Tiagra 4500 short cage mech – what max T rear cassette?

I am putting a short cage Tiagra 4500 mech on my sons dh bike. What is the max size of rear cassette I can put on? All in 9 speed and front is 32t if that matters…

Thx Simone

11-25 9sp I think. Might be 27 max. but I’d be surprised if it was any bigger.

27 is the recommended max with a maximum total capacity of 29 for the short cage road mechs

what does max total capacity of 29 mean? Sorry, new to this gear stuff…

It’s pretty much irrelevant if you have a single ring up front

a 11-27 cassette needs a capacity of 16 – I think – so nowhere near

still not sure on that, but does this mean i cant use my 12-32 cassette with this mech?

No

27 is the rated limit – you can try though

The total capacity is the amount of slack chain it can take up

You normally subtract granny ring size from big ring size (so let’s say 20 in a 22-32-42) set up. Then, subtract the smallest cog from the biggest cog at the back (11-32 would give us 21), then add the 2 together – 41 in this example, so would need a long cage.

For you with a single ring, there’s no front difference to take into account, so just do the calculation on the cassette difference. Or to put it another way, you’ll probably be able to run whatever cassette you like 🙂

Capacity is the difference between the largest and smallest sprocket plus the difference across the chainset. So if you had for argument sake an 11-32 rear you there have 21 and a 38-28 chainset so 10 there meaning your mech needs a capacity of 31. This is how much loose chain the mech arm can “wrap” and release as gears are changed. However mechs also have a maximum sprocket size. Basically this is how big a cassette sprocket can be fitted without clashing with the body of the mech. For best shifting the top jockey wants to be as close to the sprockets as possible. However with bigger and bigger rear sprockets this becomes tricky while maintaining good shifting. You will often find MTB and Road mechs with the same capacity but the road mech will usually be designed to get in close to the block and so may have a stated max sprocket of 27/28. If you have the patience it’s always worth a suck and see.

still not sure on that, but does this mean i cant use my 12-32 cassette with this mech?

If the 32 big cog causes issues and you don’t want to buy a 27, I guess you could wind the stop in to prevent it going that far in

cheers for your advise guys. I will run a chain device, does this help with taking up the slack?

I ran a 34/11 cassette with a 105 short mech (36t up front) without problems , but it does run smoother with the 25/11 that’s on it now. The smaller gaps in the gears are better too.

Chain device is in a fixed position so does not take up any slack.

Try the mech with the 32T sprocket with the bike on a workstand…you may be surprised to find it will work if you adjust the B-screw correctly.

As someone suggested, if it fails, use the limiting screw to block the shift to the 32T.

PaulD