I've been using my MTB on the road for a while now and think it's time to change my cassette for a one more suited to the road. I'm running a XT(9) 11-32 at present and have been looking at an Ultegra(9) 12-25, does anyone have any ideas if this is a good choice of ratios & cassette type? At the moment I'm running out of gears on the flats and downhill, (I want to keep the chainset I have.)

Ratio's are normally a personal preference type thing. I put a road cassette (tiagra) on my mtb, but i used a gear ratio chart to find the best ratio for my own preference.

If you find the gear that you use most of the time, check what sprocket the chain is on then count the number of teeth on that sprocket. This is the one that you want near the middle of your new cassette.
As the Ultegra cassette is smaller, you will get a harder ratio anyway, so you will have the extra bit that you need for flats and downhill. Also the gears will be closer together which makes shifting smoother and acceleration faster.

Just been out to the shed and looked at mine!
I'm using a 12/23

Thanks for the reply, would I feel much of a difference between 12-25 and 12-23? I chose that set of ratios because it seemed mid-range.

Yeah you possibly would, the gears would be closer together, and you would not have the lower end gears to use.

I currently use an ultegra 12-27 on my mtb and a ultegra 12-25 on my road bike. I was using the same on both for sometime.

Here you go, see which one you prefer

11-21 - 11.12.13.14.15.16.17.19.21
11-23 - 11.12.13.14.15.17.19.21.23
12-21 - 12.13.14.15.16.17.18.19.21
12-23 - 12.13.14.15.16.17.19.21.23
12-25 - 12.13.14.15.17.19.21.23.25
12-27 - 12.13.14.15.17.19.21.24.27

Sorry to be so stupid but the higher gears (low numbers?) are almost the same as on my 11-32 cassette (11.12.14.16.18.21.24.28.32), so how does changing to a 'road' cassette (12-23 or 12-25) increase my speed on the flat and not run out of gears?

If your running out of gears,keep an 11 tooth !!

A road cassette will be nicer in terms of fewer big jumps in the gear.

No matter what you do to an mtb,a road bike,even a cheap one,will always be faster/more enjoyable to ride.

Thanks, that makes sense, so really the only way (apart from a road bike) would be the expensive route of changing the chainset to something larger.

Why not get a bigger outer chain ring for about £20 from Wiggle or somewhere?

I just replaced my aging 42 tooth Shimano STX-RC ring for a 44 tooth Truvativ Chainring. I found a BIG difference, and as the other guy says, the 11 tooth cog on your cassette is the smallest you can get anyway.

Just to clarify...The 11 tooth on the cassette is the one that will allow you to go the fastest. To go faster with a replacement cassette you would need a 10 tooth or less. 11 tooth is the smallest I've ever seen. Changing the the cassette to a 11-23 will make it harder on hill climbs and pack all you ratios down to a tighter range (not good for mtb's/off-road).

Ive seen 9 tooth sprockets.



But that was on a bmx!

"I'm running a XT(9) 11-32 at present and have been looking at an Ultegra(9) 12-25"

"At the moment I'm running out of gears on the flats and downhill"


Eh?!

As I'm sure you're now picking up, the smaller the numbers at the rear, the bigger the gear. So swapping from a 11-32 mtb cassette to a 12-25 or 12-23 road cassette will actually give you a smaller top gear (12 rather than the bigger 11). It will however give you a narrower set of ratios and make it harder to get up the hills ;-)

To get bigger gears, you will need to swap the front chainrings. Road bikes usually use something like 53-39, rather than mountain bikes 44-32-22. As you can see, this is a huge difference! However, I'd imagine that you'd struggle to get a 53 onto a mountain bike as it's huge and might hit the chainstay.
You could try something like a 46 which will give you a slight increase - certainly noticable but nothing like what a road bike would give you. Are you very ungeared or just slightly?

I run an old XTR 5-Arm chain set on my race bike which is 26/36/48, my play bike has an XT compact 22/32/44 chainset on it. they both have 12-34 cassettes and I find an incredible difference between the two, personally i prefer the smaller rings, I like to spin. They do make a Deore non-compact chainset with 26/36/48 rings if you want to go down that route.

I'm using a Truvativ 44t as my large ring, I've seen a 48t for about £25, maybe I'll give that a try. I'm slightly undergeared and want to increase my speed on the flat and keep pace with the traffic. Will I have any problems just changing the large ring and leaving the others the unchanged?

Yeah,i'm doing the same to mine except 26/36/46. I've been told that you need to keep within a 10-12 tooth difference!

Why not get a road bike with top gear ratios 12-53 and 700x20mm tyres? Mine flies along and I keep the off-road bike for off-road. Works for me.

Yeah, from what I've just read I agree with Pete!