I'm sure this is a common question, but search is acting wonky on me. I'm just wondering:

1 - at what speed aero wheels start to make a difference?

2 - how much depth do you need to make a difference (does something like 30mm make much difference over 24mm)?

I currently average ~20-21 mph riding solo over my regular 30 mile training rides and sprint & olimpyc length tris are the only races I currently compete in. I obvioulsy have a lot of room for improvement on the "engine" & I know a good aero position on the bike is more important, but it would be useful to know when down the road aero wheels might make sense for me.

You need to define "a difference." Any depth makes a difference at any speed. More depth makes a bigger difference, and more speed makes a bigger difference.

Generally, it takes 40-50mm of depth to be significant. At 20mph, you wouldn't feel it, but could probably see the results in a TT.

At 28mph, one study found going from conventional wheels to a tri-spoke saved 15W of power. That's a fair bit.

they make a difference at all speeds, esp if they have dimples

placebo







that's me stirring the pot to watch it boil...lol

it's not just speed it's speed and distance.
aero makes a big difference.

there was an excellent article in bicycling about the MIT cycling squad which had computed all the differences and could using an excel sheet predict their finish time to incredible accuracy on a time trial based on drag and wattage output.

wheels made very little difference. if memory serves, helmet made the biggest difference, along with waterbottles and position on the bike. Wheels made very little comparative difference.

https://www.bicycling.com/article/1,6...4995-1,00.html

The MIT group has disavowed some of the conclusions as published in that article, and Jim Martin has been accurately predicting performance for over 10 years. Wheels make a very significant difference.

holy cramolies. 16:51 for 10 miles... 35.6 mph for a TTT.

cdr

In a one hour time trial, a guy riding 30 mph could save several seconds with aero rims, if there is no side wind. A guy riding a 20 mph is likely to save a second or two every hour...important to a pro, meaningless to real world cyclists.

If there is a cross wind, the high profile of the aero rims can cause stability and handling problems, which costs more seconds than the aero rims normally "save". So, pro teams try to predict the level of cross winds before deciding which rims to use during a time trial.

The most significant time savings in a time trial come from the rider's position on a bike. However, without access to a wind tunnel, a "Joe Average" cyclist has no way of measuring how much time he could save riding in "Position A" versus "Position B".

This makes it sound like the benefit from aero rims would be less with a cross wind. In fact, more time is saved in a cross wind than a direct headwind. Several wheel and frame manufacturers are now designing their products for optimal performance over the wind angles they believe are most common.

In addition to a flat TT at 20 - 21 MPH...

How about in a hillclimb race...

I have been contemplating a semi-aero rim...

But does it really help when going up hill at 7 - 8 mph...

I have been going for light weight wheels more than aero...

What say you...

this is what i have come to believe as true, correct as required:

aero rims can definately improve performance.
while i don't know exactly how much, it is significant and worth far more than an aero helmet or not wearing gloves (the mit test was just looking at frontal area if memory serves).*
*it isn't just about minimizing frontal surface area (see asgelle's crosswind post)
the slower you go, the more time you save with aero wheels.
there is a pretty clear pattern that as rim deph increases, so does aero-ness.**
**but not all rims are created equal (zipp's 58mm 360 rim seems to be the weapon of choice for rr).
slowtwitch.com has lots of good stuff about this.
i am a climber, and i would much rather have 404's than 202's or 303's.
asgelle knows more than i do about this stuff.

And what angles might they be?

https://www.analyticcycling.com will let you play around with different scenarios and answer your particular questions.

Right now different manufacturers have different opinions based on their data. 5-10 degrees would be a reasonable range without specific data to show otherwise.

i know this isn't scientific by any means but i used to have a pair of 50mm deep wheels and i couldn't "feel" a real difference between them and a 'standard' wheel until i got them into the mid 20's.

0-10 degrees depending on who you ask. If you want to run a scenario this will give you effective wind angles:
https://www.hedcycling.com/technical/yaw-calculator.php

Aero wheels would be nice eye candy but to quote "my weight has more metal bending Karma then your weight". I'll stick with my 700 Velocity Synergy rims and 36 - 14 gage Wheelsmith spokes.

They make a difference at all speeds, they just make a much bigger difference at speeds above around 25mph (and that difference isn't massive).

Even a cursory look at the equation for aero drag shows that the effect of changing wheels or other equipment is the same at all speeds (over the range of normal cycling speeds).

Well, my non scientific results showed me a few things when I put on some Rolf Vector Pros, high profile rims with minimal flat bladed spokes. They replaced some Mavic Open Pros 32 spoke wheels. My average speed was about 1 to 1.3 mph faster or so said my cycle computer. They got up to speed faster, or so it felt, they seemed to make me faster rolling down hill, particularly in a tucked position. I found that I could roll up on and pass a similarly sized rider who was pedaling. The down side is that the ride was a bit rougher with the high profile rims than with the low profile rims.

Please provide the equation then. If you can reduce the CdA by whatever means, it will make an exponentially larger difference as the velocity increases. Power consumed by drag is CdA*V^3 blah blah blah.....but, wouldn't the airflow attach to the wheel past a certain speed? Before that the airfoil effect wouldn't be as effective? I know Zipp dimples aim to keep the airflow attached but I would think a certain air velocity would be required for that to actually happen.

It looks like you already know it.

F_d = 1/2 rho Cd A V^2

"A" is a purely geometrical parameter and Cd is independent of v over normal cycling speeds (a range of well under an order of magnitude in Reynolds number). Therefore, there is no speed dependence on the effect of changing from one set of wheels to another.

OK, I get that, there is a uniform change in CdA, but wouldn't there be a certain velocity where that changes when the airflow attaches to the wheel instead of flowing in an non-aerodynamic manner? I think it may be a little more complicated than that equation.....

Where's that Zipp aero wheel test that shows their disk with negative drag?

Look at the Reynolds number for typical riding speeds and the range of Re over which separation occurs. Even taking the range as 5-30 mph (when 15-30 is probably better for TT's), that's only a factor of 6. Changes in the general characteristics of a flow take place over an order of magnitude or more in Re.

Oh, and here's the best Zipp data I've seen https://forum.slowtwitch.com/gforum.c...engine#1460321

Odds are your experience had more to do with the weight difference between the wheels than their aero profile.

I prefer a well built lightweight "box rim" wheel over many deeper profile wheels. This is mainly because of a few factors:

1. In my price range of wheels the deep section wheel always seems to be substantially heavier (200-500g heavier - 0.5-1lb) - I'll take the feel of lighter wheels any day.

2. I just don't spend enough time riding at higher speeds (25+) where the aero benefit seems to start to kick in to outweigh the benefit of the lighter wheels.

3. I don't get paid to ride. I don't get paid if I finish my century even a minute or 2 faster - let alone the 30 seconds or so the aero would save me. Again the snappiness of lighter wheels is something that I can feel and appreciate all throughout a century in comparison.

Now...make some lightweight aero wheels available to me for free and I would ride them - of course, but so would anyone. Well anyone except for ABH....