Respacing rear wheel 6sp to 8sp
 

Hey folks,

I m trying to work out what's involved in putting a 8sp freewheel where there currently sits a 6sp freewheel.

ATM there's not enough room to accommodate the 8sp. If I understand things correctly, I'd have to re-space the axel? And realign the rim? Would I need new spokes and to rebuild the entire wheel for this?

I would just replace the wheel, but it's a tandem wheel so has 40 spokes.

What's the hub/frame spacing? Given this is a tandem increasing dish may result in an unacceptable tension differential. If your bicycle frame is steel, perhaps consider also having the rear spacing respaced and the dropouts realigned. That said, you'll need 5.9mm more length on the drive side.

Dishing over approximately 3mm you can PROBABLY accomplish without new spokes--if you start seeing threads exposed on the NDS or you can't tighten the DS spokes any more then you'll probably need to respoke, at least on that side. Because you're likely increasing dish and making the spoke tension between sides on the wheel more unequal, it becomes more critical that this is done to high, even tension, requiring a good wheel builder.

How wide is the rear of the frame spread?

If you have six to ten mm between the drops and the closest part of the freewheel...... maybe. Otherwise you might have to spread the stays to give your dropouts more room. Steel? If not, I wouldn't attempt it.

Frame spacing is 135mm. A 700c wheel. And a steel frame.

Having said all this though, there's a drum brake (Arai) so i would need to make sure there's enough room on the NDS.

I think given the potential for escalating costs... I'll give this challenge a miss. 5.9mm is probably just over the edge for me.

Thanks. The Arai drum brake is gonna prevent this. thanks for the details.

The length difference is 1.4mm between a Shimano 6 speed and 7 speed freewheel. If you have indexed 8 speed shifters you're trying to use, a 7 speed freewheel might fit with acceptable clearance, and you can usually get an 8 speed shifter to index acceptably on a 7 speed freewheel as the cog spacing is 5.0mm 7 speed an 4.8mm on 8 speed, which over 7 gears gives you a maximum error of 1.4mm--you can average this out so your error is lower, and shifting is generally good.

Current Shimano tandem cassette hubs are 145 then you have room for greater width of those cogs..
6 speed, screw on freewheels?
maybe a 135 standard cassette hub & 36 spokes, 4 cross, ? & rely on the rim brakes ? which can be improved..

Arai drum brakes use a screw on thread it's RH , but being on the left & braking forces tightens it..




...

I have 8sp shifters on the 6sp freewheel at the moment - which isn't perfect but kinda works. Do you think it's possible to change the the 6 to a 7 freewheel given the included pics?

And thank you!

https://i.imgur.com/rjXtLJE.jpghttps://i.imgur.com/eKVcqhL.jpg

7 is narrower spaced 6 wider, overall width is similar... (8 is 7+1 with similar narrow spacing) and should be a simple swap , (9 is even narrower spaced)
may need 1 freewheel remover to take that off and a different one for the new one.
so better to have the bike shop do it , rather than a specialty tool you use once.

+ a new chain of course..





...;)

Ooooh... I like the sound of this... I have a spare 8 sp chain anyway ;)

While it is not difficult to re-space the frame to accommodate a wider hub, 8-sprocket freewheels are more prone to axle breakage than 6-sprocket freewheels, and on a tandem this could be a significant concern.

If it were my bike, I'd either replace the wheel with a freehub wheel to provide better axle support for an 8-sprocket cluster, or just live with the 6-sprocket wheel.

7 speed spacing is 5.0mm and 8 is 4.8mm. Cog to cog.
The more speeds your FW has, the greater the unsupported portion of the axle on the DS. Axle bending increases.

I suspect a 7 speed freewheel will fit. If not, maybe try just a 1mm spacer on the DS--being 1mm out of dish probably is imperceptible.

As I mentioned, 7 speed and 8 speed spacing is .2mm different, but being 1.4mm off in total means that if you center it over the middle of the cassette it is only .7mm off at most, which is much less than the play in the guide pulley, and in my experience works more than acceptably.

I would not go 8 speed freewheel, those had a tendency to break axels at a greater rate than the lesser cog freewheel hubs, due to the dishing required.

I sent back the only 8 speed freewheel bought. 15 years ago.. 1st PITA point_ you have to take off 2 cogs to get to the freewheel removal tool ..
{Sachs-Mailliard ARIS}

Many thanks for the replies everyone.

A few thoughts:
1) 135 is MTB standard, so no problem there, except for the drum brake threading, but 135mm tandem freehubs do exist.

2) If it's a freewheel hub, 7 speeds are the most you're gonna get. The only 8 speed freewheels I'm aware of were the Sachs ARIS. (And I think I remember hearing about a Regina, but have never seen one.) ARIS freewheels were good, but they haven't been made since the late 1990s, I think, and the 8 speed ones are basically unobtanium.

3) Shimano 8 speed shifters index just fine on 7 speed clusters. I've done this many times, using 8-speed bar-end shifters. You just have an extra click. Put it on the high side.
(Fun fact: The opposite way also works. I ran 7 speed thumbshifters on an 8 speed mountain bike, because I hate trigger shifters. Shimano thumbshifters have an overshift "below" the 1st click, and since the inner position is set by the limit screw, you just set them up so that the 1st click is on the second cog, and it just works.)

4) If you're really unlucky, you have a tandem-specific 6 speed freehub, which only very-hard-to-find 6 speed Uniglide cassettes will fit. Supposedly, you can make Hyperglide casettes work by filling out the one skinny slot. Saint Sheldon has a write-up, and it's supposed to work, but I've never had to to try it.

Bottom line, there are several good ways to remove the hide from this feline.

--Shannon