This detail shot of the Empulse RR that 2012 TTXGP World Champion Steve Atlas will ride this year shows the front sprocket behind the swingarm pivot, a reverse orientation to standard practice. This detail shot of the Empulse RR that 2012 TTXGP World Champion Steve Atlas will ride this year shows the front sprocket behind the swingarm pivot, a reverse orientation to standard practice. Photo courtesy Brammo

Brammo Empulse RR: Unconventional Design

Written by  on Thursday, 04 April 2013 16:38

In the current issue of Inside Motorcycles, my article covers some of the electric racing motorcycles and how improving battery technology is leading to some very different design paths from what we consider conventional.

One electric bike I find particularly interesting from a chassis design point of view is the Brammo Empulse RR. On that machine, the motor is located under the swingarm rather than in what we consider the traditional engine bay, and this turns out to be a favourable arrangement when anti-squat and chain pull is considered.

In a previous blog, I discussed https://www.insidemotorcycles.com/blogs/item/822-gearing-and-anti-squat.html">gearing and anti-squat characteristics. On any motorcycle, the relationship of the top chain run and the swingarm can have a drastic effect on handling. When the motorcycle accelerates, weight transfers to the rear wheel and acts to compress the rear suspension - this is commonly referred to as squat. At the same time, however, the driving force at the rear wheel acts to extend the rear suspension (due to the swingarm angle) and the chain pull is also exerting its own forces.

By juggling gearing and the swingarm angle - either through an adjustable pivot or by changing ride height - a precise amount of anti-squat can be generated to offset the weight transfer from acceleration and the motorcycle will stay nicely level and composed exiting a corner.

There are several books that go into more detail on squat and anti-squat, and there are different methods of calculating a representative number to use. Most refer to anti-squat as a percentage of squat; for example, a particular layout may give an anti-squat value of 100 percent, indicating that the squat from weight transfer is perfectly offset by the anti-squat forces. Another layout may give 80 percent anti-squat, indicating that not all the squat tendency is offset.

Of course, that is not the full story and finding the perfect amount of anti-squat is not an easy matter. As the suspension compresses through its travel, the swingarm angle and the relationship between the chain run and swingarm pivot changes; on a chain-drive motorcycle with a conventional layout, the end result is that anti-squat decreases as the suspension sinks in its travel. As the rear end squats more and more, there is less anti-squat to offset it, and that nice balance of forces is soon overcome.

Many motorcycles are designed with 100 percent anti-squat at approximately 30mm into the suspension's travel - roughly equivalent to rider sag - with the sacrifice being more anti-squat at the top of the travel and less at the bottom.

Our intuition says that all would be well if the swingarm pivot and countershaft sprocket were aligned. Chain tension would never change over the suspension's travel, and the chain run would remain constant relative to the swingarm. But in actual fact, once the changing swingarm angle is factored in, the optimum location for the countershaft is a few centimetres behind the swingarm pivot. With this layout - the same layout seen on the Empulse RR - anti-squat can be almost perfectly constant over the suspension's travel, leading to improved suspension action and handling.

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