My roadbike has 9-speed Tiagra gearing at the moment with a compact up front and an 11:25 on the back. Generally that's fine but I'm thinking about trying to do my first 100 miler soon, and would prefer considerably lower gearing for some of the climbs later in the route when I'm knackered.
Can I just slap one of my 11:32 or 11:34 cassettes and a 9-speed MTB mech on there temporarily? Will that work ok with the Tiagra STI?
I have fitted an 11-34 on the back of a road bike with Shimano 105 STi shifters and an XT rear mech when doing a sportive and it really depends on the size of the mech.
If you have a medium or long cage rear mech. then as long as you adjust the length of the chain to suit it should work ok but if you have a short one, I don't think so.
Can I just slap one of my 11:32 or 11:34 cassettes and a 9-speed MTB mech on there temporarily? Will that work ok with the Tiagra STI?
Will work fine, as long as you also lengthen the chain considerably too. Will look a bit odd though!
Will a 12-27 cassette not do the trick perhaps? I know what you mean though, I've got exactly the same gearing setup as you on my road bike, and being new to it all, am very surprised there aren't more bail out gears on a road bike, and have found myself REALLY struggling to get up maybe a 1 in 10 climb at the end of a 50-60 mile ride even! Bad that ain't it... 
Cheers. I've got long cage MTB mechs so I'll give that a try.
MTFU 
Back in the day we only had 52/42 up front and 13/21 on the back and it was plenty.
My other road bike has an 11:28 on the back and a compact up front and I do find that helps a lot when I'm either tired or doing climbs into the wind. I've only done rides up into the mid 50's so far though, so I'm expecting to be properly knackered if I double the mileage.
34:25 is LOW.
i wouldnt worry too much about going lower otheriwse people walking by may start overtaking you.
jesus missed that last post. 34:28 is LOW LOW. DAMN!
if you have a short cage mech then no - you'll have to get a longer chain and a medium/long caged mech to go above 28 tooth (the total tooth capacity might be OK - haven't checked that ) but the rear is up to 28 tooth on the 105 SS mechs (already researched this for my wifes bike).
You will hear the MTFU argument quite a bit.
But...
All the SRAM Pro Tour teams will now regularly use a 32 tooth rear cassette on stages with steep hills.
if its good enough for Contador...
(Dave Millar used a 36 tooth XX rear cassette with XX rear mech in the Giro - this is because he uses those ovoid chainrings, which are 130mm OCD only but 42:36 is the same as 36:32)
having the low gears gives you the option of spinning and using a different muscle group and you will get tired later.
I have had this conclusively proven to me in races in France where young riders with a jey 34:29 (haha benders) have then comprehensively spanked me up mountains by time gaps that are measured in geological terms.
You didnt say if you had a compact chainset?
All the SRAM Pro Tour teams will now regularly use a 32 tooth rear cassette on stages with steep hills
that are 30kms long.
You didnt say if you had a compact chainset?
I do indeed have a compact chainset, on both my road bikes.
I know that by roadie standards even my 34:28 low gear is regarded as horrible, but I'm by no means a typical roadie (lets just say my power to weight ratio is somewhat below par).
I've actually yet to encounter a hill I couldn't get up using 34:25 but that's when I'm reasonably fresh. For 100+ miles I'll definitely be wanting a bail-out option as the route I'm planning has some short but steepish climbs near the end.
that are 30kms long.
Those riders weigh half what I do and put out three times the power.
i wouldnt worry too much about going lower otheriwse people walking by may start overtaking you
They'd have to be walking quite fast as according to Sheldon Brown at my normal climbing cadence of about 90rpm, 34:34 with 700x25 is 7.1mph...
that are 30kms long.
and surfaced with gravelly mud.
ie, they are at best fire roads and so MTB gears make sense.
It's one of those things that always get people hot and bothered.
All the technical advice is good. I'm hopeless at climbing (too heavy), so tend always to use a 27, whether that's with my standard (39/53) chainset, or compact (34/50).
Have a quick look at Sheldon to work out the gear inches. this might help direct you when determining what you need to change (if anything) on the bike. For example, 39x27 is around 1 gear bigger than 34x25.
I say try fitting a 27t casette and see how you get on. I reckon it will do the trick.
I say try fitting a 27t casette and see how you get on. I reckon it will do the trick.
My other road bike already has an 11:28 and while I find that helps a lot compared to 11:25 I still want lower for this particular case!
Can I just slap one of my 11:32 or 11:34 cassettes and a 9-speed MTB mech on there temporarily? Will that work ok with the Tiagra STI?
the sti will be fine.
your rear mech might not have
a: the ability to accomodate such a big cog (use a long cage mech)
B: the capacity to accomodte the extra chain you will need.
Hope that helps.
Well I was glad I had a compact on the Wiggle Cornwall Tor ride with a 1 in 3 - I was climbing in 34:28 at a cadence of about 30 (of felt like it) on the 1 in 3 and the guy behind just fell off his bike!
I made it up - most didn't! perhaps they were not running triple or compact!
yeah 15 1/2 stone ex prop 46 year old jey boy here
use a compact with 12 / 27
can still sit down and pedal comfortably down to 5 mph or so, whereas I'd be standing on a 39
looking at 11 / 28 cassette, more for the 11 than the 28 - I span out the other day but it was at 38mph in a time trial
I've used 34/28 plenty on the road when I've been really unfit. I could manage higher but it means that I can spin rather than be forced to grind in too high a gear for my fitness which is the worst thing you can do on a long ride.
Another vote for sensible gearing, I ran 34/28 for the Fred Whitton and was very glad of it (despite having a good supply of almost-as-steep hills to train on down here in Somerset).
7:26, since you didn't ask 
I stuck an 11-28 on my roadie for the Lakeland Loop this year, which hits Hardknott and Wrynose 70 miles in.
I made it up, but only just. I was in a small minority of the people around me still cycling.
I've left it on since then. I've only just upgraded from 7 speed to 10 speed at the back so even with 11-28, I don't find the gaps between gears particularly noticeable.
If you go for 11-28, you'll probably get away with swapping just the cassette (although watch out for chain length). Anything bigger and you'll probably need to change the mech too.
Going from 25 to 28 is over 10%, so it does make a difference.
Whats wrong with a standard compact of 34/50 with 12-25????
I also use tiagra 9 speed.
Done plenty of 80-100 mile hilly routes (with long 3-9mile climbs at a time & lots of em over the course of the route) with no probs.
i find a 25 gives me a nice climb speed of 8-11mph depending how quick i spin. I find gradients of 19% ish no probs with the 25 sitting. When it gets a bit steeper in parts than this i just stand up to push on, then sit and spin when the gradient drops off again.
I guess living in north wales makes you used to climbing 
I find the 12 lacking sometimes on flat line speed in places, but for 99% of the time its fine.
Done plenty of 80-100 mile hilly routes (with long 3-9mile climbs at a time & lots of em over the course of the route) with no probs.
Given you're not me, I'm struggling to see the relevance...
relevance... Is whats wrong with a standard 12-25 with a compact of 34/50???
I would have thought the 25 with a 34 would be plenty low enough, plenty low enough to spin out IMHO. you never said what you are running up front thats all.
To actually answer the question:
If you fit a Shimano MTB cassette and mech then yes, they are fully compatible with both the hub and shifter and it will work absolutely perfectly, provided you have sufficient chain length to allow for the larger sprockets and longer cage.
Ignore everything else. If you want low gears, fit 'em.
as I said above, I now find 34x28 quite useful.
thing is I used to do the same rides if not harder ones, just fine with 42x23 when I was much fitter.
that's why the relevance is being questioned.
To clarify:
1) Both my road bikes already have a 50/34 up front
2) One of them (but not the one I want to use for the 100 miler) already has an 11:28 (it's 10-speed and the bike I want to use is 9-speed so I can't just swap the cassettes)
3) I don't want to run 11:32 or 11:34 all the time, just for my first 100 miler, hence wanting to use MTB kit I already have rather than buy stuff
4) I've yet to have to get off and push even with 11:25 fitted, but I've also yet to do much over 50 miles in a day and know I'll be wanting lower gears at points along the route I have in mind (as I've already ridden it over 2 days on my touring bike and it has a bottom gear approximately equivalent to 34:34)
5) I'm not fit, so telling me how easily you can climb hills at the end of 200 mile days with using 42:21 doesn't help!
Anyway thanks for the advice - sounds like I can borrow a long cage mech and a 11:34 from one of my mountain bikes so I'll give that a go.
I just did a very hilly Dales 100 mile sportive and did just what you're asking
I had an old XTR mech so banged it on with an 11-32 cassette and added a few links to the chain
My knees aren't what they once were so I'll probably leave it on
I really don't give two hoots what the 'macho' men have to say about it, I passed a few of them climbing up Park Rash whilst they were busy zigzaging and trying to keep the cadence over 10, I was able to maintain a liveable pace - still 'kin hurt though
Steve , you do what you want mate - tis your bike afterall ! Sounds like a good plan
We used to fit loads of MTB cassettes and mechs to road bikes for people doing the Etape. Good idea, why limit yourself.
Well it now has a scaffy old LX mech fitted and a 11:32 cassette. Not pretty, but it works.
good luck, hope it works out for you and you get up dem hills you are worried about.
Am having a similar dilemma myself after signing up to a road "holiday" in Belgium. Roadbike was my turbo trainer and the 11-25 cassette and standard double chainrings are filling me with fear (I don't run a granny on my MTB but think that this combo on my roadbike will make me walk).
Will a compact be enough or should I go for a new cassette as well, I understand that at most I would only be able to go as high as 26 or maybe 28 at a push on the short cage at the rear.
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