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Old 06-06-11, 06:25 PM   #1
expassenger
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Replacement Crankset Question

I bought a new Carrera Subway 2 hybrid from Halfords about 7 weeks ago. Last week my left crank fell off! I was surprised I hadn't noticed it getting loose during the ride. The bolt is partially de-threaded (may have happened while working loose) and Halfords said the bottom bracket may have also dethreaded, but they are not sure yet. As the bike is new and came from the factory with the cranks attached, I guess there was some assembly fault. I took it in at the weekend and they have agreed to repair it for me. They will order a new crankset and said they will see if it needs a new BB too. I was shocked that all this was needed just because the crank came off!

The current crankset is a Truvativ Isoflow 42/32/22 175mm, but they said that they don't sell those. The replacement on their system is a Romax Tracer RBR544 42/32/22 170mm. After returning home and looking up both cranksets, I have a some concerns/questions. I am a novice to bike maintenance but keen to learn and would be grateful of advice.

Firstly, will the 5mm reduction in crank length on the Romax make much difference? I presume I will have a less power? (or torque?) I don't think I like the idea of that...

Secondly, is the Romax Tracer an equivalent spec replacement for the Truvativ Isoflow? From a quick search it seems to be a cheaper product.

Finally, I have read that it is better to have a higher cadence, and I may not be pedaling right, but I have felt on this bike that I would prefer a higher ratio top gear and I find that I don't often use more than the top few gears. I noticed that there is a Truvativ Isoflow 48/38/28 175mm. http://www.sheldonbrown.com/gears/ tells me the 48 teeth will give me a max ratio of 8.5 rather than the 7.4 I've had so far. The problem is I don't know how this will 'feel'. Will it make much difference? Will it be too much? I guess at least I won't be in top gear all the time. N.B. My cassette is 11/32T.

I did ask in the shop if it would be possible to get a crankset with more teeth. They mentioned that I may have get a longer chain too. Will the bigger Isoflow 48 fit my bike? Will I have to replace other parts too? e.g. the front derailleur. Do you think this is worth doing this or should I stick with the original ratios and learn to pedal with a higher cadence? perhaps I should have bought a road bike after all

It seems that with the 48T chainring I'm considering upgrading to, my to gear ratio would go up from 7.2 to 8.5 i.e. an 18% increase. I have 24 gears. 3rd from top will have the same ratio as my pre-upgraded top gear. So, I effectively get 2 extra top gears by upgrading. I imagine this to be about right, but I was wondering what others think? do you think it might be too much?

Anyone else have a hybrid with this sort of gearing? i.e.
chainset 48/38/28T, cassette 11/32T, 26" wheels, 170mm crank

As I'm not too concerned with the low gears because I rarely go up any hills, do you think it will be ok? or is there other factors I'm overlooking? Any advice gratefully received as I can't really try before I buy.

Sorry for the lengthy question. Thanks in advance for advice.
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Old 06-06-11, 09:49 PM   #2
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Seems odd they'd replace with a different crank arm length. 42/32/22 is really low...mtb gearing even. A 48/38/28t might work. Depends on the front derailleur.

I'd love to add more, but I don't know what Halfords, a Carrera Subway 2, or a Romax Tracer is. Sounds like a grocery store, fast car, and Star Trek death beam, respectively.
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Old 06-07-11, 12:44 AM   #3
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Quote:
Originally Posted by expassenger View Post
I bought a new Carrera Subway 2 hybrid from Halfords about 7 weeks ago.
The answer is in the first line of the question, you should take the bike back, and demand a replacement, as it was not built / set up correctly / or the chainset failed. They should not offer a substandard replacement (dependant on the orginal chainset model the Romax may be un upgrade, hard to tell), it should be like for like at leaset.

If the bike was correctly build / had a pre sales inspection, the loose chainset would have been picked up, and fixed at the time, they failed on this.

For the gearing, what you have sounds fine, and is normal for a hybrid bike, you have only had it a few weeks, see what you think of it after a few months, for any major upgrade at this bike proce point, a replacement bike would be more cost effective; you would notice the 5mm difference in the chainset length.

For anyone who dosen't know what Halfords is, they are one of / the UK's largest bike retailer, Carrera is one of their brands, the Subway line is their urban / commuter bike line, which are well spec'd low to mid range bikes. Romax Tracer is a brand / model of crank / chainset
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Old 06-07-11, 12:49 AM   #4
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42/32/22 is really low.
.. with cassettes starting at 11 teeth , the high will be sufficient.

Halfords is primarily an Auto parts store chain.. right?
they do sponsor cycle racing, but I doubt they use the
Hack mechanics in the stores, to take care of the Pro's bikes.
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Old 06-09-11, 07:29 AM   #5
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Can you replace an 8-speed crankset with a 9-speed one?

Halfords say they are planning to replace the original Truvativ Isoflow 42/32/22t on my Carrera Subway 2 with a Truvativ S650 48/38/28. My only concern is that they said this is a 3 x 9 speed crankset and my bike is a 3 x 8 speed. They told me that it will work fine.

Anyone know if this will work or not? Can you just replace an 8 speed with a 9 speed?
What is the difference between an 8 speed and 9 speed crankset? What effect does this have?

Appreciate your help.
Paul
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Old 06-09-11, 08:28 AM   #6
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What is the difference between an 8 speed and 9 speed crankset?
the chain is slightly different, thinner as the space tightens to add #9 cog
in the back
they may shave a half MMin space between the rings, in front.

it does not matter , there may not be any old stock, to put on the same stuff,
as before.

Quote:
What effect does this have?
more significantly your low gear is a 28t rather than a 22t,
you will feel the low gear being higher, on the hillclimbs.

Last edited by fietsbob; 06-09-11 at 08:32 AM.
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Old 06-09-11, 08:43 AM   #7
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In my own experience, there is no meaningful difference, in terms of performance, between an "8 speed crankset" and a "9-speed crankset". An 8-speed chain will work fine on either crankset.

While it's true that 22-32-42 is adequate for urban/hybrid use, the 28-38-48 should also suffice. When mountain biking, I almost never use the 22x32t combo, as the bike moves so slowly with that gearing that it's hard to maintain balance (and traction for that matter). Point being, for it's intended use, 28x32t ought to be a more than adequate low gear. The taller top end will be nice for paved descents, and may save a little wear on the smaller rear cogs.

For the OP, I would insist on getting an exact or minimally equivalent replacement, unless it's clear that your either getting an upgrade or something that you would otherwise prefer.
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