First of all, Tractive Effort (TE) has nothing to do with weight on drivers. TE is a function of boiler pressure, cylinder dimensions and driver diameter.
The factor of adhesion, which determines how well that tractive effort can be utilized is a function of weight on drivers. Calculating the factor of adhesion for a Garratt can be a challenge, as TE is expressed in pounds but weight on drivers is in tons, so the question is, are they imperial (long) tons or 2,000lb (short) tons? Also, is the weight on drivers empty or in working order with full boiler and half full tender bunker and tank?
The boiler pressure constant for calculating TE is often 75% in the UK and the rest of the world but is usually 85% in the US. Some internet figures for Garratt's appear to have been stated at 85% MBP.
Using the Wikipedia figures for the East African 59 class, a modern and powerful Garratt, the factor of adhesion seems to work out to be 4.22. Generally 4.0 or higher is desirable. The Wikipedia article appears to have converted the TE figure to 85% MBP. No way to know if the weight on drivers is empty or working order.
Using the erecting card reproduced in Holland's "Steam Locomotives of the South African Railways" for the SAR GMAM 4-8-2 + 2-8-4 Garratt, which uses 75% as the TE constant, the factor of adhesion works out to 4.15, by subtracting the axle load of the pilot and trailing axles from the listed locomotive weight, assuming the weight on drivers is expressed as Imperial tons. Again, there is no way to know if the locomotive weight is empty or working order.
Having photographed and ridden on Garratts in Zimbabwe and South Africa, they don't appear to have been particularly slippery. The GMAM class in South Africa used a water bottle, leaving the rather small forward water tank full as ballast, so the factor of adhesion would only drop as the 14 tons of coal carried in the bunker were used.
Michael Allen
Edited 1 time(s). Last edit at 04/26/2015 06:24PM by trainrider47.