Bicycle Mechanics - Question on rear hub

I really am a newbe to all of this so could someone explain. Is the hub on the rear wheel of a 9cog, 8cog, 7cog, or 6cog the same width? Just trying to figure out how the wider cogsets are still centered in the frame. Thanks...........Sam

The overall width of hubs (known as OLD or "over locknut dimension") and the corresponding distance between the inner faces of the frame's rear dropouts varies depending on the number of cogs that were current when the bike was designed.

Old (pre-early 80's) frames, designed for 5-speed freewheels, had 120 mm OLD hubs. Later 6 and 7-speed frames/hubs had OLD's of 126 mm. Current 8,9 and 10-speed cassette hubs have OLD's of 130 mm for road bikes and 135 mm for mountain bikes.

6 and 7-speed cassettes fit on the same width freehub body with the 7-speed cogs closer together.

Shimano 8, 9 and 10-speed cassettes all fit on the same freehub body with the cogs being thinner and closer together as the number increases. For Campy, 8-speed is unique and 9 and 10-speeds fit on the same freehub.

Rims are kept centered between the lock nuts by "dishing" the wheel, that is the rim is made closer to the drive side hub flange. That is why the drive side spokes are tighter and more vertical than the non-drive side.

To make things more fun, lots of touring frames are made with 132.5mm spacing. This means that you can use both 130mm (8-10 speed road) and 135mm (mountain bike) hubs. Fun, huh?

126 mm frames, if steel, can be easily cold-set to 130mm. 120mm can usually be expanded to 130mm as well. Often, you can simply bend 126mm frames a little and wedge the hub in there without issues.

Well, as long as we're getting into obscure spacing, many early 90's frames, made during the transition from 7-speed to 8-speeds, had 128 mm dropout spacing. They could use either 126 mm OLD or 130 mm OLD hubs.

As noted, older steel frames can be respaced to widen the dropouts. 120 mm to 126 mm is fairly easy and 126 mm to 130 mm is too. Going from 120 to 130 is a lot less certain, particularly if the frame has short chainstays. Ti frames can also be respaced but are very difficult to do. NEVER, EVER respace an Aluminum or carbon frame.