.e. how much you're throwing the bike around under you.
Snacking on carrot sticks - Where did it all go so wrong?
-
Finsbury Park CC

CEVelo
Posts: 116
Joined: Tue Mar 24, 2015 5:22 pm

by CEVelo

my Enve 3.4 clinchers - no limit it would seem, never had break rub, no flex, I can put out very high W in short bursts.
my Enve 2.2 tubulars - would flex and have significant break run, even for a 120lbs female with modest power.

by Weenie


Visit starbike.com Online Retailer for HighEnd cycling components
Great Prices ✓    Broad Selection ✓    Worldwide Delivery ✓

www.starbike.com



BugsBunny7788
Posts: 67
Joined: Fri Jul 10, 2015 6:48 am

by BugsBunny7788

Thanks for all the expert advice and feedback guys. Definately a good eye opener for me about the torque side.

I think scientifically and mathematically the torque route is the way to go, but for simpletons like me, then the only somewhat quantifyable number would be our power and our body weight.

Post Reply