GP 9 Braking System

[GP 9 lover]

We have just run across a GP 9 made in the 50's and we found it to have only inboard brake shoes, is this an option??? Hardware shows it had both inboard and outboard shoes...is this safe?????

[daveklepper]

Break shoes differ in the amount of friction they produce, and possibly the effective performance of the single shoes you saw equal that of the double shoes that were original equipment.  If that were the case, using two of the newer shoes could result in lots of sliding wheels.   However, using double shoes greatly reduces truck wheel bearing wear, even with modern roller beearings.    This would be far less important if this GP-9 was equipped with dynamic brakes as well as air.

[rrboomer]

The GP9 (and other 1st gen EMD units) had two cast iron brakeshoes per wheel.  The cast shoe coefficiency of friction increased as speed dropped. Below 5mph wheels were prone to slide unless brake cylinder pressure reduced.

As composition shoes came into favor (higher coefficiency of friction at higher speeds and no peak at stopping) some roads reworked their EMD trucks to only one shoe per wheel.  Results were less wheel wear, less flat spots. The downside is they take longer to stop when flat switching resulting in more switches run by and more back up moves. Composition shoes also  not as effective in snow and bitter cold weather. A composition show will last longer than the two cast irons replaced.

Other roads have kept the two shoes per wheel, and switched to a lower efficiency composition shoe.  Most roads that made the two for one switch did it on second generation geeps. 

[GP 9 lover]

Thank you for your insight, it has helped me to understand the different systems a little better. I have done some more research and found the engine is actually a GP7.

I was wondering if you had the resources ( or could tell me who might be able to) to get the actually original specs of the GP7 braking system and how it would compare to this common 2 brake per truck modification. I am trying to prove that the new system is either equal to or better than the original braking system.