Gradeability and Cruiseability

Startability and Gradeability. We all know that startability and speed are a tradeoff. But let’s look at what these terms mean, and what impacts them.

Startability simply is the ability to move the load by letting out the clutch and idling to motion. The engine torque should be sufficient to move the combination forward, no throttle. The big question is, on what slope, or grade? Most combinations of over-the-road, Bridge Formula (80K) vehicles have no problem with this in a wide variety of combinations, where common startability exceeds 35%. When things start to get questionable is with heavy loads, exceeding 180K, or with diminished traction. In these cases, the percentage for startability requirement increases, from maybe 15% to 20% or 25%. 

While 20% may not seems like a lot, the impacting factor is rear axle ratio. A much slower ratio (deeper gearing) is required to move the load. But the trade-off is top speed. So, truck enthusiasts are always trying to balance this, or they use an auxiliary transmission to provide a wider range of gearing. There comes a point where the auxiliary is an absolute must.

Gradeability and Cruiseability, look at the achievable grade in top gear. Traditionally acceptable numbers have been around 1.5% of Peak Torque (Gradeability) or 0.5% at desired cruise speed (Cruiseability). The result of missing these targets is extremely frequent shifting. Not only is this a driver dissatisfier but it is also hard on the equipment. Just like how Startability is impacted, as your ratio gets faster, these items are affected. If you do not meet the targets set forth by the transmission manufacturer you will be forced to slow down the ratio. 

If you aren't comfortable with gearing check out our YouTube video: https://www.youtube.com/watch?v=EWUOV6ElOWQ

NOTE: Speed and Grade numbers are used for relative comparison and are not a guarantee of actual performance. Application is not considered, which is why the numbers are not an exact replication. 

I have attached a larger graphic for reference.