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dinsdag 12 juni 2012

Mango racing tweaks, practical for everyday?

It' s not exactly the first time I'm fiddling with my Mango to make it a bit more aerodynamic and stable in the corners so I can be faster in a race. It gets much more interesting when such tweaks can be permanent. If not for going faster than it will make for a safer and less exhausting ride. For that to happen the impact to practical features like maneuverability and serviceability should be very little impacted. That's why I don't do much effort to find ways to extend the tail like for instance the Germans Ludwig and Holger have done in the past for races.  While it is good for aerodynamics and not very complicated to make, it makes the Mango less compact and therefore less practical for everyday.

Aerodynamic tail added to a Mango+ (foto: Theo Zweers)
I (stop! Who: "I".  In fact it was the infamous David Hembrow and Roelf W who initiated this) have tried to use a less inconspicuous way to improve the tail's air resistance with little strips. See: http://youtu.be/OIIGPLlFsHM It looked promising in theory. But then I rode with it and made some comparative rides and concluded that in reality it didn't make significant difference. It seems that the amount of vacuum behind the Mango did not change much.

Anyway, a long tail DOES make sense for races and I might try it myself one day, it's just not handy for everyday.

But like I wrote before there is more to speed than aerodynamics because racetracks and roads alike can have nasty corners that can have a major impact on the overall speed. What usually happens in corners is that, depending on vehicle stability and agility of the rider, one either has to stop pedaling or (Sturmey Archer forbid) even brake. Either way one has to get up to speed after the corner and then the weight of the vehicle works against the rider. My Mango is probably one of the lightest around at 26,5kg but it still is hard work, especially in a long race resulting in many a corner to overcome. Now there are some tricks to get fast through a corner but I don't want you to get too crazy ideas and make loopings in the air (like I have), so I'll discuss the vehicle itself for now.

At some point I got the idea that stability and aerodynamics could be improved in one go. In earlier attempts I had put strips near the front wheel arches like for instance in the photo below, where I put a white strip above the wheel. Other strips were taped on the underside of the body to prevent as much as possible that air gets into the wheelarch.

aero strip taped in the wheel arch

This made not nearly as much aero difference as "da Hood", but to me there was no question after several training rides and races that such strips for the front wheels made me a bit faster. It was almost practical too, but it could get into contact with the tire when the suspension strut of the outer wheel got compressed in a corner, the tire could hit the strip and when the tire wears through that can cause a flat or even a blow out. I had two flats that way in last year's Cycle Vision three hour race. That was a bit depressing, since I was doing rather well. Some people race "all or nothing" and I guess that's the attitude one needs to win now and then, but I prefer to make it to the finish line without troubles. That's how I got champion of the NVHPV summer competition of 2007: I won only one race that year, but I was present at most of the races of the competition, always finished and got me some points.

So how to get the tire away from the strip? The answer lay in the suspension strut. Hold on for the longer explanation: As I was thinking of a way to get more stability it made sense to try and make the track greater, so more distance between the contact patches of the front wheels. Simply extending the tie rods was not going to cut it, because the wheels would get outside the wheel arch and into the airstream. You can say goodbye to aerodynamics when you do that, but... when you put the top of the suspension struts inward (closer to eachother) the wheel will go inside the wheel arch again. In effect this means more negative camber. Now somewhere in the back of my head I knew that too much negative camber means more rolling resistance. So I did some roll-out tests at my regular test-bridge and was relieved to see that I was still rolling about the same distance as before.

I had to drill two holes, 9mm next to the old ones. This does make fitting bigger tires inconvenient, since they will scrape the inside of the wheel arch when cornering sharply. Oh well, I was really getting into the fast Durano 28-406 tires anyway....

to the right of the nut, light coming through the old hole
The result of this can be seen below: left is my modded Mango Sport and right is a standard Mango Sport. My wheels are clearly at a greater angle to the vertical and the track is widened by 2,5cm
Hmmm, but wait a minute, looking more closely at the foto, isn't the white Mango lower than the other?

white Mango: more negative camber

The plot thickens now, because I did something else to improve stability: lower the centre of gravity. I lowered the Mango by about 2cm by shortening all the springs in the suspension struts.

white/grey Mango is 2 cm lower

To make things even more complicated and to prove that I am in no way doing a scientific research here, I also gave the wheels a bit more positive caster by shortening the tie rods between the front of the wheel arch and suspension strut. I already found in an earlier temporary modification that this makes the steering a bit heavier and thus less sensitive to undeliberate or unconscious steering input and other influences. In the flat Netherlands I do not feel much diference from more positive caster, but it might be very good in a fast downhill. For an online explanation of camber and caster: http://www.yospeed.com/100304-wheel-alignment-explained-camber-caster-toe.php

So what have I achieved now? Well amongst others the wheel at the top are more inward in the wheel arch and this means that when I put an aerostrip over it, it will not touch the tire easily. I can even make the strip bigger.



This should give me a greater aero advantage than the previous, smaller strips. I can steer sharply (although turning radius has increased a bit) and even under load the tire dos not touch the strip. Ok, so the stiffer shorter suspension also helps there, granted. 

So how practical is my machine now, is it worth it? The future will tell, but one thing is already clear: It won't be fun to get on a sandy trail, since the narrow high pressure tires will sink easily into the sand. The smoother the road the better the comfort is with these tires. Then again, when the changes are a bit less drastic I am sure that 35-406 Kojaks will also fit and that is a pretty standard velomobile tire.
Ohw, I almost forget: stability did indeed get significantly better. All the corners on my commute can be dealt with faster than before and it is easy to control the Mango. I am even thinking of picking up an old habit to try and beat my fastest commuting time. If only the other commuters and schoolkids wouldn't look so angry in my mirror when I swoosh by them..... I meet them everyday, so I'd better stay friends.

Can something be done to get the full range of up to 42-406 tires for the Mango Sport back? Yes there is, but it is not simple, because it means changing the molds of the Mango. These molds are very capital intensive, since it took very long to make them. Changing the wheel arches of the Mango molds to allow for more negative camber will mean a big investment.... For now I will keep on making small adjustments to the wheel alignment to make the most of what we have now.

8 opmerkingen:

  1. Regarding the tails, you should check out what a german enginner found in 1930. The result is to be seen in aboutt every modern car. it's called the Kamm-effect.

    Wikipedia : While the realities of fluid dynamics dictate that a teardrop shape is the ideal aerodynamic form, Kamm found that by cutting off / flattening the streamlined end of the tear at an intermediate point, and bringing that edge down towards the ground, he could gain most of the benefit of the teardrop shape without incurring such a large material, structural, and size problem. The airflow, once given the suggestion of the beginning of a turbulence-eliminating streamlined teardrop tail, tended to flow in an approximation of that manner regardless of the fact that the entire tail wasn't there. This is called the Kamm effect

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  2. And remarkable: also a blog yesterday about the Kamm-effect:http://s098.blogspot.nl/2012/06/een-reden-waarom-kamm-effect.html (sorry, in Dutch).

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  3. No coincidence Theo. Strada98=S098
    Small effects for cars can still be important enough for velomobiles. Maybe not for the everyday commuter, but for the racers it's worth to explore. It's difficult though to find what really works because the differences are small and it certainly cannot substitute training.

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  4. Oh well, I didn't expect that an English writing person could be the same as the Dutch blogger. (And what am I doing here? ;-) )

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  5. I just came to your post and reading above thing it is very impressive me and it is very nice blog. Thanks a lot for sharing this.
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  6. Thank you for this post. Its very informative.... Thanks for sharing.
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