What outsized chainring to use?

[XmisterIS]

Hi,

I have a Giant XTC composite which I'm very happy with; however I find that when I run with my slick tyres I often can't get a high enough gear ratio to keep power going to the chainset. For example, the other day I was going down a long, steep hill and I managed to top 40mph, but I had to pedal like a man possessed in order to reach that speed!

My groupset is LX front mech, XT rear mech, SRAM PG950 11-34 cassette, 22/32/44 chainrings, 27-speed rear shifter.

I would like to upgrade my outermost chainring to a 46 or a 48 in order to get a higher gear ratio.

I would have no problems switching to a 48 outer ring because I tend to run the bike on quite skinny tyres anyway - my off-road tyres are a pair of Panaracer Trailblaster 1.95's (I am currently running on my winter tyres - Panaracer Pasela TG 1.5's) and I bought the bike to ride it as it was intended - as an XC bike! So I feel that having a nice big outsized outer chainring may be just what I need to get me going nice and quick when I need it.

My questions are:

(a) Will the front LX shifter be able to cope with 22/32/46 or even 22/32/48?
(b) Will it really make that much difference? I am well aware of the simple arithmetic required to calculate gear ratios, however I wonder what difference I will actually feel as a ride.

UPDATE! - This would probably fit my cranks just nicely in the 48 version: http://www.chainreactioncycles.com/Models.aspx?ModelID=5700

TIA,
Mike.

 

[gclark8]

A 48 tooth big ring will work well with the bike and shifter is OK.

The Front Derailleur, you need a MTB 48 tooth FD for 9 speed. See Page 209 of the 2007 Shimano Book for selection criteria. Possibly FD-M581A-3 or-6.

 

[alfeng]

A 48 tooth big ring will work well with the bike and shifter is OK.

The Front Derailleur, you need a MTB 48 tooth FD for 9 speed. See Page 209 of the 2007 Shimano Book for selection criteria. Possibly FD-M581A-3 or-6.

While Shimano might spec a different front derailleur for use with their 48t chainring, I would suggest that you try your current front derailleur, first, since it might work ...

I am using a "regular" top pull, 8-speed XT front derailleur with a 52t chainring on a 130BCD road crank (see attachment to get a sense of the juxtaposed outer cage & 52t chainring -- yes, the red alignment "tape" was still on when the pic was taken!) ... the "regular" Shimano MTB front derailleur also worked fine (on the same bike) with a pair of 105 shifters (I needed 3-clicks to move the derailleur, sufficiently, because it's a 9-speed chain), too.

 

[John M]

While Shimano might spec a different front derailleur for use with their 48t chainring, I would suggest that you try your current front derailleur, first, since it might work ...

I am using a "regular" top pull, 8-speed XT front derailleur with a 52t chainring on a 130BCD road crank (see attachment to get a sense of the juxtaposed outer cage & 52t chainring -- yes, the red alignment "tape" was still on when the pic was taken!) ... the "regular" Shimano MTB front derailleur also worked fine (on the same bike) with a pair of 105 shifters (I needed 3-clicks to move the derailleur, sufficiently, because it's a 9-speed chain), too.

Interesting build there. Looks like an MTB frame. What did you do for that rear brake mount? Was there a bridge that you drilled, or did you fashion a mounting bracket of some sort? 650c or 700c wheels?

 

[alfeng]

Interesting build there. Looks like an MTB frame. What did you do for that rear brake mount? Was there a bridge that you drilled, or did you fashion a mounting bracket of some sort? 650c or 700c wheels?

It is indeed a MTB frame (YOU, of course, are one of the few-if-not-only people who noticed the brake bosses for a 26" wheel which are invisible to most people!) -- the frame is one of my old/extra/spare hardtail frames which was languishing for components (note the non-replaceable rear derailleur dropout) ...

The wheels are 700c & the brakes are TEKTRO 521A (I think that's the model designation) ... the rear rim is an old NISI (622-13) rim that I had.

Several years ago, I made the observation that the brake bridge on some (older) MTB frames & older (as in "threaded") forks would probably allow a LONG REACH road brake + 700c rim combination ... so, this was a build that was waiting to happen. On the particular Mongoose frame, I simply had to enlarge the backside of the fender mounting hole on the existing "brake" bridge to accept the barrel of a "standard" recessed nut.

A NORMAL/(old "short") reach brake might work with 27" rims.

The rear spacing is 135mm (of course), and I opted to lace the rim to an ACERA hub (why spend more?) whose freehub body I swapped out with standard 8-/9-speed freehub -- yes, it would have been easier to use an LX hub (or, an older XT hub), but I wanted a plain/("silver") hub.

The biggest decision was whether to use the road fork that is currently on the bike, a tandem fork with cantilever bosses, or a solid MTB fork with disc mounts ...

If I competed in CX, I suppose that I might have considered getting a CX fork (the tandem fork is too beefy to lug around) & use it for training ... the rear will handle a 700x38 tire without any problems.

Basically, other than the rear hub, the bike was cobbled together with existing "spare" parts (some pretty nice spare parts, eh?) ... it was unofficially designated as my "climbing bike" during the planning & build because of the oversized downtube & relatively short seat stays.

In case you were wondering, despite the STIFF aluminum frame, the ride is actually great ... as I had projected/hoped ... because of the slightly longer chainstays AND because of the longer overall wheelbase length than on most current road bikes.

 

[artemidorus]

If you are going to use a 48T large chainring, you will need to lengthen your chain and lock out your granny chainring with the limit screw or else make sure that you NEVER accidentally shift into big/big or small/small, as you will be exceeding the tooth capacity of any rear derailleur.

 

[XmisterIS]

Hi all,

thanks for your replies! I think that to start with I am just going to go ahead and buy the 48T chainring (seeing as it is not particularly expensive in itself!) and see how it runs. If I get problems I will start fiddling with deraileurs and chains and the like.

I do sometimes use small/small for steep, muddy and gnarly scree sections on hills, but I never use anything near big/big anyway and I can usually tell when my rear deraileur doesn't like my gear selection because it starts making clicking noises (in other words - change to a better ratio NOW!). So I think my current chain length should be OK.