A bit of a witter about riding position, weight distribution and centre of pressure.

The first place to look was obviously the web. I was truly astonished at firstly, just how many GN250 café racer projects there seemed to be and, perhaps more, by how many of them retained the forward set footpegs.
“ RANT ALERT”
I’m going to have a little rant now, it’s just my opinion and is in no way intended to offend anybody or denigrate their efforts. There are some great looking machines out there.
I can’t imagine why anybody would build with clip-ons and forward pegs. Forward set pegs are ok on a big cruiser for two reasons. One is that the riders weight is relatively small compared to the bike and the other is that it’s not meant for aggressive cornering.
If you’re building anything with even the mildest sporting pretensions, one must bear in mind how to corner a bike. Namely with your weight firmly on your feet, your bum just brushing the seat and the lightest of grips on the bars.
I was told that one’s grip on the handlebars should be as if one were holding a sparrow. Just enough to stop it flying away, but not enough to crush it. I’ve had little success with that myself, but found strapping gerbils to the grips pretty effective and I think they enjoyed the ride, well, the bit before the emergency stop perhaps.
With forward pegs your weight is firmly on your backside and the only way to lift it would be to pull yourself up by the handlebars… nasty. On the upside, looking at the youtubes of some of the bikes built that way, the rider would never suffer from trapped wind!
“RANT OVER”
In defence of all those who’ve built them that way (especially in areas where frame mods are verboten) there appear to be no rear sets on the market that come anywhere near fitting the GN250. A problem to be solved…and cheaply.
Getting moving
Planning these great ventures is fine, but there comes a point where one must actually lift the backside and start doing stuff. Especially in this case. It was already half past October and the ST1100 was due to go off the road in December. I was going to be working under a plastic garden awning in the back yard, so both light and heat would be in very short supply, which pretty much rules out late night sessions or a lot of evening work.
Work would start on three fronts, design, unbuild/build and procurement. Needless to say the three would interact to a fair extent, so I’ll wander back and forth between them as I see fit.
Design, whilst never actually finishing, needs to start first, not least because that’s the last time one has a complete bike to stand back and look at until it’s finished.
Starting at the front, the forks look too long and the whole thing has a rearward lean. It’s the cruiser style of course and almost exactly the opposite of what I was aiming for.
I will sound a long, loud note of caution here.
To me (though seemingly not to a good few builders out there in web world) building a café racer is not a styling exercise. Yes, I want it to look good and to look the part, but the word “racer” didn’t get there by accident. The style was a result of the function, not visa versa.
I know that a lot of folk will correct me and say that all café racers ever did was hang around outside coffee bars looking cool, but that’s not quite true.
I was fortunate enough to grow up in the North East of England (spiritual home of all café racers, there were more Bonnevilles per square motorcyclist there than anywhere else I the world!) in the latter days of the “original” café racer era.

When I was serving my time (apprenticeship, not jail) I would come out of night school on an evening and race my mate over the North Yorks moors to Whitby, for the last pint.
Neither of us had fast bikes (mine was a BSA C15 Star) so we took to the twisty back roads to keep the challenge up.
I still remember one bend between Egton and Aisleby, where you could set it up right over on the wrong side of the road going in, tip it in early to keep the front end down, hear the revs go up as you launched over the hump in the middle and unloaded the back wheel, then (if you got the speed just right) you would land on the last six inches of tarmac on the right side of the road, the back end would tramp down with the impact and you’d shoot out never having taken the throttle off the stop.
I think I got it right twice, but I still remember it forty years later.
Anyway, I digress. The point is that it has to work well and far too many people seem to think that designing a café racer is about looks.

Once the front end is dropped a bit, at least from an aesthetic point of view, the seat will have to go (it looks like a bleedin’ settee) as will the giant indicators and rear light. The tank is the wrong shape, but for the moment I’ve seen a few people just jack up the rear end of it and it looks quite a lot better. That’ll have to do for now.

Any changes to spring rates etc. can wait until it’s been ridden. The fork alteration and the change to the handlebars and footpegs will be a big enough handling change to assess in one go.

That may sound a little surprising/wimpish to some, but the effects of even shifting the bars can be quite dramatic and while it’s difficult in a rebuild such as this to stick to the old rule of only ever changing one thing at a time, one should try to get as near as possible.

In terms of forks, for example. Shortening the forks does a few things:
• It shortens the forks (well gosh!)
• it reduces the effective steering head angle
• it usually shifts the centre of gravity (CofG) forwards and downwards
• it reducess the trail
• it shortens the wheelbase.

That’s a quite a lot of stuff for one ten minute tweak. All of those things will serve to reduce longitudinal stability. That’s why cruisers, designed for long distances in a straight line, tend to have longer, more raked forks.
Now in this case, less stability is exactly what we’re after and we can look at it in terms of a more aggressive (well, less lazy) turn in. But. it will also reduce stability in the bend and in a straight line. So the bottom line is that the bike will be a bit more nimble, but take a bit more riding. That’s good, within limits and if you know what you’re doing. Remember, it also reduces stability in situations where you’d like to keep it, like under heavy braking for example.

Handling and Weight Distribution
Now I’ve got onto handling and weight distribution I may as well stay there for a bit. I looked at one web page where the chap lowered the front and then bought longer rear springs to raise the tail. The text suggested that he did it for looks, or maybe he wanted to save fuel by going permanently downhill, but for me raising anything on a bike (unless it’s because you don’t fit on it) is bad news. I want the CofG as low as possible.

Which brings us back to the clipons and rear sets, often a much misunderstood change.
It’s easy, and intuitive to assume that fitting clip-ons is going to bring the CofG forwards. It doesn’t. What it does do (if, and only if, accompanied by rear set foot pegs) is to bring it down and to stabilise it.
Just as importantly, it will also drastically move, and stabilise, the Centre of Pressure (CofP) which brings us into aerodynamics. Back to that in a moment.
In order to understand the effect on CofG we must first completely ignore the shape of the rider. With clip-ons he looks longer and lower but… His weight can still only act on the bike in three places. His bum, his feet or his hands.
As we’ve already discussed, putting weight on the handlebars is just plain wrong. It’s going to happen under heavy braking, or when you’re asleep, but in control terms it saps all the finesse from your riding and hurts your wrists.
So, for the majority of the time, most of your weight will be on your backside with a bit on your feet. When cornering most or all of that weight should transfer to your feet.
Here’s a simple thought experiment (or you can get some bits of stick and try it for real) to illustrate why.
When we corner, we are very rarely in line with the bike. We usually lean either a little bit less (flicking it through the town traffic mode) or a little bit more (playing racers).

motosmall   152259

So, if the bike is the bottom stick (blue), and pivots on the tyre and the rider is the top stick (red), and pivots on his bum, try to control the lean angle of the bottom stick by pushing down (remember you can’t pull) on the top one. Not easy!

Sticks

Now lets fit a  pair of footpegs to the bottom stick and rearrange the rider as shown, easy! And it is both simple and instinctive to move your weight from one foot to the other.

pegs

At this point I am expecting some fans of  Keith Code’s California Superbike School (and I am one) to pop up and remind me of the piece of film showing that you can’t control a bike by shifting your weight. He is of course, quite right. You can’t really tip a sportsbike into a tight bend with your body weight and counter steering is always the right way to do it. But…  Watch the film carefully.  The demonstration shows that with fixed handlebars, the best the rider can manage is to veer over the track in one direction or the other. That’s quite a lot of veering. To put it into context, only last week I watched a wall of death rider swerve the bike from the bottom of the wall to the top, several times, aiming at different spectators each time. He did it all standing on the footpegs with his arms outstretched. Definitely no counter steering there!  In short, counter steering is the way to lean a bike but a lot of the fine adjustment and stability, is through your feet.

So, back to the weight distribution.  Generally the rear footpegs are more or less under the riders backside. It’s not hugely critical (as long as they’re not too far back) as usually, when the rider lifts his weight onto his feet for a bend, he will scootch his backside back along the seat slightly to keep his balance over the pegs.  Ideally the less the better, so that the end result is that the weight shift is purely downwards (from seat to pegs) rather than fore and aft. This causes minimum disruption to the set up of the bike.

Similarly, because the riders torso is lower and flatter, when the bike accelerates or decelerates it the fore and aft weight shift acts nearer to the bikes overall centre of gravity than it  would if he were more upright. That means it has less pitching effect, so braking and accelerating are flatter. Again, it’s all about stability.

That deals with weight, but what about the aerodynamics? The effect of aeros on a naked bike is way more than most people imagine. Especially on a little one. First, let’s establish whether even considering the aerodynamics is worth it.
It’s well documented that even at as little as sixty mph roughly half the bikes power is used in pushing it through the air. Simply turning that equation backwards says that something equivalent to roughly half the bike and riders combined weight is pushing back against it.  That’s a lot of pressure and just where it pushes on the bike has to make a big difference.

So lets say that the rider is sitting bolt upright, as on a standard GN250. That’s a sizeable, meat flavoured, sail we have there. The wind is going to hit him square in the chest and the only thing he can do to stop himself rotating is to hang on to the bars. The net effect of the pressure against him (way above the CofG)  which is transferred to the high handlebars, is going to rotate the bike backwards. That in turn is going to move the CofG back, which is going to make the bike rotate backwards even more. It’s a positive feedback loop and inherently unstable.  When the rider brakes or slows down, his weight is projected forward, the wind pressure reduces and the CofG moves forwards. Another positive loop, in the opposite direction. The result can be  something like riding a rocking horse and on the little GN, it certainly was.

With clipons two things change.  Firstly, and pretty obviously, some of the air goes straight over the rider, so there is less drag to deal with. Secondly, and more importantly, the rider is leaning forward. With no wind he’s braced against the footpegs and using a combination of his thigh, abdominal and back muscles to maintain his position.  Because his body is now at an angle much of the air pressure is trying to lift him up, rather than push him backwards (think of it like pushing against a wedge)
wedge

Which gives him a really easy way to stabilise it. As the wind speed increases he reduces the amount of muscle power being applied and dissipates the wind energy in holding him up.  If it increases even more, all he has to do is lean forward a bit more, so that more of his weight acts downwards.  As he slows down, he uses more muscle power to hold himself up, or sits more upright.  It’s an absolutely natural balancing act that uses gravity to counteract the wind energy, so that it doesn’t have to be reacted through the bike.  In short, the CofG doesn’t have to move. More stability!

It really is hard to believe that a simple change in riding position can make such a huge difference, until you ride a bike and then change it.

 

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