Can I change the chainring?

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Can I change the chainring?

Postby dub » Mon Apr 14, 2008 10:02 am

I've got a 53/39 Bontrager Sport front crankset (12-26 rear cassette). I've been trying to do a few hills recently but the "granny" gear doesn't seem to be "granny" enough to get my 110kgs up any decent hill (latest challenge - Red Hill for you Canberrians). The current set-up is fine for riding around the flats and some shorter hills, but on the longer or steeper hills I'm struggling.

Can I simply buy at 34 or 36 tooth chainring and replace the 39 leaving the top end untouched, or does the whole crankset need to be replaced. I know I can also change the rear, but to me the change to the front may be easier? What other considerations do i need to have.

I know losing some weight may be on some peoples mind, but at 6'7'', there's not much to lose. OK, there might be some!

Dub
Last edited by dub on Wed Apr 16, 2008 9:43 am, edited 1 time in total.
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by BNA » Mon Apr 14, 2008 10:20 am

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Postby sogood » Mon Apr 14, 2008 10:20 am

I think you'll need to consider changing your crankset to that of compact or triple.
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Postby MichaelB » Mon Apr 14, 2008 10:33 am

You can get a 38T smaller chainring, but that is it unless you go to a Compact setup as suggested by Sogood.
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Postby dub » Mon Apr 14, 2008 10:36 am

Triple is out due to the shifters only being capatible with a double.

If I go the whole hog and get a compact crankset (as opposed to my hybrid thoughts), will I need to change the bottom bracket also?
Last edited by dub on Mon Apr 14, 2008 10:37 am, edited 1 time in total.
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Postby mikesbytes » Mon Apr 14, 2008 10:36 am

Adding to sogood's post, the 53/39 crank set has (I think) a 130mm BCD, which is the spacing of bolts and you can't fit smaller than 38t on that spacing, where a compact crank has (I think) a 110mm BCD so you can fit a smaller chain ring.
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Postby dub » Mon Apr 14, 2008 10:38 am

Thanks Mike, thats the detail i was after.
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Postby LuckyPierre » Mon Apr 14, 2008 11:07 am

You could change to a 12-27 cassette which is quite a bit easier to pedal up Red Hill. You'll probably need to lengthen your chain a bit (I had to when I went put mine on).
Try a couple of easier climbs first - Red Hill is deceptively difficult. Most people consider Mt. Stromlo to be easier.

ps. If you're using 9-speed Shimano gear, I can lend you one so that you can 'try before you buy'.
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Postby dub » Mon Apr 14, 2008 1:09 pm

According to the Bontager site, the Sport double crankset is 110mm. I will measure to be sure when i get home. If this is the case, as long as I get a 110mm chainring, will I be OK? Teeth width an issue?[/quote]
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Postby sogood » Mon Apr 14, 2008 1:12 pm

As you also noted earlier, you have to make sure your FD has the capacity to make the jump. The bigger the teeth gap the more likely for problems.
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Postby LuckyPierre » Mon Apr 14, 2008 2:13 pm

If you mean this informationon the Bontrager site, it would still pay to check. I think that the 110mm they refer to is the bottom bracket width, not the 'bolt circle diameter' (BCD). Most compact cranks have a 110 BCD in comparison the normal cranks 130BCD (or 135mm for Campagnolo cranks).
Edited
I missed that Mike had already explained the BDC dilemma.
It's a pity that the Bontrager (if you wanted to stick with Bontrager) compact cranks are all splined, so you would be up for a new bottom bracket to go compact. And if you change your bottom bracket, then you could consider triples .... and the fun goes on and on. :wink:
Last edited by LuckyPierre on Mon Apr 14, 2008 2:30 pm, edited 1 time in total.
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Postby thomas_cho » Mon Apr 14, 2008 2:27 pm

Hi Dub,
I am no where as tall as you, but still in the clydesdale category, and therefore by definition overweight!!

I think it is unlikely that you have a 53/39 combination on a 110 BCD. But in the even that you do, there is another thing to take note.

If you swap the 39 for a 34, then potentially when you do use the 34/12 combination (not that you will as it represents extreme crossing of chain), the rear derailleur may not be able to take up the chain slack. Consult the shimano website to see what the max teeth difference your rear derailleur can take in respect of the chainset.

I am in Canberra as well ... I had a go at Black Mtn but had to turn back after 1km of climbing. I am guessing that Red Hill has the same % gradient?

Still trying to build up my fitness after my spate of injuries. I once did Mt Ainslie on a standard 53/39 chainset. But was crawling at 6km/h at the top.

I think its far easier to just get a new Compact crankset.
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Postby 531db » Mon Apr 14, 2008 2:39 pm

Going from 39 to 34 teeth on the front chainring is abou the same as going from 26 to 28 teeth on the rear cogs.

It will be easier to get a new cassette.
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Postby itsaghostcar » Tue Apr 15, 2008 4:38 pm

Even if the crank is 110BCD, be careful with the front derailleur capacity. By switching your 39 for a 34, you'll end up with a tooth difference of 19 (53-34), which is probably too much for the front mech to shift.

A compact crank would be my suggestion.
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Postby dub » Wed Apr 16, 2008 10:00 am

I think that the 110mm they refer to is the bottom bracket width, not the 'bolt circle diameter' (BCD)


I might have got a bit excited when I saw the 110. Haven't had a chance to measure it, but I'm not holding my breath.

By switching your 39 for a 34, you'll end up with a tooth difference of 19 (53-34), which is probably too much for the front mech to shift.


That sucks.

Going from 39 to 34 teeth on the front chainring is abou the same as going from 26 to 28 teeth on the rear cogs.

It will be easier to get a new cassette.


I think you might be right.
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Postby dub » Wed Apr 16, 2008 10:05 am

I had a go at Black Mtn but had to turn back after 1km of climbing. I am guessing that Red Hill has the same % gradient?


I drove up Black Mountain on the way home after my unsucessful attempt of Red Hill, just to ram home the defeat. Red hill is only just over the 1km of climbing, alot like the first section of Black Mountain (ie pretty steep).
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