Converting 7 Speed to 8 speed

[HobbesOnTour]

Hello people with mechanical minds.
Here's my dilemma;
I'm running a 7 speed cassette (with a spacer) on a 8/9 speed Deore Hub.
For love nor money I can't get a 7 speed cassette (I'm in México).

This is an old Trek MTB with indexed trigger shifters on a flat bar.

I've seen this thread: https://www.cyclechat.net/threads/i...7-speed-shifter-with-8-speed-cassette.262895/

If I'm understanding correctly I can add a spacer and remove one cog. Job done. Is that correct?

Can I put the full 8 speed cassete on (no spacer) and still use my 7 speed shifter? I've read elsewhere that the differences are so small for 7/8 speed indexes that there is no practical issue.

If I wanted to make my gear changes unindexed can I modify my shifter or what kind of a shifter do I need?

Depending on other developments this may all be moot, but if it's not, this bit of fettling will be the basis for some serious climbing on some dodgy surfaces with a decent load. Reliability and strength will be key. Ideally, I'd like to keep the flexibility of being able to swap 7/8 speed into the future.

Many thanks!

 

[Ajax Bay]

I have just (yesterday) set up a 7sp trigger shifter with an 8sp cassette, with reasonable success. I have 'lost' the smallest sprocket (no big loss). The cassette is on an 8/9sp freehub (obv). The centre to centre spacing is only 0.2mm different 7sp > 8sp. I tried to get it dead on in the middle of the cassette (4, with 8 'lost'). Leave the cassette as is: no spacers required.
Don't have the guide pulley wheel too close to the sprockets (use of 'b' screw). Keep the spacer you're using now so that you can fit a 7sp cassette on, when you find one.
Don't think you'll be able to convert a flatbar indexing trigger shifter to friction.
HTH

Brand and model	Center-to-center Spacing	Sprocket Thickness	Spacer Thickness	Total Width

Shimano HG 7-speed	5.0 mm	1.85 mm	3.15 mm	31.9 mm

Shimano 8-speed	4.8 mm	1.8 mm	3.0 mm	35.4 mm

 

[fossyant]

Yes all OK, and it works. I ran my MTB and road bike with 7 speed using the shiny spare Dura Ace 3.0mm spacers from old 8 speed cassettes.

I just removed a sprocket I didn't want and fitted 3 x 3mm spacers on first, so used 7 of the 8 sprockets, plus 1 extra spacer.

Lowest and top gears indexing is sorted by limit screws, but it's only going to be 0.5 of a mm out at worse if you index perfectly on the middle sprockets, and add in a bit of wear and tear on equipment, that's not noticeable. Very different case if 12 speed as that's picky even brand new.

 

[HobbesOnTour]

Thanks, @Ajax Bay
Fortuitous timing!
I have no problem "losing" the small gear, my real concern is being able to use the big one.

That looks like a reasonable, practical and flexible solution.
I'd be interested in a long term review if you can keep it in mind. Forewarned is forearmed and all that.

Thanks!

 

[HobbesOnTour]

Thanks @fossyant that's my understanding from the other thread.

Now, I don't want to start WW III but I wonder which is the better option:
Use full 8 speed cassette (and not use smallest sprocket) or
Use spacers and 7 of the 8 sprockets?

The latter I have (some) choice as to what sprocket to lose, but to my mind the former seems stronger.

Many thanks

 

[fossyant]

Lose the extra sprocket - 1 more to clean, and is less weight.

 

[Milkfloat]

To keep things simple, why not replace the shifter too - should be very low cost in Mexico - after all you do (or will) spend a lot of time on the bike, a few pesos extra could be worth it.

 

[HobbesOnTour]

Lose the extra sprocket - 1 more to clean, and is less weight.

This is the bike - a sprocket is neither here nor there!

 

[HobbesOnTour]

To keep things simple, why not replace the shifter too - should be very low cost in Mexico - after all you do (or will) spend a lot of time on the bike, a few pesos extra could be worth it.

That is a possibility. However, I'd like to keep as much flexibility as possible. Swapping to an 8 speed shifter will have an impact on a 7 speed cassette, no?

A non-indexed shifter is possible, I presume, but I know nothing about what that looks like on my setup.

 

[fossyant]

Good luck finding a cheap 8 speed shifter to match. I've left the old MTB, similar vintage to yours, with the 7 speed LX STI's.

Drop the sprocket. Also easier to clean the biggest sprocket with a little more spoke gap.

 

[HobbesOnTour]

Drop the sprocket. Also easier to clean the biggest sprocket with a little more spoke gap.

That's a very good point! Thanks!

 

[Ajax Bay]

You won't be able to drop the 12t (assumed) - which you said was not an issue to 'lose', so you'll end up with a big step down to the 12t (?from the 16t), and effectively it'll be a 6sp drivetrain. Faffing around with an extra 6mm of spacers with the suggested (scrabbling) advantage of 'easier to clean the largest sprocket' and 'saves weight' (of a 14t) seems sub-optimal. Like the OP cleans his largest sprocket much?

 

[mickle]

What shifter are you using @HobbesOnTour ?

 

[HobbesOnTour]

You won't be able to drop the 12t (assumed) - which you said was not an issue to 'lose', so you'll end up with a big step down to the 12t (?from the 16t), and effectively it'll be a 6sp drivetrain.

That's a very helpful point.
I'm currently a bit incapacitated and trying to work this out in my head - and my head is not mechanically minded.
I hadn't thought that I'd need to keep the smallest sprocket.

 

[fossyant]

Have you got any spare 13T - I don't bother with the 12's on 7/8 speeds ? What's your range of teeth you plan to use ?