Bicycle Mechanics - Q about disc brakes

OK, I've never touched them, but after a really scary experience yesterday involving my 190 lbs (+ bike), on a 12% descent with hairpin turns, I am seriously considering building a bike with them.

I see units such as the Avid BB7 Road listed in sizes 140, 160, 185, 203mm.
My Q is: Are frame/fork mount points designed for a specific size,
or can I buy any disc-capable frame/fork and install whatever size brake I choose?

The mounting points are not size-specific, but many forks (at least in the MTB world) have design limits to the max size of rotor "allowed". In practice, I would think a 160 or 185 in front would be plenty, and a 160mm in back definitely plenty. The larger-sized discs will usually require some sort of mounting adapter to correctly position the caliper further from the axle, so check whether the specific model brake you are buying includes the adapter you need, or whether you will have to buy it separately.

i have a older aluminum frame mountain bike 99 model, can it be upgraded to disk brakes?

i have a older aluminum frame mountain bike 99 model, can it be upgraded to disk brakes?

Depends on if it has the mounting tabs or not. The fork can be replaced if it hasn't got mounting tabs, and for the rear there are various adaptors available, some which has gotten decent reviews despite all looking rather kludgy.
Reworking and welding tabs on to an existing aluminum frame is generally frowned upon due to the material properties of aluminum making it prone to weaken in the process.
It can be done, but if you have to pay someone the regular rate it'll be unreasonably expensive WRT the value of the bike. Then there are new wheels/hubs and brakes to consider too.

ok thanks for the info, i was still on the fence on whether or not i wanted to do that, but there is one thing i need to do and i dont know where to order the parts from, i have the twist shifts on mine, and one of them is cracked and i think going to break soon, i need to replace it or convert over to thumb shifts, any help is appreciated

Maybe upgrading to better pads will help.

what do you mean by upgrading pads? im new and dont know much of anything bout bikes

what do you mean by upgrading pads? im new and dont know much of anything bout bikes

http://www.biketiresdirect.com/pkshld/kool_stop_v-type_holders_with_replaceable_pads/pp.htm

ok thanks for the info, i was still on the fence on whether or not i wanted to do that, but there is one thing i need to do and i dont know where to order the parts from, i have the twist shifts on mine, and one of them is cracked and i think going to break soon, i need to replace it or convert over to thumb shifts, any help is appreciated

There are number of on-line retailers you can try, here's one.
http://www.jensonusa.com/store/sub/146-Shifters.aspx
You need to match the number of speeds to your current shifter. You also need to know the make.
If yours are Shimano, you will need Shimano compatible. If your shifters are SRAM, there are two different types, one Shimano compatible and the other is the X series that need to use matching SRAM rear deraillers.
Replacing the shifter is usually done by putting the shifter in the highest gear,detaching the cable at the derailler, pulling the cable back through the housing then pushing it out of the shifter (sometimes need to remove a cover on the shifter. Then the grip is removed and the shifter taken off the bar. Installation is the reverse, with a cable tension adjustment when it's all together.

One thing to note about disc brakes is that for road use at least, a disc brake on the front only is sufficient. My previous commuter was set up like this and my next one will be too. I ride through a hilly area and for some reason, many intersections are at the bottom of big hills. Riding in the rain especially, I'd chew through normal brake pads in less than a year and even then had to do frequent adjustments. I also did a number on my rims. The disc brake seems to be the perfect remedy.

OK, I've never touched them, but after a really scary experience yesterday involving my 190 lbs (+ bike), on a 12% descent with hairpin turns, I am seriously considering building a bike with them.

I see units such as the Avid BB7 Road listed in sizes 140, 160, 185, 203mm.
My Q is: Are frame/fork mount points designed for a specific size,
or can I buy any disc-capable frame/fork and install whatever size brake I choose?As others mentioned, upgrading your brake-pads to a better compound would help.

But let's backtrack, what kind of a "scary experience" did you have? Downhill braking is particularly treacherous due to the physics involved. The weight-shift to the front-tyre is even more dramatic than braking on level ground. This would actually give your front-tyre more grip than on flat ground, however, going over the bars has more potential. To get maximum braking on a downhill, with rim- or disc-brakes, would require practicing the technique of sliding off the back of the seat and resting your belly on the seat to prevent flying over the bars. This then lets you grab the brakes even harder for faster deceleration.

If you're trying to get any significant amount of braking out of the rear, you're doing it wrong. Learn to use the front effectively. Disc-brakes won't help you much on the rear for faster braking.