Shimano 7/8 speed compatibility

[curzons246]

Hi, thanks for looking in. I'm building up a bike to cope with the peak district. The shifters I want to use are shimano 8 speed 105 sti's ( drop bars ). I'm looking to put a 7 speed shimano cassette on the rear 32t or 36t. There are plenty of shimano mtb sgs ( long cage ) rear derailieurs but will they be compatible with my shifters. I would much appreciate your advice cheers Bill.

 

[hamster]

The derailleurs are all the same for 7/8/9 and 10 speed road. 10 speed MTB is different, also later Tiagra (4700) 10 speed road.

Note that 8 speed cassettes are NOT the same as 7, having a 4.8mm pitch. 7 speed has 5.0. It sort-of works, but no better than that. So you will find things get more and more rattly toward bottom gear.

If you want a 32T or 36T cassette you will have to use an MTB rear mech as road rears struggle with anything over 30T. I don't know of anyone who makes a 36T 7 speed, although 12-32 exist from Shiman and SRAM. The 12-32 is OK but the steps between gears are big and nasty. I would go for a smaller chainset instead to get lower gears.

 

[pigman]

Re:

Why are you sticking to a 7 speed cassette when you have 8 speed shifters? Like hamster says, it won't index properly. Just get an 8 speed cassette. If your freehub will only fit a 7, then I'd still buy an 8 and lose the smallest sprocket

 

[curzons246]

Re: Re:

Why are you sticking to a 7 speed cassette when you have 8 speed shifters? Like hamster says, it won't index properly. Just get an 8 speed cassette. If your freehub will only fit a 7, then I'd still buy an 8 and lose the smallest sprocket

Hi Pigman, my rear dropouts are set at 126mm so the 7 speed hub fits. I have in the past eased a 130mm into a 126mm frame and I'm aware of cold setting but I will see how it goes and will certainly consider losing the smallest cog on an 8 speed cassette. Thanks for the tip cheers Bill

 

[Jonny69]

That option works quite well. I’ve also done it with a 10-speed with the smallest one taken off.

 

[curzons246]

Re: finish

Just to let you know, the outcome I got an 8 speed deore long cage rear derailieur. Tried to run that with my 8 speed 105 shifters and the 7 speed cassette. As I was told above it ran dog rough. I then cold set the frame to 130 got a shimano 8 speed wheel/frehub. Fitted an 8 speed cassette and the set up runs like a dream so with 50/ 34 on the front and 11/32 on the rear I should cope much better with the Derbyshire Peaks. Thanks for your help.

 

[hamster]

Glad to know it all worked out!

 

[curzons246]

Cheers Hamster, thanks for your sound advice, Bill