Rishi Posted November 16, 2012 Report Share Posted November 16, 2012 I've recently been in a "discussion" with some friends about what actually happens to the rear under acceleration. It started like this: A discussion about trail braking included this line: On shaft-drive bikes it's particularly usefull as the arse-end of shafties rise on acceleration, rather than squatting. So I replied, as per the advice in the Twist DVD: Chain drive bikes also rise on acceleration, the same as shaft drive. Well, that created a furore. Here were some of the responses: Only some do. Most don't. It depends on the angle of the chain pull Still believing that the world is flat Rishi? Stand beside a chain-drive bike.Imagine a triangle formed by three points- the top of both sprockets , and the swingarm-pivot. As you apply power, the top line (the chain) tries to shorten by pulling the top of the rear sprocket towards the top of the front sprocket. Most of this results in the wheel turning, but some will shorten that line causing the swingarm to go up, and the back of the bike to squat. All chain-drive bikes with suspension will do this. Trailbikes try to minimize it by running the chain close (it not on) the swingarm. BMW/Husky try to minimize it with their weird co-axial swingarm-pivot/counter-shaft. But it is always there. Like much of what is taught in Code's stuff, it is simplistic. And not necessarily correct. The issue of rear squat has been explained by a number of people, including Tony Foale. Chain pull is just one of the factors. The location of the COG of the bike also comes into it, as does the position of the rider and his impact on the COG. Suspension settings also come into play. Much of it has to do not only with chain pull, but also the correlation between the rear contact patch and the swingarm pivot, and how that relates to the COG. Therefore wheelbase and swingarm length also play a factor. As does rider weight. Watch a video of a bike on a dyno (any bike) and the rear will rise under acceleration. That is because it is a static test and some of the real world forces, such as weight transference and rider position are not at play. Put the same bike on the road and it will most likely squat. There is a reason Ohlins invented the Rebound Separator Valve, and it was to reduce the impact of rebound damping when driving out of a corner, or in other words, to help stop the rear squatting under power, which makes the bike run wide as the front end geometry is lengthened. They would not have invented it if it wasn't necessary. Terry Hay fitted the valve to the rear shock of my TRX when he overhauled the suspension to help limit the squat under power. If you want to argue with one of the best known suspension experts in the country, then go right ahead. If you look at bike racers, they will get their weight as far forward as possible under power when exiting a corner. This is not just to reduce the bike's want to lift the front wheel, but to try and compress the front (and stop the rear squatting) so the bike still turns well and holds its line. Using any sort of static test for this situation is meaningless. I know Code has done a video that supports his assertion, but my point is simply that rider position and suspension setup have a major influence, so the assertion that all bikes will rise at the rear under power is simply wrong. Some will, but most chain driven bikes will squat, and not necessarily simply because they are chain driven. For example, my Aprilia hardly squatted at the rear at all under power. In fact if it did, it wasn't even noticeable. It was even hard to wheelie as it had so much weight over the front. Consequently the weight transfer under power was minimised and the rear didn't really squat that much. This was all due in unequal parts to the suspension setup, my body position on the bike and the geometry of the bike. The TRX squatted considerably, but much less so after Terry Hay installed the Rebound Separator Valve. The new Ninja1000 squats more than all of them. Respringing the rear end will help it, but it is a bike that will always squat, even with a new shock, purely due to its weight distribution and the riding position which limits where I can place my weight. Truth be told, I've never ridden a chain drive bike that noticeably raised its rear end under power, and I've ridden a LOT of bikes in the last 20-plus years. Some of that will be to do with my weight, which is higher than the average rider, and the effect it has on the bikes COG (namely raising it and moving it rearwards). Suspension engineers in race teams spend a lot of time working with rebound settings to limit the bike wanting to run wide under power, and it does this because the rear end is squatting. If Code is right in all cases, then they are all obviously wrong? It is beyond my knowledge to refute or judge any of these claims. And with a wealth of mis-information on the web I won't be using Google to answer the question. Can any of the coaches or engineers comment on the claims above? Quote Link to comment Share on other sites More sharing options...
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