bbarend

Can a rear derailieur be tuned so that an 8 speed sti shifter will work on a 7 speed cassette? The chains are the same size. It seems all you would have to do is adjust the stops on the derailieur.

Chi

I think you need to have 7 speed shifters and adjust cable tension as well as the stops.

dwatson

Yes, you can do it. Just like you said you will have to adjust the stops. Just remeber that you will have an extra click in the shifter.
But with the said, it is best to run a complete group, 7 or 8 speed.

FOG

The cassette spacing is different in 8 speed versus 7.
https://www.sheldonbrown.com/k7.html#spacing

bbarend

Ok, so who is correct. Sheldon Brown also says "Modern indexing systems are so forgiving, however, that many theoretically incompatible combinations actually work fairly well." Has anyone ever tried it?

Chi

My bike came with 7-speed components out of the box. However, the previous owner had upgraded the cassette to an 8-speed, but neglected to do the same with the shifters. All I had to do once I got the bike was buy new shifters (and a new chain--but the same kind, IG90). It works almost like a charm now, except for the front derailleur which I think has seen its last days ...

BikerRyan

its really fairly easy to do. Just put the shifter in the 8th position so that all the cable is out. Then move the rear der. to the smallest cog and make sure that the limit is holding the der. right underneath that small cog. Then pull the cable finger tight and anchor it under the bolt. A couple of left turns on the barrel adjuster should have it tight enough after that. Then shift up to the largest cog again checking the limit so that your last shifter click wont send the der. flying into the spokes and you should be done. Setting it up this way keeps the shifter from having that last annoying extra gear although your gear indicator, should you have one, will be one number off.

-Ryan

Chasbo

Keep in mind that the spacing is different and your indexing will require more tune-ups than if you used the proper cassette. Other than that you can fudge it so it will work so-so. It will never work as well as full 8 speed would.

Chuck

chip

You could rig it up but you'll have an extra click at the end like has already been mentioned?What about the resale value it could be harder for you to sell or trade in when it's like that.

uciflylow

I put drop bars on my MTB commuter recently and wanted to use bar end shifters. The 7 speed index shifters are almost impossible to find and when you do they are more used than new 8 speed stuff!
I got a set of 8 speed index off eBay with the intentions of using friction if I had to. Long story short, they work like a charm with the 7 speed IG system. You just have that extra click at the bottom of the travel limit. I set mine up to where the low speed click is at the end of the limits and the high end "little sprocket" has the extra click, you just get extra slack if you click one too many on the high end.
Good luck

bbarend

Thanks for all the input!

Grand Bois

I don't know what uciflylow is talking about in that thread. There is no extra click. The derailer stop screws and cable tension won't allow it if it's set up right.

I have a 7 speed cassette and 8 speed shifters on my wife's bike and it performs perfectly.

caloso

I did something similar -- old set of 8spd RSX brifters on my 7spd Rock Hopper. It works okay. Not perfect, but good enough for a grocery-getter/kid-trailer-puller.

Retro Grouch 

7-speed and 8-speed cassette spacing is only a hair off. Many riders report mixing and matching 7-speed and 8-speed components with good results. If you do this the best thing is to adjust the cable tension so that the derailleur lines up spot-on when it's in the middle of the cassette. That way your "fuzz factor" will be spread over two different directions.

Personally, I've fooled around mixing 7-speed and 8-speed components, both road and mountain, in various combinations. I've never been able to get them to work to my satisfaction over any significant period of time but maybe I'm just too picky.

Metzinger

I've used a 7/8 combination happily from time to time, and even raced on one. Each time it was with an 8 speed cassette and 7 speed downtube shifters. I'd pull the shifter past the last click to get the largest cog. Only problem was the chain would sometime shift out of that cog when I hit a bump.

SuperFatDave

If you have the ability, the know how and the patience, you can make a variety of Shimano shifters and derailleurs work, despite what the specs say.
Your results may vary from perfect to passable.
The specs are for setting things up so that it is the best there is/it can be.
That's why folks who do the own repairs may try and suceed at things that mechanics in bike shops won't do, as the guy in the shop has to sweat specs, warranties and customer satisfaction. Hence he stays away from "jury rigs".
The home/own mechanic tries knowing what he's willing to do/invest. If it works, he's happy. If it doesn't then he goes the spec route.
The issue boils down what you want done is determined by who is doing the work.

Sixty Fiver

Sometimes it works and sometimes it doesn't but usually, if the difference between the shifter and cassette is one the drive can work reasonably well and then, some combinations of shifters, derailleurs, and cassettes just don't play well together.

Ran my mtb with a 9 speed shifter and an 8 speed cassette for quite some time and was pretty pleased but it is flawless now that it is running 9 and 9.

The rule of thumb is that old Suntour and Shimano won't play well together either but if you run a shifter that is plus 1 you can often get nice results because Suntour spacing was a little tighter.

FastJake

I put a Shimano 7S shifter on my dad's 1983 Schwinn World Sport equipped with a Suntour RD and Suntour 6S freewheel. It works surprisingly well for a freewheel and derailer not designed for index shifting. It's not great, but he rides his bike about twice a decade so it's a non-issue..

avhed

Wrong info and wrong link.
https://www.sheldonbrown.com/cribsheet-spacing.html

70sSanO

I’ve been running 8 speed shifters when the 7 speed cassettes for years. The reason... vintage 8 speed trigger shifters are better than the old bowl type 7 speed shifters.

I have never had a less than perfect shift. The reason... I split the cassette apart and sanded the plastic spacers to give me the correct center-to-center distance. It helps to have Harbor Freight digital calipers.

I’ve run 7 at 8, 9 at 8, added a 10 speed 40t to a 7 spaced at 8. The only caveat is individual cogs and not spidered/carrier setup up.

John

Edit Added: DOH! 20 year old thread! I need to look closer at the dates. Of course, secretly I'm hoping the OP responds and thanks me becauase his shifting has been off for a couple of decades. Or, what's a 10 speed 40t?

Kapusta

I have run 7 sp cassettes with 8 speed shifters on two bikes. One still in service. Works fine. I set it up so that 1-7 all match up and shifting into 8 just makes the cable go slack. But the other way works too

If the spacing is different, it is so close that it doesn’t really matter.


EDIT: Crap, another zombie thread.

Reynolds 

I have done the opposite - 7sp shifter with 8sp - 1 cassette. Worked OK for me.