Disc brakes and rim brakes offer similar levels of aerodynamic performance in a head-on wind but rim brakes are slightly more aerodynamically efficient in a crosswind, according to research conducted by Specialized.
Researchers from Specialized trialled two Tarmac road bikes in their wind tunnel in Morgan Hill, California, one with rim brakes and one with disc brakes.
The differences between the two bikes were the brakes themselves, the hoods, and the rims. The testing was conducted with the same rider aboard each of the bikes.
“At zero degrees, or a head-on wind, there was actually no difference between the two setups,” says Chris Yu from Specialized’s Aero R&D.
“When we introduced [a] 10° yaw angle to simulate a crosswind to expose those disc brakes, we started to see a little bit of a difference. Over our standard 40km distance we saw that the disc brakes were about 8secs slower than the rim brakes.”
Those were the only angles that Specialized tested: head on and 10°, so you couldn’t say that this is the most comprehensive research ever conducted.
According to Specialized, the aerodynamics doesn’t give the whole picture.
“If you’re doing any rides with a lot of descending or tight corners, that little bit of extra confidence that the disc brakes will provide may wipe out those 8secs over the 40km. So it’s really the whole package and not just the aero that you have to look at.”
We’ve still not seen any research into the relative descending speeds of riders with disc brakes and riders with rim brakes. Most people probably suspect that discs will save you time on a descent by allowing you to brake later, but we’ve not seen any research that quantifies this. Point us in the right direction if you have.
33 thoughts on “Video: Disc v rim brakes: which is more aero?”
Maybe the question should be:
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.
KiwiMike wrote:Maybe the
Is there an echo in here?
DrJDog wrote:KiwiMike
Not really. The current conversation is mostly around performance benefits. That you can brake later, go harder, stop quicker. This wind tunnel test is about proving to TT fiends that the critical 16 seconds they gained by changing to a different type of jersey back-pocket zip won’t be completely negated.
For most people who don’t ride at the ragged edge due to a) not being that fit b) having a day job and c) not loads of cash to spend repairing broken bikes, this ‘enhanced performance’ sell passes them by. No wonder they think it’s all about marketing for the sake of it.
But if you tell someone they can maintain their normal, comfortable speed down a long hill in pretty much any weather and not worry about crashing due to poor brakes, or not get sore hands – that’s a reliability/safety sell – which is much more relevant to the masses.
KiwiMike wrote:
But if you
Thing is that doesn’t really sell it to me either because I will drag the brakes when descending regardless of the conditions and type of brake. I just wouldn’t fast descend a narrow country lane that is covered in potholes, gravel and grass up the middle however powerful the brakes are.
Besides they are only more powerful in the wet. My rim brakes on carbon rims are as powerful if not more than my BB5 discs.
The argument that works better for me is the lack of rim wear and no chance of the rim overheating.
Not able to watch the video
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?
Direct quotes of Chris Yu on
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!
I love my bike wrote:Direct
No way I am reading all that guff.
Plasterer’s Radio wrote:No
And you felt the need to not only tell us, but to quote it in its entirety?
truffy wrote:Plasterer’s
Well I assumed you were not clairvoyant.
Plasterer’s Radio
I can’t be arses to read it either. Sounds tiresome.
This isn’t an independent
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.
Quote:Real conditions in the
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.
‘I wouldn’t ride a disc
‘I wouldn’t ride a disc equipped bike’
This.
8 seconds over 40km doesn’t
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.
whether you like it or not
whether you like it or not …. 11 speed, disc brakes & electronic shifting will all become de rigueur on all top end bikes.
Wait… is someone with
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!
With slick tyres in wet
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.
i can get a 8kg bike for
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.
birzzles wrote: …… Cable
Why do so many people talk in such exaggerated, hyperbolic terms? Surely if cable disc brakes slow and stop a bike in a reasonable time they must be of some use. Perhaps you meant to say they are not quite as good as hydraulic?
mike the bike wrote:birzzles
Two reasons (there may be more)
1. ‘cos their brains can’t deal with the nuances
2. ‘cos they think it makes them sound more interesting. Like swearing.
:B
Sorry. A bit off topic.
I think in most real world
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!
Mid-season I switched to
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.
Bought a. Ultegra equipped cx
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.
Read what?
Read what?
@earth
Avid bb5 one of the
@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 😉
hampstead_bandit
Agree on the BB5’s I have them on my CX, and they are a royal pain in the arse! I have just put new campy brakes on my road bike, and they are much better than my old campy’s, but pale into insignificance next to even poor disc brakes.
For those proper cyclists who
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.
I wish I was good enough to
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.
Having used various 105 and
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 <:P
Seems most road/cross
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?
“aeroiseverything” or not?
“aeroiseverything” or not?
When you look at data for
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
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???