Bicycle Wheel Power Data 50km/h Disc
Over several years, bike wheels have been tested for aerodynamic performance. To date, this is the largest independent dataset available. This dataset was originally derived from wind tunnel test data but has latterly moved over to Dinitriev number with a back calculation to give “wattage”. It is not a perfect test.
It should be noted that bicycle wheels are particularly sensitive to tire size and having a wheel and tire combination that bulges will cause a significant increase in drag. Ideally, the tire and the rim should be of the same width. These graphs are now updated on a regular basis and are plotted for interactivity. You can see some of the background for this data here.
Small differences in watts should generally be ignored as these will be affected by arbitrary things such as shoe overlap and the size of bike frame. A bigger differential will produce a bigger drop in drag but it diminishes as the wheel tends towards a disc. In general, a deeper rim will always have less drag. The spoke area which is of high turbulence is much smaller on a deeper wheel – hence the primary drag reduction.
Warning
A number of cyclists get infatuated by very small differences between the wheels. Guidance in this regard is to adopt a 2.5SF rule. That is to say that for an appreciable difference, there should be a difference of 2.5 significant figures.
eg a rider is unlikely to notice a difference between a 190W wheel and a 180W wheel (2SF). There is some error in there as well as geometrical differences based on the rider and their bike. When the difference becomes 190W vs 175W, that is effectively 2.5SF and would be noticeable.
This is a rule of thumb and requires considered application.
Thanks for the data! Would love to see some data on differences between front and rear. So e.g. how much slower is 50mm front and rear compared to 50mm front and 80mm rear. Do you have any data on this?
It’s not easy to split that data out because the airflow hitting the back wheel has already been man handled by the front
Any thoughts on the 9Velo Disc?
I would get them without even thinking twice.
Your ‘guidance’ to rely on a 2.5 significant figure difference is completely nonsensical!
What on earth is a 2.5 significant figure difference?!??
How can you even have 0.5 of a significant figure???
By definition, you cannot have a non-integer number of significant figures because a you cannot have a non-integer number of digits…
For someone who loves to criticise others for their lack of technical knowledge and precision, this is really very ironic.
That’s called practical engineering and a way to account for error for non technical people. I could just say 15w but it does not scale.
So your definition of “practical engineering” is just making up your own completely meaningless and nonsensical terminology in the hope that “non-technical” people will lap it up because it sounds official?
To be crystal clear to anyone else reading these comments, the “2.5 SF rule” is not some well-known rule for estimation used in engineering; Hambini has just made it up.
Irrespective of the above, it’s still completely unclear to me what you’re even trying to mean when you say 2.5 significant figures? Can you please explain, precisely.
Even if 2.5 significant figures did make sense, your approach of using significant figures as a means of quantifying a “noticeable difference” in this instance is wholly illogical, as I demonstrate below.
Referring to the power values you used as an example, both 180 W and 190 W round to 200 W, when rounded to 1 significant figure. One might be mistaken into thinking that because these numbers both round to the same value, we can say there is no noticeable difference between them (as you appear to be saying?).
In reality, every number between 150 W and 249 W rounds to 200 W, when rounded to 1 significant figure. As any cyclist that has ridden with power data knows, there is a huge difference between 150 W and 249 W. Clearly, rounding to 1 significant figure is far too imprecise. I’m sure we can both agree on that.
You state that most riders probably wouldn’t be able to notice the difference between 180 W and 190 W. You don’t supply any further explanation for this other than writing 2SF in brackets. It’s wholly unclear to me what this is supposed to mean. When rounded to 2 significant figures, 180 W rounds to 180 W and 190 W rounds to 190 W. Because these numbers now round to two different values, does this mean there is now a noticeable difference? You seem to be saying the opposite.
As a somewhat unrelated aside, there should be a space between any values and their associated SI units (i.e. 200 W, not 200W as you use).
You are quite right, it is made up. It’s called an engineering rule of thumb and it’s developed through experience in the field. It is like accounting for a radial bearing clearance when you don’t know the number, it’s 0.001″ per inch. It’s a rule of thumb.
The modern engineers think their CFD systems and what not can account for everything – they can’t So the SF rule – it’s not always 2 or 2.5 is an old school way of accounting for the unknown.
In this case as above, you’d need a double digit difference to have a high chance to be outside the error margin.
Rule of thumb is stated in the article text but in case you’re wondering, the definition is “a broadly accurate guide or principle, based on practice rather than theory.”
Thanks, I do know what a rule of thumb is. I think we’ve probably been talking at cross purposes, as often seems to be the case with internet discussions!
It now appears to me that your original comments regarding 175 W vs 180 W vs 190 W were almost entirely based off experience. For what it’s worth, I agree that 180 W vs 190 W is probably much of a muchness and that 175 W vs 190 W would most likely be noticeable.
I think the point I’ve got stuck on is the fact that you seem to be ‘applying’ this 2.5SF ‘rule’ as a means of justifying the above comparisons when, in reality, the ‘rule’ doesn’t make any logical or technical sense whatsoever and the comparisons are instead based almost entirely off experience.
Despite the critical tone of my previous messages, I genuinely do appreciate you taking the time to respond.
For info, there are some useful engineering rules of thumb. Gnat’s cock is a widely used one. Effing Tight (for flogged tight), Double Effing tight for hydraulically tight. Wizard’s cuff for a H7. Shagging a bucket for H7. The list is quite long
I think you’re getting mixed up between “useful engineering rules of thumb” and all too typical engineering lad-culture/misogynist ‘banter’…
And you’ve assumed that it’s always about women, some are into men too.