Ok guys, I have been to everyone’s web page to read the numbers and read many of your postings. Someone give me the gouge, disc vs trispoke. Main race is IMF (flat) Seems discs win with head on drag head, but trispokes aren’t far behind and start to exceed discs with winds of 30 degrees. Rode Nimble crosswinds on my last bike and loved them but had to sell them with the bike. Considering another pair vs zipp 909’s. Opinions?
I like how you phrased your question: You asked for opinions. Thank you.
This is my opinion: Nothing is faster than a disk.
I made a big, 4-6 minute mistake by not having a disk at IMNZ. I did it because of the road surface. Stupid error. I should have run the disk. I would have been a little faster. I could tell. The guys with the disks, Bjorn, Cameron, etc.- just motored. 99.999% of that is because those guys do just motor. .001% is becasue they used a dsik (speaking symbolically and metaphorically of course).
There is this malignant, on-going diatribe that goes “what about cross winds?!?!”
What about them?
The only reason a disk would ever be slower in a cross wind is when the bike is buffeted by the gusty wind (it is) and it becomes momentarily unstable (it does) the rider expereinces anxiety and backs of the pedals. Speed is your friend in this case. It is like some airplanes- just add power and it sorts itself out. The only reason the disk slows down is becasue YOU slow down. If you keep the power on that “buffeting” sensation my be speediing you up as the disk is pushed forward by the crosswind. That is not the case with all yaw angles, but it is with some.
My understanding of this is admittedly anecdotal and rudimentary. We need a real expert on this like John Cobb or Steve Hed. However, I might predict their empiracle, scientific research may largely support my hearsay.
There is a reason why every significant time trial in the Tour de France except uphill time trials is contested with a disk rear, regardless of the weather. It’s because it is the fastest wheel IMHO.
I’ve always read and and have always been told…use the disc!!! The only time you won’t use the disc is when the wind is SO bad that you fear being blown off the road! The disc is the fastest - RUN IT!!!
“Speed is your friend in this case. It is like some airplanes- just add power and it sorts itself out.”
That’s always been my attitude as well. When in doubt, gun it.
Exactly. The projectile almost always sustains less damage than the target.
There’s a good thread about this on Cobb’s forum. Most wheels rarely see the high yaw angles so I wouldn’t worry too much about those numbers. Run a disk in the back and a deep rim up front and you will be set.
Front tri-spokes are fast, too, but paradoxically they see their greatest benefit when it is scariest to use them - windy days.
As all sailors know, “apparent” wind direction is not the same as “actual” wind direction.
If you were standing still, and the wind was hitting you at a right angle, it would be an “actual” wind of 90 degrees. But, if you are moving forward, the wind would produce effects that ACT AS IF it is coming from somewhere up front of 90 degrees, depending upon your speed and the speed of the wind.
When standing still, if the actual wind speed is 25 mph, then you start riding and also reach 25 mph, the “apparent” wind would feel to be coming at a 45 degree angle from the front. If you sped up to 30 mph (you’d be a stud, for sure), the “apparent” wind would be coming from less than a 45 degree angle.
Similarly, if the wind were 10 mph and you were going 20 mph, the “apparent” wind would feel to be coming from a 22.5 degree angle from the front.
So, it is correct to say that the faster you go, the less yaw effect a cross-wind has on you and your bike.
In other words, if you can handle your bike in the wind that is present for that day, ride the disc. Tri-spoke front, BTW.
Good explanation. It’s basically a vector analysis exercise.
Bravo! glad someone remember some vectors. Of course, the next question is: does the apparent wind provide lift as is courses across the airfoil of the trispoke? I think I remember that it does. If this is true (that might be a big if) then a Trispoke might help your propulsion in some sidewinds.???
I don;t know about this. I understand the concept of vectors with the combination of wind direction and the direction of your bike as well as its forward velocity- the combination of the two- but I don’t know if Bournelli’s principle applies to the profile of a trispoke strut.
Bournelli’s principle, as I understand it, works chiefly because the distance from the leading edge to the trailing edge of an airfoil is longer on top than it is on the bottom(except in the case of a supersonic, symetrical airfoil that generates lift by varying angle of attack). I think the pressure on top of the airfoil is lower than on the bottom, so the airfoil rises.
That is why a wing generates lift. I don’t think a trispoke works the same way.
Yes, that is correct. That’s the working concept of the Wells turbine. The point at which it provides propulsion only happens for extreme yaw angles, angles that are not in the working range of a bike wheel. But it does provide a decrease in drag for more “normal” angles.
Also I can’t understand how can someone forget vectors! For example, in an out and back course, a side wind is the worst wind because you have a headwind component both ways!
Paulo
Yeah that’s what happened at my recent testing. The course is usually a little downhill with some pretty strong headwind one way, then the opposite on the way back. I’ve seen people go 28:00 there for 20k. Last week there was a really strong wind, but it was from the side and it was aweful.
I like wind (even headwind), but sidewind is badddddd. The headwind forces you to become as aero as possible and it really helps get set up aerodynamically on your bike because you can immediatly feel if the change you’ve mde (ie get lower) has any impact on your riding.
Thanks, Tom. If you hear otherwise, post it.
Actually that’s not it.
In potential flow (“Bernoulli flow”) around an airfoil, if you take two adjacent fluid “cells” in the flow upstream of the foil and one goes under the foil and the other above the foil, they’ll will up at the trailing edge at the same time. Why? Because the one that went above the foil, even though it had to travel a longer path, was accelerated through it, its velocity increased. Because of the that velocity increase, the pressure dropped (Bernoulli’s principle), and that pressure differential between the two sides of the foil is what causes lift.