Quick release rear wheel

[Attitudeless Badger]

I have a knocked together a fixed gear machine using an old raleigh frame and some Mavic wheels.  The rear wheel still has the QR skewer arrangement in place, which I have been using for the moment. I have seen that it may not be the best thing to do and could be better to replace them with 'proper' nuts.

Any thoughts for or against?  The bike gets used on an occasional basis (no commute or anything interesting like that) so I am not looking for a costly solution.

[border-rider]

if it's a decent q/r it should be fine. The old internal cam ones will hold a fixed wheel in place just fine.  The external cam type are a rubbish and will slip - with fixed or gears, but with vertical dropouts & gears it's not an issue

Bicycle Quick-release Skewers

I've used a Goldtec wheel with a q/r before (they have a hollow axle) for ease of turning the wheel round, and climbed the Gospel Pass (the steep side) on it with no problems.

[oncemore]

What MV said!

[border-rider]

Good question though

I might stick it in the FixieFAQS

[Si]

I've been doing this so a while.  As has been said, there are some that are better than others. I'm using a 1970s steel campag one.  I tried a newer Alu one (which had internal cam) and it was woeful - forward summersault across the road at the first T junction.

I've also remanufactued a cheap chain tug so that it hooks around the back of the dropout for added secuity.

Initially it was fitted on the inside of the dropout as that was where the axle was but this might be counter productive s it stops the axles biting the inside of the droput.  Now it's on the outside and as caused no problems despite the hole being bigger than the squewer. Note, it's not used to tension the wheel- only to stop the wheel moving should it start to do so, thus in normarl use it's not pushing on the thin squewer - it's there only to act should the wheel move (a last chance measure should everything else have failed.)

[oncemore]

Good question though

I might stick it in the FixieFAQS

Is it not there already? Or are you being MischieVous?

[border-rider]

Is it ?

I had a quick look and couldn't see it.  I did have a feeling it was, I must say

[oncemore]

If it isn't then the authors should be cast....igated.

[nuttycyclist]

Franken fixie came with QR.  30mph skid stops often resulted in slack chain^TM.

A loaned chain tug (only the one on the drive side) has fixed (ha ha) the fault.

[rogerzilla]

The chain tension on a fixie is about twice as high at its peak (when climbing the steepest hill possible on a medium gear) as on a very low-geared bike.

Assume 5 mph, which is a reasonable climbing speed for a 1:6 hill, 170mm cranks, a 26" wheel and a power output of 250W*:

776 wheel revolutions per mile = 24 rpm for the fixie and 94 rpm on the granny gear

Each pedal revolution is 1.07 metres, and force = power/speed, so for the fixie the average pedal force is 250/0.43 = 581 N and for the gearie the average pedal force is 250/ 149 N, which is why gears were invented.

However, the chain tension is the pedal force multiplied by the ratio between the crank length and the chainring radius, which for the 48T fixie is 581 x 1.75 = 1016 N and for the 22T gearie is 149 x 3.82 = 569 N.

Is the difference enough to pull a wheel over?  Probably not, if the QR cam is lubricated and it's properly fastened.  Interestingly, all decent MTBs with granny gears have vertical dropouts, presumably to stop a wheel pulling over, while horizontal dropouts persisted on road bikes for a lot longer.

Feel free to check my arithmetic.


*implies a bike-and-rider weight of about 70kg; maybe it's Pantani

[border-rider]

for the fixie the average pedal force is 250/0.43 = 581 N....

Feel free to check my arithmetic.

If a 100 kg rider puts all his weight on one pedal (and I'd argue that I can apply more force than my own weight, though I weigh under 100 kg) that's 1000 N

[rogerzilla]

Ah, but you'd have to almost double the power for a 120kg bike-and-rider, so that's OK.

[robgul]

I've also remanufactued a cheap chain tug so that it hooks around the back of the dropout for added secuity.

... be interested to know what you made here ... do you have a picture/sketch please? 

My fixed is a Claud Butler Audax 531 frame with forward-facing dropouts - and a nutted wheel tightened with two spanners, a great deal of force and a final tighten with a foot on each spanner ... but I'd really like some insurance too in the form of a chain tug of some sort

Rob

[Crumbling Nick]

There is a simpler analysis: -

For a given hill & rider+bike weight, chain tension is directly proportional to the ratio of sprocket diameter to wheel diameter. For a given wheel size it is thus inversely proportional to the number of sprocket teeth.

That assumes that the cyclic torque fluctuation at the cranks is the same in both cases (in percentage terms, that is), and I'm not personally sure whether it's greater or less at 94 rpm than 24. Does anyone know of any measurements over this cadence range?

[border-rider]

Ah, but you'd have to almost double the power for a 120kg bike-and-rider, so that's OK.

But a 70 kg rider can exert  a lot more than more than 700 N.  which is what matters.

[border-rider]

There is a simpler analysis: -

For a given hill & rider+bike weight, chain tension is directly proportional to the ratio of sprocket diameter to wheel diameter.

Is it ?

Doesn't it also depend on chainring diameter and crank length, and hence speed ?

[hubner]

I've never had a QR pull over.

If a proper (not the modern lightweight type designed for vertical ends) and correctly fitted QR gets pulled over, for whatever reason, then a solid axle and nuts will also pull over.

I thought a QR can actually be tighter than nuts.

[rogerzilla]

There is a simpler analysis: -

For a given hill & rider+bike weight, chain tension is directly proportional to the ratio of sprocket diameter to wheel diameter.

Is it ?

Doesn't it also depend on chainring diameter and crank length, and hence speed ?

Nick's right.  For a given speed on a given size wheel, the chain pull needed to turn the wheel over just depends on sprocket size.  Everything else cancels out, although my calculation is correct.

Now why didn't I think of that?

The moral is...use 61 x 23 to keep the chain pull down, not 48 x 18.

[border-rider]

Nick's right.  For a given speed on a given size wheel, the chain pull needed to turn the wheel over just depends on sprocket size. 

yes, but the amount of chain pull per unit force applied depends on the leverage at the pedal

[rogerzilla]

Yes, but the unit force needed depends only on the road speed, wheel size and sprocket size.

Longer cranks? Higher force on the chain per unit of pedal force, but for a given road speed you'll be applying less pedal force.  If you applied the same pedal force as for shorter cranks, you'd be developing more power and going faster.

[border-rider]

for a given road speed you'll be applying less pedal force. 

A smaller sprocket on the back means the same chain pulled makes you go faster  - if the chainring is the same size.  if it's smalller then there's more leverage

[giropaul]

Sounds a lot like the old "sprinters use 47x14, pursuiters 50x15" arguments.

Apart from the added stiffness of a smaller chainring and the very slight mechanical superiority of the larger chainring and sprocket I think we were convinced by the experts that it didn't matter at all.

[Attitudeless Badger]

Thanks for your thoughts, folks.  Didn't mean to start a physics debate, but it does no harm to get the old grey matter going.

I'll have  alook at the QR skewer to assess durability and maybe invest in a good quality one, if needed.

[Si]

I've also remanufactued a cheap chain tug so that it hooks around the back of the dropout for added secuity.

... be interested to know what you made here ... do you have a picture/sketch please? 

My fixed is a Claud Butler Audax 531 frame with forward-facing dropouts - and a nutted wheel tightened with two spanners, a great deal of force and a final tighten with a foot on each spanner ... but I'd really like some insurance too in the form of a chain tug of some sort

Rob

It is just a cheap one a bit like:
http://www.bikedock.com/posit/images/products/0000022443.jpg
but the bit for hooking over the back of the track ends wasbigger and curved rather than straight backed.
I just opened it out so that it would hook over the back of the forward facing drop out, and then hammered the threaded bit straight.  Thus when put on the bolt ran down the side of the dropout. Not exatly elegant but it worked.

[Si]

So, another question for the mathers:

can you (ie an average rider) put more pressure on the QR holding the wheel in by climbing up a real steep hill, or by trying to do an emeragncy stop by leg braking?

I ask, because being weary of the back wheel dislodging, I don't tend to leg brake much but if leg braking is much less likely to move the wheel than climbing then I might start again.