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Virtual pivot point technology, from a performance perspective
Editor’s note: This entry was submitted by Adrian Howard. Adrian is in the final stages of a graduate Masters in physiotherapy at the University of Queensland. He has a Masters in sports science (human performance) and has worked in the field of strength and conditioning for twelve years. Adrian currently is living in Brisbane, Australia with his wife and his interests include human performance, musculoskeletal rehabilitation, snowboarding, surfing and, of course, mountain biking.
In this age of full-suspension technology, there is an enormous amount of debate as to which suspension design works best for mountain bikers. There are plenty of reviews all over the ‘net that discuss which design is best, and why it is so. I feel there is a fundamental problem with many of these suspension arguments: very few people ever really seem to consider the rider and how they interact with the bike on the trail.
I recently purchased a Santa Cruz VP-Free for downhill duties and took it on a trip to Mt. Buller in Victoria, Australia. In this article, I want to discuss specifically how a rider interacts with a Virtual Pivot Point (VPP) bike, rather than the usual score of the bike did this well and that poorly.
The location: Mt. Buller’s International downhill course
The International downhill course in Mt. Buller is extremely technical; in fact it is one of the most technical runs I have ridden, and I’ve been to places in France, Scotland and Spain. Each place presents its own challenges but none are quite as challenging as the downhill I was about to face.
After a couple of warm-up runs on the blue track I set off down the first section of the International downhill, a very demanding two hundred metres of planted rocks — some the size of basketballs. As I bombed my way down a strange thing occurred. I felt the bike lengthening and shortening like I had never felt a bike do.
Working the VPP
After a number of runs it suddenly occurred to me that it was the design of the VPP linkage performing its duty and sucking up most of the bumps that came its way.
In what seemed like a second it also occurred to me that the length of the chain stays were constantly changing as I rode the bike. The chain stays are around 17 inches and when the suspension compresses the chain stays move backwards around 0.5-1 inch before the rear wheels begins its more vertical path over the obstacle you have mowed into. Also, I realised that as I went into a corner with my body weight forward and then moved it backwards as I exited the corner I needed to shift even further backwards in order to offset the horizontal component of the suspension and effectively weight the rear wheel. I realised that, in shifting my weight back, I was in fact increasing the effective length of the chain stays and increasing the need to move my body even further back to maintain my centre of gravity over the rear hub.
A moment of dismay occurred while I was thinking about this. If the rear wheel on a VPP-equipped bike moves backwards before it moves upwards, then the rider constantly has to adjust your body position to cater for the change in the centre of gravity over the rear axel. This is the case for all VPP bikes that have a horizontal movement component in their suspension design.
VPP systems and a dynamic base of support
If you consider a VPP bike as your ‘base of support’ while riding, you’ll see that you have a very dynamic system at play. What I mean by this is that when you change your centre of gravity on the bike you change the position of the rear axle relative to your centre of gravity which then necessitates another change in body position to offset the first change. This is an infinite process in the initial stages of the travel, as quite a lot of rearwards movement occurs before the more vertical path of the axle begins.
While riding in Buller, I found this dynamic base was a factor during cornering. As the rider weights the rear of the bike, the wheelbase lengthens as the axle moves backwards. This characteristic can be quite disconcerting, especially as one enters tight technical sections, because, as the bike lengthens, it becomes more unwieldy and less manoeuvrable due to its longer wheelbase.
This situation required subtle but deliberate changes to maintain optimal force on the rear wheel in and out of corners. Every bump I hit magnified the constant forwards and rearwards movement of the rear axle, particularly in the early stages of the travel. As the damper returned the rear axle and chain stays to their starting position it happened again and again.
Have you ever tried counting the number of times the damper moves in and out on a downhill course? If you imagine this you will get the picture.
Dissecting the dynamic base of support
In the world of physiotherapy and performance training this forward and rearward change in axle position, relative to the rider would be considered a dynamic base of support. Dynamic bases of support are the most difficult to manage because they require more finely tuned movements of the body to cater to the changes in the rider’s centre of gravity.
To put it simply, these bikes require more effort to move around. During the situations described above, the rider’s body must work harder to coordinate the fine movements that are necessary to maintain an optimal centre of gravity over a constantly changing base of support (see figure 1). Furthermore these body position changes are necessary to perform the tricks of the trade, like the “bunny hop”. We have all heard reports of people finding the Santa Cruz V10 impossible to bunny hop. The reason for this difficulty is obvious if you consider what I have said about the effective lengthening of the chain stays and the constant need for the rider to change his centre of gravity to stay over the rear axle.
Figure 1. In order to stay in an optimal riding position, the rider wants to maintain the same relative position, relative to point ‘Z’ on the diagram. As the effective length of the chainstays on a VPP-equipped bike increases, the rider (red triangle) must move between points A and B to maintain their centre of gravity relative to the horizontally displaced rear axle (points C and D) in order to stay in that ideal position.
Any rider out there will agree that, as you get tired, the little movements that are required to maintain an optimal centre of gravity relative to your bike become harder to perform. This is one of the reasons so many really bad crashes come at the end of the day; riders’ muscles become tired and they become less able to fine tune the movements required to stay on top of the bike. When you add in additional factors, such as a changing base of support, you are going to fatigue more quickly, and this can’t be good for anyone.
Figure 2. On bikes with a vertical rear wheel path, the rider (red line) does not need to move between points A and B because point B does not exist. The vertical axle path makes it far easier a rider to orientate their self above, and to maintain their centres of gravity in, an optimal position.
On shorter travel bikes I think the VPP trade-off is worth it. The energy saved during pedalling is worth the subsequent loss of energy on the downhills. For pure lift assisted riding I believe that most riders would benefit by sticking to a bike where the base of support changes very little. The typical four bar or single pivot performs this way because these bikes have a more predictable rear wheel path, something that makes positioning on these bikes less difficult to figure out.
And in the bigger scheme of things that means you can put your energy into what you are really on your bike to do… RIDE!
If you have an idea that you think would work on The Biking Hub, don’t hesitate to drop Cory a line.
1. Posted by Mr. P | 7:28 am, 20 April 2006
Interesting and new perspective on the VPP’s rear axle movement. I can’t imagine taking the bunny hop skill away on a DH run. Gret point about late day fatigue too.
One of the marketed benefits of the rear axle movement is it’s ability to handle square edged bumps at speed. The rear movement supposedly allows more time and an easier path for the wheel to roll over the bump, resulting in a smoother “hit” and more maintained momentum (and less fatigue). This is something one could feel in the body. Did you experience this?
Mr. P