Mountain Biking - shimano deore lx question

I recently got a '92 bridgestone mb-4 and noticed on the top of the rear derailleur shifter it has the option of being on either F or 515, what is the difference between the two? I have rode using both, and dont seem to notice any difference. thanks.

Sure its not "SIS"?

yea it probably is SIS, im pretty blind. I see SIS means shimano index system, but what does the F mean?

Friction. No indexing.

thanks, im not very knowledgeable in this department, but what is the difference?

Index, when you hear the "click" the derailer should have shifted the chain to the next gear.
Friction, there is no "click". You feel your way into the next gear by moving the shift lever slowly until the chain shifts to the next gear, then trim to eliminate any over-shifting.
Prior to index shifting every bike had friction shifters.
EDIT: Late 80s and early 90s bikes had an option to return to friction from SIS because of two reasons:
1. Shimano didn't know how popular SIS would be so they left the friction option on their shifters in case SIS index shifting was not popular. I know it sounds crazy now, but back then Shimano was just coming off an early attempt at index shifting (FFS) that was a flop. They wanted a safety net.
2. Back then, there were a lot of die hard friction shifter lovers who did not like index and only rode their bikes in friction mode.

ah cool, thanks a lot for the info. is there any benefit to friction over index or vice versa? I've been using it in friction mode, i guess. I didnt really notice any clicking when i had it in index.

The benefit to friction is that you have some wiggle room on dialing in the cable tension as you can correct it yourself. With indexed shifting, the benefit is that you just click it and it shifts, but it has to be dialed in correctly or you'll get a fidgety drivetrain.

The benefit to friction is that you have some wiggle room on dialing in the cable tension as you can correct it yourself. With indexed shifting, the benefit is that you just click it and it shifts, but it has to be dialed in correctly or you'll get a fidgety drivetrain.

+1, I have a couple of vintage, friction shifting road bikes that I use as gofer bikes. They're a lot of fun when you're not trying to keep up with a pace line. The advantage of friction is you can fine tune the chain line in any gear so that any rubbing can be dialed out with the shift levers.
The advantage of index is, the shifting is fast and I can place the bike into the exact gear I want by the feel of the shifter clicking. IMHO, this is very important with mountain biking where I may suddenly down shift by two or three gears and I want to know exactly what I did. Also, when mountain biking I don't usually have time to "listen" to the chain to dial it in. I'm focused on the trail ahead and the next turn or two or three, not on what the drivetrain is doing.
While index shifting was developed for road biking, IMHO it has greater advantages for the average mountain biker compared to the road biker because the mountain biker is usually not riding a straight smooth road and has to focus on the ride path.

I find when I'm down a hill and want to go back to harder gears, that I have to click several times to get the harder gear which causes me to lose time which causes me to wonder about the cars that want to turn right behind me. I'm building a touring bike right now and I got Shimano Ultegra 8 speed bar-end shifters that I intend to use in friction mode. EDIT: mounted on Paul Thumbies :D

I find when I'm down a hill and want to go back to harder gears, that I have to click several times to get the harder gear which causes me to lose time which causes me to wonder about the cars that want to turn right behind me. I'm building a touring bike right now and I got Shimano Ultegra 8 speed bar-end shifters that I intend to use in friction mode.

Or you could shift as you go down the hill while you pedal, loosing no time and staying in a useful gear. It's not hard.