Matt Russell + Omni in the Wind Tunnel
article & images by Nick Salazar
Mar 19, 2019
Side Force + Handling Torque
Upon analyzing the data from our trip, we discovered a couple of very surprising facts about our new rig. The first of these relates to side force, which is the component of the wind that conspires to topple you over directly from the side. The second is the Yawing Moment, which I will refer to as handling torque. This is the component of the wind attempting to twist the entirety of the bike/rider complex away from the direction of steering (in either direction). These are numbers that the San Diego LSWT provide, but not all tunnels do. And they bear some additional explaining, because the data we collected was quite profound.
You may have heard of the "sail effect" - the notion that if you add something like a rear disc wheel, it can actually reduce drag, as well as move the bike's center of pressure rearward. Additionally, when the wind pushes against that disc, that actually steers the back half of the bike into the wind, making steering easier. Put another way, the Yawing Moment (what I'm calling Handling Torque) has gotten lower. So that's good, right? Well, yes. However, this ignores the important element of side force.
Side force is, as mentioned above, a force orthoganal to the rider (directly to the side). It's the component of the wind trying to topple you over. As the wind picks up, and particularly when you experience big gusts, that component can be very significant. So if you've added that rear disc, and a huge gust of side wind hits you, you'll notice that difference. The more surface area available to the wind, the greater (in general) the side force.
So, in our example above, the disc wheel lowered Handling Torque, but increased Side Force. But does that always need to be the case? Is the only way to lower handling torque going to come at the expense of side force? Our testing definitively says no. In fact, the organic shapes on Omni help decrease both Side Force AND Handling Torque, simultaneously! How much better are they? Have a look.
In short, our tests showed that Omni suffers from about 20% less side force and about 30% less handling torque compared to the double-diamond frame. This is huge. Ultimately, that means you don't have to work as hard fighting the wind while riding Omni. During windy conditions, the rider can focus more energy on moving forward, and worry less about steering and handling effects. And the less bothered by the wind the rider is, the more likely he or she will stay in aero. This effect is rarely discussed in modern tunnel tests, especially for new bikes that have more and more surface area, and perhaps higher side forces. But ultimately, this means Omni riders get to stay more aero for more of the time. And the windier the conditions, the more pronounced this effect will be.
But why does Omni exhibit this rare characteristic of lowering both side force AND handling force, while ALSO exhibiting very low drag from the rider-axis position? That's all thanks to its fundamental design. Omni's low-drag characteristics were well documented in our first wind tunnel studies with it. That has a lot to do with its Monofoil shape, which has virtually no trailing edges from the front to the back of the bike. But when you view Omni from the side, you can begin to intuit why is has done so well in this second context. Put simply, Omni doesn't add much side area compared to the rider alone. Only a small portion of the frame is 'visible' to the wind from the side, because the rider's own legs/body conceal it. Even crazier, Omni's side force actually goes down once the rider gets on board. That's right, Omni exhibits less side force with a rider on board than with no rider at all.
And this test was only against a traditional double-diamond frame. We suspect the effects would be even more pronounced when considering some other radical frame shapes that add lots of side area for the wind to grab hold of.
(Editor's Note: We want to express a big thanks to Josh Poertner, president of Silca for helping us mine through the data and understand these concepts. Josh made time in his tight schedule to help us out. That bing said, any errors, omissions, or odd opinions are ours alone, and we wouldn't want Josh blamed at all for them. We have always loved reviewing Silca products, because they are truly great pieces of kit. But the quality of their products is surpassed only by the quality of the people who work there. Thanks so much, Josh!)
Hit the jump and we will pull out a few conclusions from the test.
Although they might not be so exciting at first glance, these graphs are the real treasure of our time at the tunnel. The two big takeaways are that, with the new equipment and position, Matt will save nearly 10 Watts compared to his previous rig, meaning that for the same power output, he could go approximately 3 MINUTES FASTER over the course of an Ironman bike leg. Second, we've significantly reduced the side force and handling torque he experiences in the wind, meaning he can spend more of his energy going forward, rather than fighting the wind's effects on bike handling.
Matt is a consummate professional. Not only does he have a very detail-oriented mind, but he can pedal in the tunnel with a stillness that makes the data very clean. Most athletes will move or fidget just a bit, causing the data readings to jump around somewhat. That's why you have to take very long data samples with a rider on board (the tunnel recommends 60 seconds per point). But with Matt, he's such a smooth pedaler that we were able to shorten the sample rate to 40 seconds per point, and still get very consistent data.
This test rig is set up exactly like Matt's rig back at his home in Florida. Because Omni and the Alpha One aerobar are so easy to adjust, it took us all of two minutes to set the bike to Matt's exact position. It was just a matter of setting the Monopost position, the tilt angle, and then the seatpost/saddle. Five bolts. Running through different hand positions was also a breeze, just two bolts to loosen, set tilt, and tighten again. The staff at the tunnel said we were perhaps the most efficient crew they had seen.
San Diego LSWT
Some shots of the facility and instruments at the San Diego Low-Speed Wind Tunnel, as well as a couple of those of Matt's prior bike that we used for comparison purposes.