A drawing of Alco's Hi-Adhesion trucks, used on locomotive #315.
Thanks goes out to Charles Tomashek
for supplying the description of how HiAd trucks work.
The problem ALCO was trying to solve with the HiAd truck design is a common
one. We have all experienced it in our cars. When we were younger we loved to
accelerate hard and the hood would rise and the tail of our car would fall, the
weight of the car had transferred to the back wheels. For those of us young
enough to have had front wheel drive cars, we noticed the same transfer
happening. If you have had the pleasure of watching an ALCO Century switch a
long heavy cut of cars you would have noticed a massive amount of weight
transfer. The AAR type B trucks would transfer weight and swing the side of the
truck accordingly, much like an old time drag racer. The HiAd design eliminates
most of this transfer and thus the term Zero Weight Transfer truck
design. The reason ALCO wanted to eliminate weight transfer is that it causes
slipping on the unweighted axles, and thus loss of adhesion and loss of
effective tractive effort.
- You will notice the side of the truck frame dips down to Axle level in a
graceful curve. This is the most important element of the design, It allows
the pushing/pulling forces from the axles to be transmitted in the same
PLANE to the bolster. In a type B truck the forces acting on the Bolster
were above the plane of the axles and thus the twisting or Yaw in that truck
design was considerable.
- The other important design feature was the spring pack location in the
center of the truck thus equalizing the weight on both axles using the same
principle as an old hand scale.
Compare this drawing to a close-up photo of #315's
truck from 1969.