Video: Disc v rim brakes: which is more aero?

When you look at data for many wheel manufactures, what you see is very little advantage to deep section wheels at 10deg or less compared to pretty much any standard wheel.
Deep section wheels have their biggest advantage at high yaw angles, hence the proliferation of wide , rounded top rims.
You wouldn't expect much extra drag at low yaw with a disk brake, because there is only a small increase in cross section. At high yaw, the air will be trying to flow smoothly across a sharp edge flat plate, that's where you expect the disk to be at a disadvantage. So what was the point of this test???

When you look at data for many wheel manufactures, what you see is very little advantage to deep section wheels at 10deg or less compared to pretty much any standard wheel.
Deep section wheels have their biggest advantage at high yaw angles, hence the proliferation of wide , rounded top rims.
You wouldn't expect much extra drag at low yaw with a disk brake, because there is only a small increase in cross section. At high yaw, the air will be trying to flow smoothly across a sharp edge flat plate, that's where you expect the disk to be at a disadvantage. So what was the point of this test???

"aeroiseverything" or not?

Seems most road/cross cyclists will not go back once they switched to disk brakes. The proof is in the pudding. If only manufacturers could now settle on standards for (thru)-axles, the price of disk-brake options would tumble. But that would prevent them from milking the consumers for a few years - through the tried and tested "new and improved" ploy - until they all converge to a golden standard that was available right from the start. 2018 maybe?

Having used various 105 and Ultegra on road bikes for many years and then extremely expensive top of the range cantis on a CX before I got a CX bike with BB5 2 years ago and another with BB7, there is no way I would ever contemplate going back to rim brakes for every day riding.
These are my own experiences from 5000 miles of commuting each year and many more miles of weekend rides and CX events
Ultegra 6700 are equivalent to BB5's in the dry, so no difference there really when set up correctly, but far better than cantis though which truly were appalling and dangerous.
Disc brakes still have a time delay in wet weather, but far better in the wet than any rim brake.
£20 or less to replace discs every 4 years (estimated on current wear) or £200 plus every year to 18 months for a new set of wheels.
Very little, if any, difference between prices for pads or longevity.
No crap all over your hands when you get a puncture in the wet.
My disc road wheels are 1400g without the discs, with a 25-30mm deep semi aero rim, 1600g with the discs, so little or no weight advantages whatsoever for rim braked wheels and not much difference in weight for the brakes either. They have been ridden all year, 90 miles at least per week commuting since February and have only needed truing once in that time
I very much doubt the alleged "aerodynamics" will come into the equation in real world riding either.
The wider 1600g cheaper wheels for off road have been abused repeatedly and have never had to be trued yet, so lack of weight does not always mean lack of quality.
I have BB5 on my commuter, very easy to set up, no disc rub, no squealing (OK, a bit in the wet for a while) and need adjusting once every couple of months I guess to keep them as I like them.

The MTB world realised eventually that disc brakes were far better.
Give it time and all road bikes will have them as the advantages far outweigh the disadvantages.
Once upon a time even cars and motorcycles didn't have them either

I wish I was good enough to notice the difference!

My Ultegra rim brakes seem to work very well for me. However my winter commuter does have disc brakes but it's more to do with all the grinding paste wreaking my wheels than improved braking performance.

For those proper cyclists who ride in ALL weathers mile after mile after mile, every day, disc brakes are far better because 1) the pads don't wear down your rims in wet, cruddy, gritty shitty conditions 2) they work far better than rim brakes when it's wet and also when hot as with rims the braking surface can get rather hot where as with disc brakes the pads and discs are not so affected by heat and brake fade. So disc brakes are far far better if you ride many miles as opposed to pro wannabee cyclists who perhaps only ride on a nice dry warm sunday on a club run who have all the Pro road gear and no idea.

@earth

Avid bb5 one of the worst mechanical disc brakes made, a swear word to bike mechanics, for sure!

Its a cheap OE choice for the bike brand product manager, cheaply made by tektro with wobbly rotor, wobbly moving piston and fixed piston/pad on inboard side meaning the brake functions by bending the rotor when you pull your brake lever. All that wobblyness means setup is vague and it needs constant attention...

Trp spyre is the best mechanical disc, but pales compared to quality hydro disc brake. There is a reason all quality mountain bikes use hydraulic discs with twin or quad pistons.

Aerodynamics aside, disc brake performance in terrible weather makes sense for UK road riders. And don't think of just the brake, but the constant wear to wheel rims and brake pads with caliper systems.

I cannot think of many rides when the weather has been so fine I've been happy to be using rim brakes, compared to sh*t weather where rim brakes have tempered my pace

Read what?

Bought a. Ultegra equipped cx bike with the latest shimano hydraulic discs a few weeks back (rose dx cross 3000) It's rained or been wet roads at least half the time I've had it.

I'd never go back. Real world performance is like night an day - I genuinely enjoy wet riding now as much as dry riding.

All for less than £1400. Next year they'll be cheaper versions - if you upgrade and are not a fair weather rider then be sensible and get one.

Mid-season I switched to discs on my cross bike - the difference to cantis is night and day (you can actually stop), but what did surprise me was when going back to a regular road bike (koolstop salmon on alu rims) after spending a couple of weeks riding only discs I really noticed the difference in effort needed to brake. It has changed my opinion on the relevance of discs on road bikes.

I suspect discs are another thing you don't need, but in the same way that you don't 'need' indexed shifting, 8/9/10/11 speeds, lycra, or carbon frames.

I think in most real world situations 8 seconds is really splitting hairs.

Its a fun piece of research however, that does at least qualify that the difference in aerodynamics is minimal. Long term however, discs will become more aero I am sure.

Everyone would benefit from hydraulic discs, I am sure of that, to argue otherwise is a bit silly.

Yes, right now, you would be an early adopter, so limitations on weight, compatibility issues, etc are going to be part of it... in 5 years though, its going to be gravy.

Which is fine for me, as I'll have stopped racing by then, and can instead enjoy hairy descents at max speed on my sportives!

i can get a 8kg bike for £1000, and have best rim brakes money can buy for extra £120. Or i can pay at least 1800 for a bike with hydraulic discs that weights nearer 9kg and has 2kg wheels that i cannot easily upgrade. I also face the steep future improvement in discs, and the obsolete axle design. Sure discs are better but i'd prefer to pay 1200 quid for my bike than £2,600 (scott solace 5 or Giant Defy Advanced Pro). Cable discs are, by the way, a total waste of time.

With slick tyres in wet weather I can still modulate to fully locking my wheels with rim brakes. What has a big bearing on my speed when descending in the wet is not so much the brakes - since I can keep them dry/clean using ghost pressure - but the tyre grip, and maybe the frame/fork combination. I descended Tiefenbachgletscherstrasse and it started raining as I emerged from the tunnel, and so I couldn’t safely do more than 30mph the rest of the way despite reasonably fresh Schwalbe Marathons and steel forks. With slick tyres and carbon forks I would have been much more cautious.

Wait... is someone with hairy legs and rim brakes faster that someone with shaved legs and disc brakes? And about about shaved legs, beard and.. OOOH MY HEAD!

whether you like it or not .... 11 speed, disc brakes & electronic shifting will all become de rigueur on all top end bikes.

8 seconds over 40km doesn't seem like very much really, much less than wearing slightly flappy clothing or sitting up higher than is optimum would be.
I'm kind of indifferent as to the kind of brakes on my bike, as long as they work well then I don't mind what they are, I think the weight/aero difference between the two is such a non issue for anyone except people who race that it really doesn't matter.

Ride what makes you happy.

'I wouldn't ride a disc equipped bike'

This.

Quote:

Real conditions in the UK don't involve consistent percentage yaws, they involve strong, often blustery, cross winds, as well as headwinds etc.

Of course they don't but then they test cars on a rolling road to get an mpg figure which bears no relation to what it'll get in "the real world" with headwinds, rubbish road surfaces, traffic etc. But it's useful as a comparison across the board.

Same here, it's a test - defined parameters, scientific and repeatable.

This isn't an independent test. It's from a company trying to get you to buy their latest model.

Real conditions in the UK don't involve consistent percentage yaws, they involve strong, often blustery, cross winds, as well as headwinds etc.

I wouldn't ride a disc equipped bike in real world cross winds for hours on end.

Direct quotes of Chris Yu on another website forum (for Triathletes):

Hey all, I just skimmed through this thread and I'll try to hit all the major points. Bottom line, we're not out to bamboozle anyone - just trying to add another data point about something that people are obviously interested in!

I was pretty surprised by the results as well. The video wasn't the first time we ran the comparison, we've done quite a bit both with and without a rider and on a few different frames. However, we try and keep the videos as mass appeal as possible, which means short and sweet. The ST crowd is much more sophisticated, so talking about yaw sweeps, etc...makes sense here, but we've found not so much for a 2-3 min YT video. Anyways, we limited the video to 0 and 10 deg yaw (non-drive side upwind) since we've found that those were representative points from our past studies with full sweeps. We've been finding that with the DS upwind, there is a very minimal difference between rim and disc. Starting at about 10 deg on the non-DS, there's a small (in this case 8 sec/40km) difference which holds out to higher (20 deg) yaw.

The Di2/Mech bike was our screw up for the video. We had a tough time finding Rim+Disc Di2 bikes or Rim+Disc Mech bikes with the exact same fit build in time for the video so we went with what we had. That being said, we've done this test with bare frames and also various component mixes before, and the results do not vary significantly (on the Mech bike, there's effectively 1x additional stretch of housing from bar to side of DT - the rear brake housing to the TT contributes a very small amount of drag if cut correctly). Have not directly tested it, but it's possible the Di2 control box and additional FD volume offsets the additional housing drag on the mech bike.

As for the disc brakes - I always say there's horses for courses. As someone mentioned earlier, if you're in Florida and mostly riding in dry weather, then there's really not a functional reason to use disc brakes over rim brakes. And yes, even 8 sec/40km can be a lot for someone depending on what you're doing. My personal opinion having ridden both quite a bit now is that there are definitely rides and conditions where I would chose the disc bike. More fun? Definitely. Faster? Harder to quantify and it's going to differ depending on the rider (remember, not everyone's at the same confidence and skill level descending/modulating).

Also, I want to make super clear that I know the big S has this death star reputation but these videos are done by us nerdy engineers because we enjoy doing it. We have to ask for help shooting them and getting them posted - we're not told to do them as some kind of marketing stunt (although one person on Facebook accused us of being "models" reading off a script - haha, I wish!). So to reiterate: there isn't anyone pointing a finger at us pushing some agenda.

In fact, we've tried to make them as brand agnostic as possible to make the reach broader.

If you guys have ideas on how to do them better, I'm all ears!

Not able to watch the video right now- but is any consideration mentioned that a few years down the line we can have "aero" bikes where the disc brakes are accounted for, rather (eg) with the Tarmac somewhat roughly retrofitted into the design?

Maybe the question should be: how much slower do you go on long descents due to concern over the performance of rim brakes? Going down a soaking-wet Furka pass a few years back I couldn't let myself go above about 30kmh, as the rim brakes were so useless in the wet. It wasn't that discs would have been an advantage, it's that rim brakes are such a disadvantage.