This is taken from discussion on the bmw-gs list, February 1996. Most of this is my text, but I have modified it based on observations from Noemi Berry and Bob Fraiser.You should also access the ST Owners Manual Supplement that describes important differences between the ST and G/S.
The ST was roughly made from the G/S by bolting on an R65 front end - forks and console. The rear shock does not have as much travel as G/S though how much is unclear. The clearance is slightly less and the wheelbase is very slightly shorter. STs have no kickstarter, claw footpegs, wide handlebars or high front fender. STs came only in red and gray. Both have the high muffler.
A note on the R65 forks. Noemi has inspected the ST forks and found numerous small differences between them and ST forks. One important difference, according to Hayne, is that the ST forks have a rebound spring, where the R65 does not. She says "The triple clamps, fork top cap and steering bearing adjuster are different too."
The R65 tank is a bolt-on, though ST tank has knee indents and double petcocks. The forks and brake on the ST are the same as the R65.
The owners manual for an ST is the G/S manual, but a supplement stating some differences is provided. The following notes are taken from that supplement, discussions, and (especially the following bullets) the MICAPEAK comparison.
Some very slight size differences (in mm) are:
ST G/S seat height 845 860 (ST is just more than 1/2 inch lower) length 2180 2230 wheelbase 1446 1465 (ST is 19 mm shorter)
clearance 179 218 curb (ST has a bigger, lower sump)
130 175 with 165kg/75kg rider
The ST main jets are 150 and the G/S is 130. The ST will run with almost anything but it does seem to do better with the larger jets, possibly because of the slightly lower compression ratio.
The ST fuel capaacity is 5.0 gal. vs. 5.1 for the G/S. The ST's oil sump is slightly bigger: 2.65 quarts vs. 2.1 for the G/S. The electricals, carburetors, transmission, and final drive are apparently the same, though the ST came with a battery with a smaller rating. (A higher final drive ratio might have been better for a street bike.) The slightly bigger oil sump might account for a few pounds.
Except for the position of the frame number, the other specifications are different only in the front end, including the instruments and front light.. For some reason, the rear brake must also be loosened on the ST when removing the rear wheel.
The ST is 403 lbs/ 183 kg DRY weight (ISO standard 6726).
The G/S is
368 lbs/ 167 kg with kickstarter,
381 lbs/ 173 kg with electric starter (Haynes Manual - Noemi)
The UNLADEN weights are 437lb/198kg for the ST and 409lb/186kg for the G/S with kickstarter. (I don't have the unladen G/S weight but suppose that it is also 41 lb more.) Then the G/S with electric starter is 422 lb. So what is the difference between the ST and the G/S with electric starter? And why should the ST with a smaller front tire be heavier? (STs have no kickstarter.)
There is 22 lb difference in dry weight but only 15 lb when tools, fuel, and lubricants are included. So maybe some of the weight difference can be accounted for by supposing a bit of "fudging": the "dry" weight of the G/S might have been a little too dry. What about the remaining 15 lb?
So the mystery of the official weights remains. It is interesting that Noemi notes that her R65's dry weight is 25 lb heavier than the G/S. Could the R65 forks on the ST be that much heavier than the forks on a G/S?