The Jump Stop clamp is sized to match the outside diameter of your seat tube or derailleur post.  It comes in three standard sizes.

Small fits 1-1/8 inch or 29mm (28.6 nominal bare tube size, before added finish thickness) and these are rarely anything but steel frames.  (Some earlier US bikes and recumbents used 1 inch tubing.  I have shim material to adapt the small clamp to fit this size.)

Medium fits 1-1/4 inch, or 32mm (31.8 nominal bare tube size) and these can be any frame material.

Large fits 1-3/8 inch, or 35mm  (34.9 nominal bare tube size) and these can be any material, though rarely steel.

If the seat tube has a uniform diameter, then the Jump Stop clamp size should be the same as the front derailleur clamp size.  If there is no size stamped on the front derailleur, or there is no front derailleur, the easiest way to determine seat tube diameter is to cut a strip of paper 4 inches (100 mm) in length, and wrap it around the seat tube. If the ends overlap by more than a quarter inch (7mm), it's probably 1-1/8 (29mm). If the ends just barely meet, it's probably 1-1/4 (32mm). If the ends have a gap between them greater than a quarter inch (7mm), it's probably 1-3/8 (35mm). If you have questions about how difficult it is to mount the Jump Stop, the instruction sheet is reproduced here.

The Jump Stop is available through some independent bike shops (primary US distributor is QBP) and is also carried by some mail order outlets.  But if you would like to order direct, you can e-mail me at ngear@gvtc.com or call (800) 659-4607 (toll free) or (830) 755-4607 (local or toll).  I will need to know what size you need and the destination postal address.  I don't require pre-payment, so if you would like to confirm that the Jump Stop is the right guide for your bike, you are welcome to try it out first to make sure it fits and does what you want. Then you can decide whether or not to keep it.  (If not, just mail the parts back.)

If you decide to buy, I accept cash, check, money order, and Paypal (to ngear@gvtc.com).  The following charges would apply.

1 unit           $12
2 units          $22
3 units          $31
4 units          $39
5th unit and beyond    an additional $7 ea.

Shipping:
North America and Hawaii    No Charge
Overseas, 1 unit      $ 2
Additional units      $ 0.80 ea.   

Paypal        $0.40 per unit

If you have any questions, feel free to e-mail or call.  (If you call my 800 number, it would help me to contain costs if I could call you back.)  Thank you for taking the time to read about the Jump Stop.