I think it’s pretty much a foregone conclusion that a rear disc wheel is more aerodynamic and faster in a straight line than a spoked wheel, but we also know they tend to be a tad heavier. At what point would one opt for a 60mm or a 90mm rear wheel over a disc? For example, I’m running HED 60/90 as my standard race set up. I’m racing IM Coeur D’Alene this summer and this course has some significant climbs. On a flat course this would be a no brainer but, what would be the better choice for a hillier course like this? I’ve ridden a Flo disc before and liked it, other then I found it to be a pain in the ass on race mornings fumbling around trying to get it inflated. I’m 5’9, 150lbs, with an FTP of 250 if that makes a difference.
Use the disc always unless it’s a straight uphill course.
this is a good post to read https://forum.slowtwitch.com/forum/?post=7346345#p7346345
Very informative, thank you!
I was told to always use a disc unless it’s purely an uphill race. I watched a few races this week while on the trainer, one race in particular was Challenge Wanaka and most of the top pros weren’t using discs. I don’t know anything about that race or the course or the conditions on that day but I found it odd for sure. Even Luke McKenzie who’s well known to be an axe on the bike wasn’t using a disc. I wonder why?
Use the disc always unless it’s a straight uphill course.
this is a good post to read https://forum.slowtwitch.com/forum/?post=7346345#p7346345
Amazing post linked there. Golf clap.
Also, if the course ever goes back downhill, your airspeed will be high enough that every little aero detail is of utmost importance. Downhill you may be going 30, 35, 40 mph. The pesky air molecules find all sorts of things to hold you back at those speeds. Minor details could mean over a whole mph downhill.
I’ve ridden a Flo disc before and liked it, other then** I found it to be a pain in the ass on race mornings fumbling around trying to get it inflated**. I’m 5’9, 150lbs, with an FTP of 250 if that makes a difference.
Not sure I understand. I can inflate my Flo Disc with any standard pump that has an L-shaped head (every pump I have except my super ancient Silca track pump). I am using the appropriate length valve stem tube and it’s never been a problem, even with CO2 fixing a flat.
Now, when you talk about what you have to do if the spoked ‘disc’ goes out of true, that’s a real big PITA.
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Use the disc always unless it’s a straight uphill course.
this is a good post to read https://forum.slowtwitch.com/...ost=7346345#p7346345
Good advice. Anyone got any calculations they can share about what slope of a hill TT (assuming net gain in elevation), or what average speed might be a good rule of thumb to swap for something other than a rear disc?
Yaw angles tend to increase on average with reduced speeds, and I think disc aero performance, relative to non-disc, increases with increasing yaw angle, right? My intuition is that the speed threshold for going with something non-disc must be down near 10mph? or maybe slower??
Use the disc always unless it’s a straight uphill course.
this is a good post to read https://forum.slowtwitch.com/...ost=7346345#p7346345
Amazing post linked there. Golf clap.
Also, if the course ever goes back downhill, your airspeed will be high enough that every little aero detail is of utmost importance. Downhill you may be going 30, 35, 40 mph. The pesky air molecules find all sorts of things to hold you back at those speeds. Minor details could mean over a whole mph downhill.
Downhill is where real world differs most from the BBS-style aero calcs. The combination or cornering and descending skills will cause the rider to go slower than their terminal velocity…and at that point the limiter isn’t the aeroness of their wheels; it’s descending skills/guts. The point at which you hit the brakes to scrub speed on a downhill if the point where you are getting no benefit from the aero gains of one wheelset vs another.
That all being said, even “hilly” courses typically have so much flat/false flat that a disc is faster. Lots of discussions about using discs on courses like Wildflower, and it always worked out faster IRL (even though it sure feels crappy on the climbs).
Use the disc always unless it’s a straight uphill course.
this is a good post to read https://forum.slowtwitch.com/...ost=7346345#p7346345
Amazing post linked there. Golf clap.
Also, if the course ever goes back downhill, your airspeed will be high enough that every little aero detail is of utmost importance. Downhill you may be going 30, 35, 40 mph. The pesky air molecules find all sorts of things to hold you back at those speeds. Minor details could mean over a whole mph downhill.
Downhill is where real world differs most from the BBS-style aero calcs. The combination or cornering and descending skills will cause the rider to go slower than their terminal velocity…and at that point the limiter isn’t the aeroness of their wheels; it’s descending skills/guts. The point at which you hit the brakes to scrub speed on a downhill if the point where you are getting no benefit from the aero gains of one wheelset vs another.
That all being said, even “hilly” courses typically have so much flat/false flat that a disc is faster. Lots of discussions about using discs on courses like Wildflower, and it always worked out faster IRL (even though it sure feels crappy on the climbs).
I do TT, not really tri. I don’t have speed as a gps field. I’d rather not know. Just hammer. I feel more stable going faster while making power than freewheeling. So, enough chainring and cog is needed.
Good advice. Anyone got any calculations they can share about what slope of a hill TT (assuming net gain in elevation), or what average speed might be a good rule of thumb to swap for something other than a rear disc?
Yaw angles tend to increase on average with reduced speeds, and I think disc aero performance, relative to non-disc, increases with increasing yaw angle, right? My intuition is that the speed threshold for going with something non-disc must be down near 10mph? or maybe slower??
I have the math somewhere but there are tons of drag charts out there showing at higher yaw angles disc wheels destroy other non deep to deep section wheels. Couldn’t find the math on a quick search through my hard drive. You can go to any wheel manufacturer website and find that data.
I’d say if I was racing and even just going to go 10-12mph average speed I’d use a disc.
What I find surprising is that so many people say “oh I’m not fast enough for a disc or an aero helmet or a deep front wheel or latex tubes or go fast only tires.” The slower you go the bigger the time savings bc you’re out there longer. The reason we test at 30mph in the wind tunnel is it’s easier to see the differences between product or position A or B. The slower the tunnel speed the more alike everything looks, yet there is a difference and it’s those differences that can mean the difference between grabbing a 70.3 WC slot or a KQ slot*.
Another way to look at it is if you’re going to be expending 4000 kJ for a 6h bike ride, I’d rather expend 3875 kJ for a 5:50 min bike ride on the same average power. **
The slower you go the larger the average yaw angle you’ll see, the more often you’ll see larger yaw angles and the more time you’ll spend at larger yaw angles. When reading aero charts think about your relative speed then look at that part of the chart. If you’re going to average 18mph for a 70.3 don’t worry about the comparisons at zero. Look at them from 5-12-15 yaw angle. If you’re going to average 28 mph don’t worry about anything >5 degrees. Think about which angles you’re going to be spending 90+% of your racing time in then buy your equipment based on that.
I’m also of the philosophy of never buy equipment for racing that’s going to make you slower than what you’re replacing. Now at times you may be guessing but there is enough data around to make a very educated guess. Buying an aerohelmet? Don’t buy anything that has wings (although the newest helmet from them looks like it has potential) buy something that’s AERO on your HEAD or CEREBrEaL.
LMK if that didn’t answer your questions.
- I had an athlete grab the last KQ slot in his AG at an IM. The difference between the dude ahead of him and the 2 guys behind him who did not get slots and whom he passed in the last 4k was somewhere between :50 & :55. That’s 4 people at the pointy end of the AG within at most :55. Think about that if you’re racing a deep rear wheel and someone has a disc. Or latex tubes and you don’t. Or really fast tires etc.
**I did consult work for a guy who placed 6 at the Ironman World Championships on what kit to wear. He finished ~ 1:10 -1:30 (can’t remember exactly) ahead of 7th place. He wore an aero sleeved suit, 7th place did not and he out split 7th by ~ 3 minutes on the bike. I figured the difference in their clothing was ~ 3:30 on the bike. You might say he would have beaten him anyway, which is true. Yet those were kJ’s he did not have to expend on the bike and could use to hold off 7th place who was a bit faster on the run. Remember folks ~ 100kJ = ~ 1 mile of running energy requirements. That 200kJ you save on the bike can be the difference between blowing up at mile 24 and blowing up at mile 26.21. The difference in prize $ more than covered my consulting fees. The difference in sponsor bonus money was > than my consulting fees as well.
Having worked with a couple of pro athletes who’ve raced Wanaka, it’s likely due to the road conditions. NZ tarmac is notoriously “dead” - lots of chipseal.
Running a rear disc (especially one of solid construction) can be a very harsh ride due to the increased rigidity of the structure. So whilst it would still be faster than a spoked rear wheel, by the time you get off the bike & try to run, your body may be so shaken to the core that you’ll lost all that time & more on the run course.
This is one area where a wider, tubeless tyre on your disc can make a real difference. By shifting the compliance from wheel to tyre, you can maintain aero benefits without the physiological penalty.
Would a wider clincher tire be helpful?
Even the weight is really not that big an influence.
My rear disk 1005gr
My rear 60mm wheel 881gr
124 grams which is about a 1/6 of fluid difference in your water bottle
Yes, especially run at a lower pressure (as you likely would with a wider tyre). You’re looking to deliver some form of ride compliance, essentially a suspension effect, through the tyre rather than the wheel construction.
Added benefit being that you can run that lower pressure and maintain rolling resistance (see Bicycle Rolling Resistance data for this).
Also worth noting that the aero penalty of a wider tyre is substantially reduced on the rear wheel if the rim profile isn’t optimised for that tyre size.
I’d say if I was racing and even just going to go 10-12mph average speed I’d use a disc.
Sounds like our intuition is similar! Thanks for the detailed response by the way.
Did you just advise something other than or close to an Aerohead (for those that can’t test)??
Yes
The Poc has seemed to become one of those helmets that works more often than not. Now not better than an aerohead but if I’m betting money and can’t bet on the aerohead I’m betting on the POC.
I did just get my hands on one of the new RP’s to test next time I go to the tunnel. Should probably start working on that since I have a few things I want to test as well.
That’s good to know and makes perfect sense. Thanks!
I can vouch for the terrible road surface at challenge Wanaka. It is a fantastic race but definitely run lower pressures!!
Wow thanks for that …I learned a lot in that reply!!