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Not that Nice If I'm Honest

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« Reply #750 on: Monday, October 7, 2013, 09:58:57 »

I recently presented an argument, based on science, that CO2 should leak more slowly that air, as it is a bigger molecule than Nitrogen.

It was a very well made argument, as TEF members would expect based on my previous posts, however, I too pumped up my tyre with CO2 and I agree it does go down quicker.

Something weird is going, on so maybe we should ask Darra O'Brien, or the smiley Manc guy ?

Having said that, the CO2 pumps are brillinat as an emergency solution out on a ride
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4D
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« Reply #751 on: Monday, October 7, 2013, 10:16:14 »

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« Last Edit: Monday, October 7, 2013, 10:21:28 by 4D » Logged
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« Reply #752 on: Monday, October 7, 2013, 18:46:53 »

http://www.pinkbike.com/video/333789/
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Not that Nice If I'm Honest

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« Reply #753 on: Monday, October 7, 2013, 22:29:00 »

A bit of self promotion -

You can see me briefly in the red shirt at 2.22 powering up Wrynose


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Nomoreheroes
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« Reply #754 on: Monday, October 21, 2013, 21:30:51 »

Without any tools, how do I tell if I need to replace my chain? I've had the same one since new in 2005. Admittedly I haven't done that many miles on the bike in that time.

Also, how often should you change the cassette and chain rings? I've got a SRAM Powerglide II cassette and some form of Shimano crankset.

I don't have any issues with gears slipping and I haven't notice that the teeth on the cassette or chain rings are shark-finned. Should I just carry on with the same chain, cassette and chain rings?
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Bewster

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« Reply #755 on: Monday, October 21, 2013, 21:57:24 »

Without any tools, how do I tell if I need to replace my chain? I've had the same one since new in 2005. Admittedly I haven't done that many miles on the bike in that time.
Also, how often should you change the cassette and chain rings? I've got a SRAM Powerglide II cassette and some form of Shimano crankset.
I don't have any issues with gears slipping and I haven't notice that the teeth on the cassette or chain rings are shark-finned. Should I just carry on with the same chain, cassette and chain rings?

Its hard to tell if the chain is worn without a tool and that will only cost you £10. The cassette should be changed with the chain - or else the chain will slip. The chain rings should last through 2+ chains and cassettes.

I replaced my cassette/chain on my road bike after 2000 miles but if you read on line some people have them for 3 times that amount and I really look after my bike. Saying that I seem to have gone through a bottom bracket on my MTB in under a year.
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Nomoreheroes
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« Reply #756 on: Monday, October 21, 2013, 22:04:16 »

Actually, I've just taken a harder look at the cassette and see that there are signs of wear. Not real shark fins or shark teeth, but the odd mis-shapen tooth here and there.

But I'm not getting any problems with shifting etc. Should I just carry on since there are no problems? What real benefits will I see if I change the chain and cassette? Also, what should I look for in a new cassette (I just read an old review on a SRAM Powerglide II and see it says they don't last long)
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« Reply #757 on: Monday, October 21, 2013, 22:38:38 »

Actually, I've just taken a harder look at the cassette and see that there are signs of wear. Not real shark fins or shark teeth, but the odd mis-shapen tooth here and there.

But I'm not getting any problems with shifting etc. Should I just carry on since there are no problems? What real benefits will I see if I change the chain and cassette? Also, what should I look for in a new cassette (I just read an old review on a SRAM Powerglide II and see it says they don't last long)

Not sure you would see many benefits from replacing them if they are ok, shifting well and not slipping. If they are in good nick, clean and well lubed they should be fine. They tend to wear out together so even thought the teeth are worn and the chain is stretched they fit well together.

Personally Id wait until they start causing trouble and then replace as part of a service along with the cables
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Nomoreheroes
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« Reply #758 on: Tuesday, October 22, 2013, 05:21:53 »

Not sure you would see many benefits from replacing them if they are ok, shifting well and not slipping. If they are in good nick, clean and well lubed they should be fine. They tend to wear out together so even thought the teeth are worn and the chain is stretched they fit well together.

Personally Id wait until they start causing trouble and then replace as part of a service along with the cables
That's what I was thinking - Thanks for that. Going to do a bit more research and file away knowledge for use later.

Been reading up on various cassettes in preparation for a change and was surprised that you can get one for around £15-25 on Amazon. Reviews seem to suggest slicker gear change with Shimano cassettes rather than SRAM. Looking at youtube, I think I'd prefer the way that SRAM chains easily join. Not sure whether mixing and matching chains and cassette makes is the right thing to do or not?
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@MacPhlea

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« Reply #759 on: Tuesday, October 22, 2013, 06:22:01 »

I recently presented an argument, based on science, that CO2 should leak more slowly that air, as it is a bigger molecule than Nitrogen.

It was a very well made argument, as TEF members would expect based on my previous posts, however, I too pumped up my tyre with CO2 and I agree it does go down quicker.

Something weird is going, on so maybe we should ask Darra O'Brien, or the smiley Manc guy ?

Having said that, the CO2 pumps are brillinat as an emergency solution out on a ride

Found this on another forum...

Hmm, this has puzzled me. So much so I had to look into the science of butyl tube inflation   My search of the web has located many misinformed articles and postings on triathlon/cycling forums. Indeed one of my earlier postings on the subject was ambiguous and plays right into sticking point 1:

jibby26 wrote:
CO2 is a relatively small molecule and will squeeze out through the holes in the rubber overnight.


CO2 is a relatively small molecule, but just to screw with your mind on a Saturday morning it is also a relatively big molecule. It all depends on the stick you use to measure it. If you think of nitrogen (N2), oxygen (O2) and CO2 as snooker balls wanting to hit one another then CO2 is bigger than N2 which is bigger than O2. If however you think of them as fast moving molecules that want to get out of the sieve they are in (just like in the example of an inner tube) then it is different. The "kinetic" diameter of CO2 is the smallest, followed by that of O2 followed by N2. Hopefully that clears up this misconception.

Now for the nitty gritty science bit. The reason the tyre goes flat is due to permeation of gases (gas diffuses through the rubber) as opposed to effusion where the gas goes directly through a small hole (a puncture). The different gases all have a different permeation rate (permeability) through the rubber, and different rubbers all have different permeabilities for the same gas.
Now hopefully that is a little clearer than an open water lake after a mass start and I can through in an equation   
The amount of gas that moves through the rubber is known as the flux and given by:

Jm=[Rm x A x (Pi-P0) x Fm]/t

where Rm = Permeability of gas m through material (in Barrer)
A = Area (in cm)
Pi = Inflation pressure (in cmHg)
P0 = Outside pressure (in cmHg, normally 76)
Fm = Molar fraction of gas m (i.e. air is 78% N2 so FN2=0.78)
t = Thickness of inner tube (in cm, approximately 0.1)

Now, for a butyl tube RN2=0.323xE^[-10], RO2=1.30xE^[-10], RCO2=5.17xE^[-10],
If we put the numbers in for a 1cm2 patch in tube inflated with air (Fm: m=N2=0.7809, O2=0.2095, CO2=0.0004) to 8 bar (608cmHg) we get:
JN2=1.34E[-07] cm3/s i.e. one ten millionth of a millilitre per second per cm2 of tube
JO2=1.45E[-07] cm3/s
JCO2=1.10E[-09] cm3/s

So from this you can see that most of the gas lost from a normally inflated tube is actually O2. If you total up these losses then the tube is losing 2.80E[-07] cm3/s per cm2 of tube. My crude estimation of an inner tube puts it's surface area at 500cm2 so the actual loss is around 0.14 microlitres of gas per second.

But if you use a CO2 inflator there is no N2 or O2, so JCO2 now becomes 2.75E[-06] cm3/s per cm2 of tube. i.e. the rate of gas loss from the tire is about 10 times greater.

Hopefully some of you have read to this point and understood some if not all of it. There are a few caveats to these numbers, they are only true for the pressure and compostion of the gas initially described, as the tire deflates both of these will change and it gets too complicated to describe here.

As for why your tire is still infalted Andy, the only thing I think can explain it is Halfords have made the tube so thick the permeation rate is very slow, and/or they have added something to the rubber which reduces the permeation of CO2.
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Samdy Gray
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« Reply #760 on: Tuesday, October 22, 2013, 06:22:11 »

Nothing wrong with mixing brands between chain & cassette. As long as the chain is the right speed for the cassette it'll be fine.

The quick links on the SRAM chains are handy for removing the chain for cleaning. KMC chains seem to be favoured amongst MTBers.
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Bewster

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« Reply #761 on: Tuesday, October 22, 2013, 07:16:59 »

You can get magic links for all chains which make them removable. However the link that came with my sram chain wasn't reusable so bought a link from KMC.

I have two chains for my road bike which I rotate after each clean so they wear evenly and I can get more mileage (ocd/geek/sad man)

I favour shimano cassettes/mechs with SRAM chains buts that's just personal preference.
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« Reply #762 on: Tuesday, October 22, 2013, 08:45:48 »

Something I found out the other day (by personal experience/cock up) was that not all rear mechs have a barrel adjuster and therefore rely on the barrel adjuster on the shifter (if you have one) for tuning.

I found this out as I upgraded my bog standard rear mech for a shimano slx model and couldn't fine tune it because of this issue and the fact that I have no adjuster on my shifter. I ended up having to swap the rear mech for a different model at an extra cost.
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Samdy Gray
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« Reply #763 on: Tuesday, October 22, 2013, 09:15:03 »

I have to index my MTB by using the shifter, because I stupidly cross threaded the adjuster on the mech  Embarrassed
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« Reply #764 on: Tuesday, October 22, 2013, 09:17:19 »

I have to index my MTB by using the shifter, because I stupidly cross threaded the adjuster on the mech  Embarrassed

I share your pain !  Grin
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