Question
 

Would it be possible to take an 89' Peugeot 6-speed cassette and replace it with a 7-speed without having to do anything but that?

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Are you sure it is a cassette and not a freewheel?

It seems that I would have a freewheel and not a cassette. Sorry for my lack of knowledge I am new to the sport, but am doing everything I can to learn.

I believe that the 1989 Peugeot model range used freewheels on all the 12 speed models. Cassettes were not employed until you got into 14 speeds with the Triathlon model.

Certainly a 7 speed freewheel will thread on in place of a six speed. Plus, it allows you to upgrade to HyperGlide's profiled and ramped teeth for improved shifting. However, a 7 speed freewheel is marginally wider. There will be less clearance between the frame and at worst there may be interference between the frame and chain. This is generally easily solved by adding an additional washer under the drive side locknut of the hub. If so, you should also re-center the axle, so equal lengths are protruding from the locknuts on both sides.

Also, you may find that you have to replace the chain. Some 6 speed chains are too wide for 7 speed spacing. Reagrdless, it is good practice to install a new chain with a new freewheel.

If it's a freewheel your chances are good. You might need to add a spacer to the axle, or it might work as is. Worth a try.

When casette hubs were first introduced in early to mid 1980's, 6-speed was still state of the art so that is what the first casette hubs had. By 1989, 7 and 8 speed was much more prevelant on high-end bikes so it is unlikely that a bike manufacturer would spec a casette hub (indicating a decent quality bike) with only 6 speed. If your 1989 bike is a 6 speed, it was probably a lower end economy model that still used a freewheel to keep cost down.
Assuming the frame is 126mm width (safe assumption for a 1989), then yes a 7 speed freewheel can directly replace the 6-speed freewheel and it will work fine friction shifting. If you have 6-speed index shifting you would at minimum need 7 speed shifters to preserve the indexing with the new FW.

126 spacing with 6 speed freewheel:
http://i985.photobucket.com/albums/a...reewheel01.jpg

126 spacing with 7speed freewheel:
http://i985.photobucket.com/albums/a...o/PA241420.jpg

It depends on the wheel. I have successfully removed 6-sp freewheels and put 7-sp on.
I did this by handing the wheel to Hal, the oldest wrench around, and asking "can you put this on?"

I repeated this success with Don, the second oldest wrench around, and it worked the same way.

That's all I know about it.

Good point about the indexing, SJX462. By 1989 it almost certainly would have been indexed. If so, either use the friction shifting option or upgrade to compatible 7 speed shifters to retain indexing.

The reason I was thinking about doing this was to be able to use brifters. From what I have read is that 7-speed brifters are the lowest speed available yet I have been unable to find any. Does anyone know where to find them?

I am not aware that 7-speed brifters exist, I think they all started as 8-speed. However, the cog spacing for 7 and 8 speed is basicly the same, the 8 speed cluster is just 1 cog wider so you can get away using 8-speed brifters on a 7-speed freewheel or cassette hub and have 1 extra unused click left in the brifter.

You should be able to fit a Shimano HG Ultra 7 speed in 126mm spaced rear dropouts. The six speed cog pitch is 5.5mm and the 7 speed pitch is 5.0mm.

check this webpage.

7 speed Shimano RSX brifters exist.... in fact I am half heartedly trying to find some

Yeah, I just actually found that out too! Although it does seem that they would be hard to find considering they are no longer in production. If you or anyone else knows where I could get my hands on some let me know!

I am also reading about them and they are meant for 3x7. Since I do not have a third ring in the front would that just become a click on the shifter?

No. They were compatible for both a double and a triple crank up front. You just need to adjust the setting (pretty similar to adjusting a SIS downtube shifter from friction to index shifting as far as I can remember). As a matter of fact they were standard equipment in several Lemond bikes (maybe even others, but that's all I know) with a double crank up front...