…how deep do rims have to be to be considered ‘aero’? Or, how deep do they have to be do have that noticeable 1/2-1mph advantage? 33mm…46mm…58mm???
I’ve seen lots of claims and heard lots of opinions…is there an inflection point where the aero advantage increases at a higher rate as the wheels get deeper? …
I remember John Cobb saying that even a 20-22mm has some advantage over a box rim - I doubt it’s 1mph though.
It would be cool to see drag plotted vs. rim depth but since different rims have different cross-sectional shapes it probably wouldn’t mean much.
When I change from my training wheels to my Velocity Deep Vs, I notice almost 1 mph difference. It may be in my head, but they seem much easier to push.
jaretj
According to Zipp - it’s 38mm minimum.
http://www.zipp.com/TechInfo/wheel_rim_discussions/index.html
Interesting study comparing Campy Shamal and Hed CX. They came out fairly close but the Hed has almost twice as much rim depth.
basically you need about a 3:1 ratio
so if you’ve got a 20mm wide rim/tire, 60mm of wheel depth is needed, or 40mm rim and 20mm tire. Zipp is pretty accurate when they say 38mm minimum.
The real ratio is actually less than 3:1. It also depends on the leading edge of the tire and really too many factors to mention. There have been attempts to “flatten” or ovalize a tire’s cross section to make them more aero, Gommitalia and Tufo both tried this, among others. They do this because you don’t want a true NACA profile. Ok I’m over my head here, I’m not trying to be the authority on the subject, just providing a guideline.
-SD
SD, so is that 60mm of total depth, or just rim depth? I mean, do you measure the tire and the rim, or just the rim?
20mm of tire and 60mm of rim is 80mm total. that is 4 to 1 ratio. deeper ain’t always better.
frank
Isn’t the tire depth supposed to be included in the 3:1 ratio? That would mean a 40mm deep rim would get you the 3:1 ratio on a 20mm tire.
I mispoke as I warned I may do. 60mm total.
total depth is approximately 3 times total width.
-Dave
If that is true then anyone who bought Zipp 303’s has been duped. 38 mm plus 20mm for the tire is only 58mm. Not Aero. Dave where did you get the 3:1 figure?
Clearly you must run 19mm tires on the 303s then, and you must run no more than 22mm on the Tiliums…
I said about 3:1 the acutal ratio is less than that. I don’t recall my source, but I can find it I’m sure. Analyticcycling or one of the tech forums should have it also.
Please don’t quote me as the Gospel according to Cobb…
I told you that the numbers were approximate and that tire shape had nearly as big a roll in the “aeroness” as the wheel depth. There are many factors to consider and if you would like irrefutable data and concrete numbers I’d need to spend considerable time and effort to provide them. Instead I’ll just recall some of the information I’ve learned after racing cars and bicycles the past 20 years and cautiously suggest that there may be some margin for error and room for interpretation.
Ok?
-SD
From Cobb’s site:
Q: What is the minimum rim height for aerodynamic benefit? Is anything below 38mm useless?
A: 28mm aero rim will show up over a box rim and even a 22mm with a good rounded profile will show some improvement.
http://www.bicyclesports.com/Snitz/topic.asp?TOPIC_ID=82&SearchTerms=aero
Here is an idea - why not train more effectively instead of tweaking out every last aspect of your equipment? At least then, when you say your bike split was slower than before, you can be reasonably sure it really is a shortcoming of the equipment and not the athlete.
It is a poor craftsman that blames the tool.
$$ can’t buy you time to train, but it can buy you 30 seconds.
C’mon I.G.T.L. be a consumer!
-SD
bunnyman institute opinion:
It is shape and number of spokes. Remember Lemond’s great TT ride? He used pretty shallow rims with few spokes and a rounder shape.
Sometimes a heavy, but aero rim xould hamper a rider who doesn’t put ouy the watts. My weak ass cycling these days was hampered heavily by the lead-filled Mavic Cosmic I was riding for the front. It was a good wheel with a GREAT hub and a very durable rim, but I was a LOT faster on an old Sun Mistral 19mm aero-section rim with sixteen spokes. It’s a shame my big fat ass bent the damned rim and the aluminium has aged considerably enough to not allow it to get too true. This is where a nice rim like a Nimble Fly laced up to my American Classic hub will work great. I think the Lew might go…
I think a total wheel depth to width profile would be this: 55-60 rim/tire combo to 20mm wide tyre. You would have somehwere around (what some call) the perfect ratio of 3 (side area) to 1 (frontal). Any more would compromise handling and add unnecessary weight, unless you are “0berbiker”.
38mm is touted as the minimum, but I think it has alot to do with speed. I seem to recall that most of the aero numbers are generated at about 30mph. Certainly faster than I ride so maybe shallower rims work for us mortals.
At slower speeds I think the air has more time to reattach to the rim behind the blunt tire and poor (non-aero) tire/rim interface, so shallower rims might be of some benefit.
I still hope to buy 60mm deep front and maybe deepr rear wheels when I can scrape together the money. besides, they look cooler!
TriDork
…if you had to choose between the Stratus DV and 404 tubulars 700c - what would you buy?
I’d get the Ritchey WCS Carbons. Same rim as the 404, fewer spokes, and they use traditional j-bend spokes which are much easier to locate should you ever need to.
Reynolds uses round spokes and an “interrupted” shape on the sidewall of the rim.
-SD