TurbineBlade

I've read that this is possible due to the close spacing of the two systems, but I've never had my 7 speed DT shifters index very well with an 8-speed cassette.

Is there a trick way to adjust them to work properly? Or are people just satisfied with having a few gears not mesh properly?

rccardr 

I find that using 6 speed shifters with a 7 speed FW works, but not so much a 7 speed shifter with an 8 speed FW. My latest project is a KHS Flight 500 frame build, and I'm using an 8 speed DT shifter (Shimano 600) with the 8 speed cassette to avoid the noisy shifting and 'missing' index at the bottom of the shift stroke. 8 speed Shimano shifters are out there if you look for them...

I_bRAD

I run an 8sp cassette with 7sp XT thumb shifters. It takes a bit of extra patience to get it just right, but in my case at least, it works quite well now and I'm very particular when it comes to things working correctly.

Make sure your limit screws, especially the high limit are set correctly.
Be sure your derailleur hanger is straight and true.

There's .2mm difference in spacing between each cog between 7 and 8 speeds, so that's a total of 1.4mm you'll have to make up. Fortunately .7mm on either end of the range is usually within the tolerance of the derailleur to work properly. I'm using an XT derailleur FWIW.

You'll have to go by trial and error to get it just right, but since the shifter pulls more cable than it should, In my case I leave the high limit screw set slightly farther out than I normally would. Basically you want it as far out as you can go without throwing the chain.

The other thing to try is to leave the cable just slightly looser than you normally would The extra slop on the first shift (from 8 to 7) will help to deal with the accumulated spacing difference over the range of the cassette.

So, hard to describe in words but hope that helped a little!

Edit- I also forgot to mention that the old Shimano thumb shifters (XT and Deore at least) have an extra click built in which obviously makes things easier.

Also also, you can always switch to friction.

Metzinger

I ran 7sp 105 DT shifters with an 8sp cassette for a couple of years. My main beef was with the largest cog. The chain didn't want to stay on it as there was no detent on the shifter for that gear. On long climbs I had to keep pushing the right lever forward. I_bRAD's set-up sounds like how I adjusted mine. I don't know if it helped, but my cassette was practically a straight-block.

TurbineBlade

Actually, your two descriptions make prefect sense to me . I appreciate it. I may try again to see if I can set it up - I kind of like index on the rear derailleur. Not 100% necessary, but I like it.

After the chain wears out it won't matter - I'll pick up a 7 speed cassette.

I_bRAD

Nice 7sp cassettes are getting harder to find! You can still get the HG70 though. I'm running a 12-32 XTR which is very nice, but it was expensive (and I get parts for wholesale even)

roadfix

I used to run an 8sp index shifter on a 7 speed cassette on my commuter.

Ex Pres 

To set it up best adjust the chain/cog/derailleur alignment in the 4th (middle) cog. That way you have the least error at both ends.

gearbasher

I've used Campy C-Record 7-speed downtube shifters with an 8-speed Campy cassette with no problems.

TurbineBlade

Ok, tried it this evening. It hasn't worked well.

Strangely, the shifters want to skip a cog (the 3rd smallest) when shifting each direction across the cassette. This makes me wonder if I installed the cassette with incorrect spacers at some point....which is possible since I basically "rigged" this one together from a couple of older 8 speeds I had laying around.

Thanks for the advice though

Back to friction mode-

Metzinger

Are either your shifter or derailleur pre-1997 Dura-Ace?
That could be the issue.

Retro Grouch 

Over the years I've fiddled with various combinations of 7 and 8-speed shifters and cassettes. I can get it to kind of work but I can't get it to work to my satisfaction for an extended period of time. My conclusion is that the guys who use such combinations are either better bike mechanics than me or they're not as picky as I am.

Al1943

Shimano 7-speed cogs are spaced 5.0 mm center to center.
Shimano 8-speed cogs are spaced 4.8 mm center to center.

That's the problem. The 0.2 mm error is cumulative as you shift across the cassette.

Al

cs1

Most of us agree that it will probably work but not well. If you're using 7 sp shifters you could pull the 8 sp cassette apart and use 7 sp spacers to give you the 4.8mm spacing. That should give you perfect shifting. Or use a 4.5mm spacer and a 7 sp cassette. Good luck

I_bRAD

As was stated in post #3

I_bRAD

I too am very particular, and it took me some time to figure out how to set it up just so, so that it worked perfectly. Once it's set up it works exactly as it should though! This may also be one of those situations where it's dependent on a suitable combination of components.

collcs

Take the 8 spd cassette apart. Discard the 3.0mm plastic cog spacers. Purchase 3.15mm 7 spd spacers (available on line) and replace the discarded spacers. This results in a 4.95mm cog spacing instead of 4.8mm....closer to the 5.0mm 7 speed spacing. The total stack height will now be slightly more than 1mm thicker, but you should still be able to engage the threads of the hub with the lockring. Alternately, you can discard one 8 spd cog and replace with a 1mm spacer (keep the cog spacer with it). Optionally, if you are using on an 8-10 spd cassette hub (this requires a 2mm spacer using with an 8 spd cassette), you can replace that spacer with a 1mm spacer and keep all 8 cogs. This gives you the option of setting your limit screws for 2 to 8 or 1 to 7 cog usage.Suggest you set the high detent on 7 on the shifter since the cable pulls for lower gears. The shifter detent for 1 and the RD limit will stop together to prevent unintentional cable stretching from over shifting.

Reynolds 

I use a 8sp cassette minus 1 cog on a 7sp hub with a 7sp shifter, works OK.

Miele Man

And yet another Zombie thread gets reborn by a Newbie. This one languished for NINE YEARS before being resurrected. LOL

Cheers

verktyg 

I did get an answer for my question though....

verktyg 50:

Kimmo 

Surprised the correct answer wasn't provided: rotate the pinch bolt washer so the cable goes around the tab, changing the RD geometry to require more cable pull.