A Simple Change?

Yes, you will be able to continue to use your Tiagra shifter & front derailleur.

Yes, it is a fairly simple change:

• replace the crankset & BB
  
• shorten the chain by a few links
  

If you have a Tiagra crank, I believe it probably uses an Octalink V2 BB -- that's the same type that Shimano's Octalink MTB cranksets use. While it would seem expedient to simply continue to use the existing BB, your BB's spindle is probably 118mm long ... and, you only need one which is 113mm long.

Does 5mm make a difference? Maybe, yes. Maybe, no.

Probably, not!?!

• Some people fixate on the Q-factor ... it matters to some extent, but not as much as some people would like to think, particularly since pedal-type and how the cleats (if you are using clipless) are mounted can effect the foot placement on the pedal.
  

The reason it may not matter is because the "factory" Q-factor on a MTB crank is closer to 170mm ... on a Road Double it is between 145mm to 150mm ... on a Road Triple it is 150mm to 155mm. Simply installing an Octalink MTB crankset on the BB with the longer spindle will result in a Q-factor of 175mm ... so, figure your pedals will be 10mm further apart on each side than they are at the present time OR 7.5mm if you change the BB.

So, again, probably, the spindle length will probably not matter as long as you lower the saddle by a few millimeters.

• I am presuming that the front derailleur's cage will swing out far enough
  

If you think the 2.5mm in the Q-factor is going to make a difference, then you can buy any MTB crankset & BB which strikes your fancy.

BUT, do you really need THAT LOW a gear ratio?!?

19 inches would be pretty much the same as walking.

BTW. You can get Shimano MTB cranks which use Octalink BBs from eBay ... you will need a crank puller if you don't already have one ... if you have an older one OR if you don't get one for Octalink-and-ISIS BBs, then you can insert a "slug" (e.g., a couple of pennies) against which the crank puller will press.

alfeng,

Thanks for the detailed reply.
In fact the existing crankset is an FSA Vero with a standard tapered bottom bracket. I think I can obtain a 44/32/22 crankset (possibly used) from Ebay for a modest sum. That would allow me to 'prove' the low-geared system. I will try that now that you confirm the changer should work. I may keep the existing BB for simplicity at least initially.

I don't think I need a low gear of 19 inches, but I know I don't need a high gear of 123 inches! Unless I start changing both crankset and rear cassette, the simple crankset change seems most easily done.
It will be nice to experience using all three chainwheels!

Steve

One further comment:
I chose 44/32/22 because it seems readily available. Ideally, I might have chosen 44/36/26 to give a tighter range of 22 - 108 inches. That, I assume is much more difficult to obtain?

Steve

Quoted post said:

Originally Posted by steveal50 [IMG]/img/forum/go_quote.gif[/IMG]

One further comment:
I chose 44/32/22 because it seems readily available. Ideally, I might have chosen 44/36/26 to give a tighter range of 22 - 108 inches. That, I assume is much more difficult to obtain?

You can buy separate chainrings and thereby have a more useable combination.

BTW. The 22t chainring with a 27t cog would be approximately the same ratio as a 26t chainring & 32t cog, so you can just think of the 28t cog in your cassette's stack as a close approximation to the low gear you want AND then think of the 32t cog as a bailout cog or something you will be more likely to use with your middle chainring.

Quoted post said:

Originally Posted by steveal50 [IMG]/img/forum/go_quote.gif[/IMG]

My road bike (Specialized Tricross Sport) has a fairly standard 50/39/30 chainset with an 11-32 nine speed cassette.
I'm a 61 years old recreational rider and I never use the large chainwheel. The highest gear on the middle wheel (96) is really all I need. On the other hand, I could sometimes use a lower bottom gear.

A simple change would seem to be to swap out the chainset for a 44/32/22 mountain bike model. This would give me a range of 19 - 108 inches rather than 25 - 123 inches.
Would this work whilst keeping the Tiagra front changer and Tiagra STI levers?

Steve

Yep, no sweat. Lower FD, probably shorten chain.

Thanks to you both for the guidance.

Steve

go to nashbar.com they have a couple of cheap MTB cranks for sale now. should be plenty durable for recreational use and probably only cost you about $50 maybe even less.