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Riblet used only four sheaves per tower to allow for steeper tower angles. Tony Sowder (Doug's father) had a design philosophy wherein the sheave angle relative to the tower tube was as close to 90 degrees as possible, so he didn't want a great deal of weight up on the cap. As Riblet progressed to building triples and then quads, it became necessary to build towers with eight sheaves due to increased loads. (I was told by Doug that it would be not impossible, but difficult to build a six-sheave Riblet assembly.) If you ride Riblets a lot, you'll notice that the only eight-sheave assemblies are the newer rectangle-box ones rather than the older squashed-tube or the really old triangular box. A clarification- I refer to the number of support or depression sheaves on a tower, not the total including combo sheaves. There are some Riblet assemblies with a total of twelve sheaves, but that includes four combo sheaves. Tower 1 on Baker's C-8 has a combo with 16 depression and 8 support- the most I've seen on any lift.

- John Holm

To add to John's post, I belive Riblet got hit pretty hard on a code addition in the early 80s. "One sheave per degree of angle change". Riblet did have a large artistic design license with this issue.

Four sheave design per tower was self governing. He needed bigger components to allow for larger angles. Design did evolve later to include larger angles and loads.

In each area there are given limits and recommendations for corrections if the limit is exceeded. Haul rope actual safety factor in ski lifts is quite high compared to other wire rope uses. To my knowledge no haul rope has broken while in normal use at a ski area.

In wire rope industry (which ski industry is a part of) there is a correlation between sheave diameter and suggested degree of maximum angle change.

Load force is perpendicular to the haulrope.

Riblets "lighter design" dictates tower must be perpendicular to haul rope. This also includes sheave assemblies and cross arms.

"Beefier" designs have more latitude with placement.

It should be noted that both types of design are well above any structural failure. Also both types have to resist deflection as the first criteria in their design. The only way Riblet can do this by angled tower placement.