Bicycle Mechanics - Savy bike mechanic wanted... Custom Freewheel!

This may seem silly but there is a true purpose behind this lengthy post:

I want a custom 9 speed SRAM ~FREEWHEEL~... Not a cassette or ~FREEHUB~ but a FREEWHEEL!

According to the spec's, this is do-able if you mod a SRAM 8-speed Freewheel.
All that is required is changing the spacing between the gears from 3,2mm to 2,54mm.
[I do have access to a machine shop to make this mod as required]
Here is the LINK (http://www.sheldonbrown.com/speeds.html) to this reference.

Why do I want to do this, you ask?

I have a Actionbent Tadpole Trike on the way and it uses the SRAM 9 speed cassette and everything else is married to the 9 speed spec's.

I want to put a Bionx 350W motorhub on this trike. The Bionx hub is still set up for a freewheel ( LINK (http://nycewheels.com/Installation-manual.pdf) ).
So obviously, I want to keep the same 9 speed bar-end shifter, same chain, etc... so I can simply change out the rear wheel for a motorless ride whenever I want without the extra 15lbs.

According to the numbers, the 9 speed cassette would be the same stackup~width as the 8 speed freewheel. This should be a no-brainer with the exception of getting the spacers thinned out.
[ (1,8*8=14,4) + (3,20*7=22,40) = 36,80mm ] or (5,00 * 7) + 1,8
[ (1,8*9=16,2) + (2,54*8=20,32) = 36,52mm ] or (4,34 * 8) + 1,8
The 0,28mm is only .011" different (well within tolerance range).
Being slightly narrower means this WILL WORK!

I believe the proper combo of sprockets is as follows:
11~13~15~17~20~23~26~30~34 (9 speed)
All of these are common 8 speed sprocket sizes

...And if you're wondering if this is a worthwile expedition?

Ask youself how many people might be searching for the same solution one their late model Bionx motorized conversion. Any bike shop worth their salt offering Bionx conversions will jump on the opportunity to buy such an upgrade. AND... it's made with common old stock parts.

Today, Bionx dealers are loosing out on sales.
I'm a great example: I won't downgrade to the 8 speed options due to the incompatability with the motorless quickchange option.

Please contact me if this opportunity interests you!
...Or I'll pay cash for clean parts to make this work ;)

You might want to consider the reason why you don't see very many 9-speed freewheels - or 8-speed freewheels for that matter.

The issue is the location of the axle bearings. With an 8-speed freewheel hub the outermost drive side axle bearing is almost in the middle of the hub. That means that all of your weight is trying to bend the axle and usually succeeds.

I can understand that. In this case, it is a motorhub designed to work this way. But you're right, it may not be an easy find. I can find a ton of 7 speed freewheels.

I want a custom 9 speed SRAM ~FREEWHEEL~... Not a cassette or ~FREEHUB~ but a FREEWHEEL!

According to the numbers, the 9 speed cassette would be the same stackup~width as the 8 speed freewheel. This should be a no-brainer with the exception of getting the spacers thinned out.
[ (1,8*8=14,4) + (3,20*7=22,40) = 36,80mm ] or (5,00 * 7) + 1,8
[ (1,8*9=16,2) + (2,54*8=20,32) = 36,52mm ] or (4,34 * 8) + 1,8
The 0,28mm is only .011" different (well within tolerance range).
Being slightly narrower means this WILL WORK!

I believe the proper combo of sprockets is as follows:
11~13~15~17~20~23~26~30~34 (9 speed)
All of these are common 8 speed sprocket sizes

I'm afraid this is most likely not possible. I don't have the detailed specs of Sachs/SRAM bodies here at home, but I think you're going to run into trouble due to the fact that they use two different spline patterns and two or three different threadings for different parts of the cluster.

The freewheel body is stepped so that the biggest two or three (can't recall which) sprockets fit on a big spline, then there's a step, and the next three or two sprockets fit on a smaller spline pattern, then the others screw on with built in spacers that are part of the threaded sprockets.

If it was like a cassette where all of the sprockets use the same spline pattern, it would be easy, just a matter of changing spacers, but I think the steps are going to sink you.

It's a funny thing...back when freewheels were the norm, it was common to do custom ratios for customers, and all good mechanics were adept at it. Now we use cassettes, and hardly anybody customizes them any more. Most folks assume that it is harder to customize cassettes than freewheels, but the opposite is actually true!

Sheldon "Probably Impossible" Brown

+--------------------------------------------------------+
| There is no conclusive evidence of life after death. |
| But there is no evidence of any sort against it. |
| Soon enough you will know, so why fret about it? |
| --Robert A. Heinlein |
+--------------------------------------------------------+

It's a funny thing...back when freewheels were the norm, it was common to do custom ratios for customers, and all good mechanics were adept at it. Now we use cassettes, and hardly anybody customizes them any more. Most folks assume that it is harder to customize cassettes than freewheels, but the opposite is actually true!

On the other hand, we used to only have 5 or 6 cogs so you had to make the best use of what you had. Today, with 9 or 10 cogs on a cassette, there's less need to customize. You might like to have a ratio to fit between your 12 and 13 tooth cogs, but you can't very well make a 12 1/2 tooth cog.

I'm afraid this is most likely not possible. I don't have the detailed specs of Sachs/SRAM bodies here at home, but I think you're going to run into trouble due to the fact that they use two different spline patterns and two or three different threadings for different parts of the cluster.

The freewheel body is stepped so that the biggest two or three (can't recall which) sprockets fit on a big spline, then there's a step, and the next three or two sprockets fit on a smaller spline pattern, then the others screw on with built in spacers that are part of the threaded sprockets. Yeah, I ran into this issue with converting Suntour Ultra-6 and Shimano freewheels to 7/8-speed too. What's gonna end up happening is the middle-cog that's typically flush with the end of the large-splines will be inset more. You'll then have to use a large-diameter spacer on top of it (instead of usual small-diameter spacer) to clear the remaining stubs of splines. Then lay a small-spline cog on top.

On the other hand, we used to only have 5 or 6 cogs so you had to make the best use of what you had. Today, with 9 or 10 cogs on a cassette, there's less need to customize. You might like to have a ratio to fit between your 12 and 13 tooth cogs, but you can't very well make a 12 1/2 tooth cog.

Hah! You've fallen for that scam too! To learn the truth behind this conspiracy, see:

http://sheldonbrown.com/carapace-2.html#fractional

Sheldon "Decimals, Not Fractions" Brown

+---------------------------------------------------------------+
| Anyone who can't use a slide rule is a cultural illiterate, |
| and should not be allowed to vote. --Robert A. Heinlein |
+---------------------------------------------------------------+

On the other hand, we used to only have 5 or 6 cogs so you had to make the best use of what you had. Today, with 9 or 10 cogs on a cassette, there's less need to customize. You might like to have a ratio to fit between your 12 and 13 tooth cogs, but you can't very well make a 12 1/2 tooth cog.

Hah! You've fallen for that scam too! To learn the truth behind this conspiracy, see:

http://sheldonbrown.com/carapace-2.html#fractional

Sheldon "Decimals, Not Fractions" Brown

+---------------------------------------------------------------+
| Anyone who can't use a slide rule is a cultural illiterate, |
| and should not be allowed to vote. --Robert A. Heinlein |
+---------------------------------------------------------------+