Bicycle Mechanics - swap 7spd with 8spd cassette?

I have a 1990 Trek 1420. I recently replaced the speed downtube shifters with 8 speed sti levers. I found a balance to where everything works pretty well, but not flawlessly. I figured that was just the comprimise I would make for the convienience of STI levers.

However, I got to thinking. Can I fit an 8speed HG cassette in place of my 7speed HG cassette? Is the overall size of the cassette the same or am I not going to have enough room for an 8 speed? My frame has 130mm rear spacing. I am assuiming that the worst case scenario would involve redishing my wheel to create some room?

You probably have a 7 speed specific freehub - if there is a 4.5mm spacer before the rest of your cassette you have a 8/9/10 compatible shimano freehub. In which case you can just slap on a 8 sp cassette.

I doubt it's the latter though. Modern 8/9/10 shimano hubs are 130mm so you shouldn't have aproblem either way. You maybe able to also just swap the freehub out on your 7s wheel for the 8/9/10 freehubs.

Ahh.. I see what you are saying. I am thinking in old terms of a freewheel. Are 8 speed freehub bodies difficult to find? That probably puts the brakes on doing this for the mean time.

While I am on the subject. I was looking at Sheldon's alternate cable routing for using 7spd cassettes with 8spd shifters. What difference does that really make? I can't see how that would affect the cable pull.

8 speed freehub bodies are the norm now, as 9 and 10 speed cassettes were engineered to fit 8 speed freehubs.

You can also buy a 9 speed cassette and remove one of the gears. This spacing will fit on a 7 speed hub and give you 8 effective gears. The only problem is that then your chain, derailleurs and crank need to be 9 speed compatable.

You can also buy a 9 speed cassette and remove one of the gears. This spacing will fit on a 7 speed hub and give you 8 effective gears. The only problem is that then your chain, derailleurs and crank need to be 9 speed compatable.

This approach will work only if you use 9 speed shifters.

8 speed freehub bodies are the norm now, as 9 and 10 speed cassettes were engineered to fit 8 speed freehubs.

Great, so they aren't tough to find. I'll keep that in my back pocket for now, maybe eventually I will swap freehub bodies... or just see if I come across reasonably priced 7 speed shifters.

Like I said, the 8 speed shifters work... just not perfectly.

While I am on the subject. I was looking at Sheldon's alternate cable routing for using 7spd cassettes with 8spd shifters. What difference does that really make? I can't see how that would affect the cable pull.

Cable pull is only half the story. Every derailleur has as actuation ratio (throw ratio) determined by the geometry of the derailleur. Fortunately most Shimano rear derailleurs are the same, 1.7. 8-speed cable pull is different from 7-speed cable pull, changing the actuation ratio with the alternate routing helps offset the difference in cable pull. Here is a good reference chart: http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3946#Shimano

Al

I upgraded a '92 Trek 1420 from 7-speed downtube shifting to 8-speed STI's several years ago.

As noted, almost any 8/9/10-speed Shimano freehub body (except maybe the newest 10-speed) can be substituted for the 7-speed on your current hub. My frame was spaced 128 mm so it would easily accept both the OEM 126 mm hub or the revised now 130 mm hub after the switch.

I had some left-over 8-speed 105 brifters in my parts box and a bunch of downtube cable stop sets so the whole thing only cost me the price of a freehub body and a new cassette.

Hi,

I have a somewhat similar situation. I have a 7 speed Hyperglide freehub with 130 mm dropouts using 8 speed Sora brifters on two of my bikes. I bought the 8 speed brifter version thinking I might want to convert in the future and read somewhere in the forum that the difference between 7 and 8 speed cassette sprocket spacing and there by brifter pull per click were close enough to work OK which turned out to be the case for me. So I do not want to use 8 out of the 9 speed cassette.

Reading between the lines in your thread am I correct to think I can convert to an 8 speed cassette
by performing a 7 to 8 speed body transplant on my existing hub shell? I also would like to decrease my lowest 13 tooth sprocket to 11 tooth, so will I need a special lockring plus 8 speed spacers between each of the sprockets? Third, where can I get an 8 speed body? from Harris Cyclery or any decest LBS?

I apologize for all the questions, but I am a relatively newbie 50 plus.

Cable pull is only half the story. Every derailleur has as actuation ratio (throw ratio) determined by the geometry of the derailleur. Fortunately most Shimano rear derailleurs are the same, 1.7. 8-speed cable pull is different from 7-speed cable pull, changing the actuation ratio with the alternate routing helps offset the difference in cable pull. Here is a good reference chart: http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3946#Shimano

Al

That is pretty interesting. Incidentally I am using an old Shimano Deore DX derailleur, it does not have the tab on the cable anchor bolt but I think I have others that I can use.