Compact chainset 28T Cassette???

[mill4six]

50/32 on the front, 11-25 9 speed on the back. Terrain dictates that 28T would be nice but does anyone know if it'll work with a short cage Tiagra rear mech? It looks like it will. It's coming from CRC but it'd be nice to have a thumbs up from someone who's already done it before I start spannering......

[gaz]

Shimano Tech Docs give a 37T total capacity and 27T max sprocket.

I'd expect you'd get away with a 28T sprocket. I've got a feeling that someone posted recently about succeeding with a 30T sprocket but I wouldn't want to try that.

[gbnz]

I've used a 28 tooth cassette with both a 8 spd and 9 spd short cage soro deraileur. Never been an issue and I'm sure the tiagra will have much the same capacity (Mines on a 50/34 chainset)

[willem jongman]

It depends on the geometry of the derailleur hanger, but there is a more than fair chance that it will work. I got 30t to work. See here: http://sheldonbrown.com/deakins/lowgears.html Interestingly (though irrelevant for you) Shimano's new road derailleurs now have a 30t maximum (so perhaps 32t in reality). This is good news now that Shimano have made road and mtb 10 speed less compatible.
Willem

[Brian73]

I used a 12-28T cassette with Sora 3300 8 Speed short cage (rec max 27T) Was pretty close, but passable, looked a bit odd compared to a 12-23T too. I was using a 38/52 chainset though.

105 5700 allows for 30T and SRAM Apex a massive 32T

[Geoffrey]

I've tried numerous setups like this and made them work but... If you try to use the gear 50x28 you will probably run out of chain and all hell will let loose (Think mashed up rear derailleur and probably chain stay). Equally on 32x11 you will have no next to no chain tension and the chain will bounce off the chainset when you go over a bump. It is not the maximum sprocket size that is the issue but the chain wrap capacity.
FWIW running a 52/34 chainset with a 11-28 cassette works with every short cage mech I've tried, Shimano and Campag. (It doesn't get you a lower gear though.)

Good luck, Geoffrey

[willem jongman]

Both maximum sprocket size and chain wrap are an issue. To deal with insufficient chain wrap you want to use a long enough chain to allow the use of the largest chainwheel and the largest sprocket. Inevitably that means that if you exceed the capacity of your rear derailleur you have excess chain if you use the smallest front chainwheel and the smallest sprocket. Of course, you should always avoid these extremes in any case, but if you make a mistake, it is better to have a chain that is too long than one that is too short. But if you avoid extreme cross chaining you are unlikely to get into trouble if you exceed Shimano's specifications by a modest amount.
Willem

[mill4six]

I put a new chain on but kept the length the same as the old. The new cassette is on and seems absolutely fine in all gear combinations. I shall give it a quick road test tomorrow to be sure as I'm racing a hilly triathlon on Sunday. It's tempting fate to start tinkering so close to an event but I couldn't resist Thanks for the reassuring replies so far......