Do your wheels pass the “flame test”?
What ? ? ? – – What the hay are you talking about ? ?
One evening a friend and I was installing a set of reconditioned tri spoke wheels for his new CAD 2.0 Bacchetta. The bike came with nice set of bladed spoke wheels however being prep’d as a Sebring racer – a more aerodynamic set of wheels seem to be in order – then use the originals as back up.
After getting the rear tri spoke set up with proper cassette spacing/alignment, I ask him to spin up the rear wheel as fast as he could with the cranks – why? I wanted to see how the newly installed ceramic bearings felt at high speed and check for wheel balance. I also wanted to do a aerodynamic test on the tri spoke wheel to see how it compared to the bladed spoke wheels the bike came with.
With the wheel spinning at high rpm, I check the bearings and wheel balance then picked a BIC lighter see how much the wheel pumped surface air at different stations of the wheel and tire. Having done this many times before with different wheels, I knew there are usually surprises to learn from. I love doing these low-tech tests as they are eye awakening and challenging to reason.
A well-known outlet store sells a budget bladed wheel that I use on my Easy Racers Gold Rush. The design of this wheel allows the flame from a BIC lighter to be as close as 1/2″ before the flame does serious flickering. Moving the flame closer to the nipples will demonstrate how sensitive laminar flow can be tripped. The flame blew out when placed as close as 1 1/2″ from the nipples. This was caused from an abundance of air turbulence from the spoke nipples. Moving on to dish of the rim, once again, the flame was able to be placed 1/2″ from the rim before it started flickering. Note – any decal edges that have come unglued will be cause for additional turbulent, so be sure to have them properly laying flat as they should be.
Now for the tires – Interesting enough, I have found that some designs of racing tires cause very little if any air pumping turbulence at all while other tires of “race” design produce quite a lot of power robbing vortices’. Remember those little mold tags on new tires – get rid of them. They cost you a drag coefficient – how much – – ? ? ? What I have found is – smooth tires are less apt to cause turbulent that create’s air pumping. Any kind of irregularities on the tire surface “side wall or tread” will cause air pumping drag.
Obviously – these findings were not what my friend wanted to see after dropping a bunch of bucks on the tri-spokes. Ironically – the bladed spoke wheels that came with the bike actually showed us they were more aero efficient than a popular brand of tri-spokes. I find these results are consistent with other bladed spoke wheels that have come through my shop. So – bummer – after spending a bunch of money, the tri-spokes flunked the “flame test” big time. We found that the flame consistently would blow out approx. two inches away from the aero design spokes of the wheel. It was obvious the big wide spokes were pushing a lot of air mass around even though they are more laminar friendly. It did get a little better moving out to the deeper dish part of the wheel. But hey – they look good – right! wink
Bladed spokes started flickering the flame about 1″ and stayed lit 1/2″ from the total length of the spoke “except the nipple” to the dish/rim of the wheel continuing on to the smooth sidewall of a Michelin Pro.
Tri-spoke flame blow out was at 2″ along the blade and 1 1/2″ at the deep-dish portion of the wheel. We could not even get with in 3″ of the tire before a blow out. I do not recall the tire brand but remember pointing out it had a lightly textured tread.
Paired round spoke wheel designs did not do well either. This test example showed 3″ blow out at the spoke area and nipples, but got better at the dish of the rim.
Standard 32-round spoke double/triple cross – flame flickers violently at 3 1/2″ and blows out 4″ at the nipples.
Hmmm – for over all club riding – I think I will stay in the bladed spoke camp. I have now replaced the round spokes on my 451 20″ front wheel to bladed spokes. The smaller 451 – 20″ front wheels spin approx. one third faster than the 700c – – so the aero benefit will be another small improvement for the over all aero package of the bike.
Thoughts: Use self-control for the glitzy bling we all get caught up in. Ask yourself – are you using the RIGHT STUFF or the LIGHT STUFF. Your bike may look faster but go slower by assuming trick wheels will be faster. I have to say I now have softened my interest for the high dollar “carbon fiber wonder wheels”.
Now that I know this part of the story – I cannot wait to test a carbon disk wheel. Will it pass the “flame test”? It will be interesting to dig out the old cloth disk wheel covers from years past to see how they compare.
I have been ask – what do I use?
For practical everyday use – I use a 28mm dish rim, 24 bladed spokes, ceramic bearings, smooth tires with light weigh tubes.
Disclaimer – There is another important element in the study of wheel aerodynamics. Different bike frame designs each cause a multitude of turbulents – this alone drives wheel engineers crazy. Developing a wheel design moving forward while spinning on a bike change’s the whole dynamic of wheel aerodynamics. The result discussed in this article was done with wheels spinning on a stationary bike stand. Open road is a different setting, however – the amount of wind pumping going on does not. It is no secret that larger spokes push more air mass and catch more cross winds. Disc wheels are down right dangerous wheels to use in moderate to high winds so users beware. Choose your wheels carefully for what your needs are for. All out racing wheels have specific purposes that are generally not suitable for daily service. It is interesting to surf the web and see controversial claims being made from different wheel brands. Many claims are out right contradictory of each other. All n all there is but one simple test that works better than terabytes of data. Long-term seat of the pants ride comparison. Then – who has the patience for that? lol
Up-date: – – It is now one year later 2013. I have just had a discussion with the rider mentioned in this article. He has concurred that the Tri-spokes are indeed are a handful in cross winds and he does not feel they are faster. This equipped high racer is down right scary in cross winds, he says. From now on he will be using his tried and tested socked Gold Rush in future racing events. His claim is the socked Gold Rush is over all faster for everyday use and racing in the wind.
It was because of his comments pertaining to his year long comparison that prompted me into publishing this article.
More later ~
Driveline efficiency test –
Aerodynamic coast down test –