Does Size Matter? | GCN Tech Investigates The Best Disc Brake Rotor Size For A Road Bike

[@gcntech]

Which rotor sizes would you prefer to use?

[@katcorot]

203F/180R Surly Big Dummy

[@Rover200Power]

I find 355mm vented discs works pretty well.

[@AnarchoKapital]

Considering that there was already a blocked back wheel, which means reduced brake performance at that wheel, I would go 160 front, 140 back. However I think bike manufacturers could even go smaller in the back. Also application of the bike should be considered: Do you ever ride in conditions in which you need more braking power, e.g. bike packing or very steep descents or in which braking power is reduced by wet or muddy discs, e.g. gravel or CX? That Aeroad is so sweet though.

[@somethingreal5042]

160mm

[@Pythonassum]

622mm

[@CentristRN]

Also shows the importance of practicing emergency stops.

[@frandalzotto7244]

I was able to save my life by training the emergency stops A few hours before he almost collided with a truck And it's probably going to end up under the truck.

[@Drago0900]

Nah I commute on my mtb and get brake checked enough by idiots to not need to. Makes me angery when they park in front of my and I can’t lock up my front wheel, while I was getting up to 20mph trying to hit 25.

[@DanTuber]

More impressed at how fast he can unclip

[@urh5734]

Gone are the days of Matt

[@Garfie1d73]

Matt certainly would agree 😂

[@dings2063]

Pls appreciate Alex's skills riding on his front wheel😂👍

[@Kimberly_Sparkles]

Those were pretty stops.

[@ronnirobert1214]

The next test should be "does price matter for braking performance?" please test for 105, ultegra and durace disc rotor, please thanks you for the test.

[@Max__apex]

No it doesn’t .... there all good enough to flip rear wheel up on 160mm rotors from the initial bite point

[@Max__apex]

They are but look carefully in the video and see the ease with which he can bring the rear wheel off the ground with 160mm.... getting to maximum braking force earlier when speed is the highest and distance covered more. In braking when travelling at 50m/s in a car if you delay max deceleration by 0.1 sec that’s will impact braking distance by 5 meters. So key is reaching maximum force as earlier as possible and that’s easier with 160mm since lever pressure is less. Also in the video he has to really queeze the levers to reach that point so due to confidence issue 160mm rotor makes it easier to get to that point. Basically 160mm easier to lift rear wheel and slow down :)

[@Minecraftrok999]

Yes they are, but your tires probably play a just as - if not even more - important role for stopping you, as you have so little contact to the ground.

[@Minecraftrok999]

105 will be good enough.

[@joshwilliams6849]

It definitely doesn't, but all I do know is that I have an Ultegra bike and I really want dooss sexy Dura Ace rotors purely for the black paint-job to match my bike ha.

[@esoxcycles]

Playing to your strengths, Ollie doing the maths, Alex riding as Ollie wouldn't have reached 40kph

[@berthull9333]

Ollie can average nearly 50 kph for over an hour.

[@abedfo88]

Alex's Canyon looks like an absolute weapon. Severe bike envy.

[@breitemasse]

but with a damaged Seatpost Like many others

[@JustAGlitchFL]

*severe bike enve

[@tomgruitt6563]

It's interesting on the last one that you started braking waaaaay before the man hole covers!!!

[@jerehada]

Did wonder if it was start of end of the man hole and it was consistently hit.

[@peterscuba]

Ollie was spot on with getting better at emergency stops being a factor

[@Dhungerf60]

Those are some impressive graphics, GCN has come a long way

[@jcsrst]

160 on the front, 140 on the rear.

[@markhaneklaus282]

seems they get limited exchange with their mtb collegues

[@TheSpinnerfish]

This, if only because the adapter on the rear looks shite.

[@antonpeterson1245]

Would have been good to see the mixed setup as Alex was locking up with the 160 rear

[@helmiwijaya7]

i love to see how Ollie is walking when he measure the distance :D

[@tflspitfire]

main takeaway: learn how to handle emergency brakes

[@ginti4725]

I tried this experiment out on mountain bikes when they went to disc's, and on Motorbikes, and i found the hole braking system and the rider combined Wil respond differently to any minor changing of components. Simply moving your hand to a position so you grab the leavers at their ends, means you can apply more power (longer leaver). How you feel or perseive mod's has to be taken into consideration, sometimes it's the rider not the bike that makes the difference.

[@grahamelliott6041]

Nice spelling

[@mbal4052]

@@grahamelliott6041 don’t be that guy

[@grahamelliott6041]

@@mbal4052 which guy is that ?

[@ginti4725]

@@grahamelliott6041 sorry, we all can't be geniuses. Some of us people have dyslexia.

[@grahamelliott6041]

@@ginti4725 you don’t have to be a genius to spell correctly or to use a spell checker

[@DirkDierickx]

and it gets more important when you do descents after big mountain climbs in the alps for example, your speed is very high there and a bigger disc cools better as well. the reason why mtb's also have very big discs, if they would have small discs they would overheat all the time.

[@DeKempster]

And on road bike the descent speed are much higher than mtb. Requiring more cooling. So yes size does matter

[@neildaniel8232]

It's like watching the same thing that MTBers went through around the turn of the millennium! The basic lesson is that you should use the disc size that best manages the brake temperatures you experience during your normal riding. That doesn't mean bigger = better, you need a big enough disc to cope with peak temperatures but not so big that it is too hard to heat up and generate the best pad grip on the disc. That usually means a bigger disc on the front to the rear. But you can also change the disc size to improve feel and reduce 'arm pump' on long descents. Basically small rotors for flat terrain, bigger for hilly areas. If you want a grabbier brake go up a size and smaller for more modulation. For the vast majority of people a 160mm up front and a 140mm out back would be a good starting point, that way you get decent power on the front (where the majority of braking is done) and better modulation on the rear (where grip is less as your mass moves forward, allowing you to reduce locking up). For newcomers to discs there is also the matter of braking technique too but that's probably another whole video in itself as it is subtly different to rim brake technique.

[@RoninCycling]

This is the greatest video on braking; full-stop.

[@davidparks8099]

I would be interested to see a Disc VS Rim brake science experiment. Keep up the great content!

[@JustAGlitchFL]

"bigger rotors* *Mountain bikers have left the chat*

[@bjoe385]

Big ol’ 203s all round.

[@JustAGlitchFL]

@@bjoe385 I run 200s on my trail/all mountain bike with SRAM codes. I love it.

[@chrisrye13]

Worth pointing out the logic in increasing rotor sizes for heavier riders, or those likely to be carrying luggage etc. When weight increases is when the braking performance of the disc becomes most important. As we saw, he was closer to going over the bars on the 160's so bigger isn't necessarily better for the same weight.

[@RicardoPetrazzi]

Meanwhile over at #GMBN we've been switching up from 160 to 180 and 200's now! When you are going downhill at speed, you need that extra stopping power!

[@thedownunderverse]

soon you'll be up to 700c brakes....... oh wait

[@piercepowers6448]

An interesting control would have been to also compare to rim brakes on a similar bike

[@prestachuck2867]

Yes! And use alloy rims with Dura-Ace integrated rim brakes!

[@lucabravi4332]

Check GCN italia for that similar compare

[@brendonnoble5227]

@@prestachuck2867 yes and in the wet.

[@rossdavies2343]

Been done I’m sure?

[@hernendezsanchez7646]

Oh there will be some tears, tantrums and long comments and posts if that happens. Lets just leave the rim blokes to themselves playing with their crayons

[@ErwinPfuhler]

Very impressive wheely! It is a pitty, that you missed considering a 180 mm rotor, which might be a valuable upgrade for some of us.

[@iangannon8543]

The benefit of 180s are heat dissipation, larger riders or lower grip strength.

[@myscreen2urs]

@@iangannon8543 and performing somersaults🙃

[@billlayer5117]

I run a 160 on front and a 140 on the rear. Having ridden racing motorcycles I know you can’t have to much front brake and the rear as little as possible due to the weight transfer during braking.

[@flightbyrd]

Yes! Rear wheel lock up and regaining traction causes high sides on road motorcycles. In this test on the bike, the rear wheel is providing limited braking and none at all when it is in the air.

[@mlee6050]

I think someone did that way or other way round to sort of balance out braking

[@damianward8138]

Brilliant pair presenting together! Ollie's love of the data combined with his facial expressions...👌

[@AnthonyLock92]

There is literally no point in Ollie being in kit in this shoot, Is it the only clothing he owns?

[@SaschaN]

From my perspective the best reason for bigger rotors is heat transmission. I had never problems that my disc brakes stops the wheel. My bigger problem is that my wheel tires (GP 5000) are slipping on the road surface (even if it is dry) or that I am afraid that they overheat. I have seen that Bosch has developed an ABS system like from cars for E bikes (I don't want all these electrical stuff on my bike, but there are innovations to solve this road surface slipping problems)

[@aberdeenal3234]

160mm both front & rear for me - always have stuck 160's on

[@amarmangaonkar7682]

I have 29 inch rotors on my road bike called rim brakes .🤣

[@tofadeisastart]

I’m sorry you’re still living in the stone age

[@All4Grogg]

Kinda doubt that, since my 622mm 24.49" are the industry standard, for better or worse. One needs decent pads and alloy rims to make them work well, even with the massive advantage in leverage. Disc systems are better at the high end, but rim brakes still get the job done just fine and with less complexity for those not entirely interested in buying into technically faster, but more expensive and less durable tech.

[@mgoo1713]

Do you have an anchor on a chain also?

[@All4Grogg]

@@mgoo1713 Alloy rims are less aero for the same weight, not heavier in most cases. Also, rim brakes absolutely do work just fine on grippier brake surfaces as found on alloy wheels. Unless you are severely lacking hand strength the tire will skid before the limits of the brakes are reached. Disc brakes are superior, but that doesn't make rim brakes dangerous.

[@TheObeseDuathlete]

And then it rained

[@tuudsavuudsa8508]

I made a presumption before starting the video. Wasnt expecting basically any difference. Result is surprising - considering the braking performance is a sum of many factors.

[@TringmotionCoUk]

Should have tested the mix option too, I suspect because you were doing endos, the stopping would be similar. To answer what do you get for more money, mainly weight savings. One set I have fitted at the lower end didn't have automatic pad advance, but back in the heady days of stock availability I frequently swapped the OEM levers for series ones. This is despite the inline connection not being sold after market - which is mad because it's useful

[@dansotelo228]

(FYI on Messages #1 & #2) If you're wondering, this is why hyper fast cars have massive disc rotors that are almost the same diameter as the wheel so as to give all four wheels as much leverage stopping power as possible, to keep heat down and extend pad wear. Now these brakes are truly like on & off light switches with absolutely little or ǹo modulatioǹ. To over come this problem they use computerized anti lock brake systems to give the driver a normal brake pedal feel. If you think about it, regular caliper brakes and the wheel is actually a giant 700c size disc rotor.disc brake. The first to realize this fact was HONDA in the late 50s as they were searching for a more powerful brake system for their roadraciǹg motorcycles besides the traditional hub drum brake. Incredibly they developed first V Brake with a fair amount of succes, exactly like the V brake on mountain bikes today! Hard to believe but this is what inspired British race car engineers to develop the first Lt/Wt disc brakes for cars. I might be wrong on this history but I read it in a Honda authorized history book 30+ years ago. I love it, Dan ( :

[@paullinnitt5450]

Suzuki ran a rim mounted disk in some GP RG500s in the 80s for maximum rotor size. Calliper was inside the disk. Search Ciba Geigy Suzuki. They also had a honeycomb composite chassis

[@dansotelo228]

Ahhh, those crazy Japaǹese they come up with great and crazy solutioǹs, lots to learn there.

[@dansotelo228]

I think it was Buel who ran those rim discs for a while on There sport bikes. When I was at Shimaǹo we looked into that same rim disc idea but it was not practical.

[@naturarum]

both wheels do completely different jobs, so for me it makes sense to give each one its own specific properties: disc brakes: 160mm front, 140mm back wheel depth: 40mm front, 50mm back (for example...) tire size: 25mm front, 28mm back tire pressure: 90psi front, 100psi back...

[@Anza_34832]

Please do a similar test, changing from resin over semi-metallic to metallic brake pads! 👏👍

[@whichdoctor4858]

Would be interesting to see 160F and 140R as a comparison.

[@NeutralGenericUser]

Thank you so much for this comparison, and thank you Ollie for insisting on retrying the test, because I agreed with your observation, and you were right. At a difference of 14%, as a non competitive average cyclist, I'll stick to the 140s that came on my Giant Fastroad Advanced 1 for the weight benefits.

[@AlexSquared]

yeah that 2 grams you saved will make a HUGE difference....lmao

[@Huttify]

Kudos for not chaning more than break rotors and changing back to the first ones in the end. A proper way to test!

[@freisianpug]

"I didn't expect that..." As a mountain biker who has ridden discs since the Naughties, I absolutely expected that. It's called Physics...

[@rongarfinkle]

Next time you encounter a recent generation tandem, check out the disc brake rotors. They’re usually quite large, 200mm or larger. It takes a lot of breaking power to bring two riders and massive frame to a stop.

[@breitemasse]

nice to know that the gcn Guys got the Same Seatpost issue with the aeroad

[@ml48963]

The larger rotors may also provide better feedback for modulation, which could help explain why they performed disproportionately better. I'm not sure of this, but it's a thought I've had for some time.

[@ayowser01]

That would be up to the master cylinder.

[@aflats]

I'm sorry to be that person... But Force X distance give a moment in Fmm not in F/mm. Gcn does science needs a scientist on board. I'm free if you're hiring?

[@rezoanalom9732]

Ollie has a PhD lol. Granted it's in chemistry and not in physics, but still 😅

[@hiddenSeeker]

@@rezoanalom9732 Ah wow, didn't know that. Now it makes sense that he fixed their explanation in the 'Critical mass' video

[@keithmaclure9101]

@@rezoanalom9732 being an expert in one chemical niche does not qualify you to pronounce on every science/ engineering topic thereafter...🤦

[@rezoanalom9732]

@@keithmaclure9101 no, but the physics in the video is high school-level. I'd expect a PhD in chemistry to be familiar with that level of physics. However, I'm sure it was just a small mistake.

[@keithmaclure9101]

@@rezoanalom9732 limited RAM filled up with chemistry knowledge?! 🤪🤣

[@kevinmills3329]

Can GCN do a tutorial on emergency stopping? I don't know how to do them and want to learn

[@Kingjay814]

My bike has two different rotor sizes and I was gonna swap everything to 160mm anyways. I already figured that there'd be better stopping performance, but I'm surprised that it's that much. Good thing I just got my new work stand.

[@christopherhood9241]

i’m building a gravel bike with campag ekar. interestingly the brake manual recommends larger rotators as rider/bike weight increases.

[@RudiJoubert]

Makes sense to have 140mm at the back and 160mm on the front. Would suspect performance to match 160mm test results, because the back wheel was airborne during each proper stop. The back rotor played no part when it mattered. Would be great if you could run the test with this variation as well.

[@tflspitfire]

I'm trundelin' , I' trundelin'.... Ollie during their 4v1 race :D ANyway, its good to see him inthese kind of videos, because he seems to be the only one who actually gets the technicalities. I loved his part in Hanks heavy hillclimb

[@SkyhawkSteve]

lessons learned: 1 - 160mm in front and 140mm in rear. 2- practice emergency stops!

[@jabehauber]

One key part of this decision is on display in the torque equation: the "N" part. If you are a heavier rider, you need more applied torque to stop. Therefore, the larger diameter rotor is the way to go. I am 88kg and run a 160 out back, only just recently upping from 140. Noticeable difference, especially since I like the downhills, which, in New England, invariably end at a junction. Note: Offroad, I run 180 on my trail bike and 200 on my enduro. I like being able to feather my rear brake with a bit more finesse to scrub speed while keeping going,.

[@unkebunktebusal9023]

The braking force that caused the deceleration was in all cases such that the rear wheel lifted off the ground and remained at about a constant height. So it was always about the same. Assuming, of course, that the speed, grip, etc. were the same. It does not matter how large the rotor is. A larger rotor only means that you have to pull less strongly on the brake lever. And in fact about 14% less strongly. It would be interesting to know what really caused the shorter braking distance. My guess is a mixture of practice and the placebo effect.

[@tomeklubomir]

You can discard the last 160 attempt of 5.6m, as Ollie says break at the “predefined breaking point” not before (4:29) and if you also discard the 140 fluke of 11m... Then you end up with much more narrow margin of 7.3% or as low as 4.4% of less stopping distance in the case of 160mm rotors (140mm - 8.7m 8.2m 7.8m avg. of 8.25m for 2highest vs 8m for 2 lowest attempts, 160mm - 8.5m & 6.8m gives avg. of 7.65m)

[@Tony-tq4tx]

Love u Olie, u're always so interesting. U'll always be our favorite presenter.

[@russstarke6004]

And now we know what size Alex should use under these conditions. What about rotor weight and aero drag over the course of a longer ride? Sprinter versus climber. Is it possible a smaller rider would do fine with 140's front and rear? A larger rider with 160s and more average size with a 160 front/140 rear? And what about a test where Hank has to stop with different size rotors before hitting an obstacle?

[@samuelchap]

I put a 180mm on the front of my gravel/bikepacking bike. It's there if I need it!

[@wayslow]

Don't. Unless your fork is rated for 180. Brakes put a lot of force on fork and it needs to have proper strength to handle those forces. I learned it the hard way.

[@samuelchap]

@@wayslow I believe it is, the current version comes stock with 180mm at least and mine appears to have the same overbuilt fork. I didn't change it to shorten my braking distance but rather to avoid brake fade when rolling down Devonshire hills while carrying (too much!) kit. I'm also running Sram Force which has a lot of modulation, you'd need to quite determined to lock the wheel from the hoods and I could easily exert the same torque by braking at speed from the drops with 140mm rotors.

[@dariuslankarian3282]

@@wayslow I believe thats only for carbon forks. For steel fork you should be able to upgrade from 160 to 180 with no issues.

[@wayslow]

@@dariuslankarian3282 I've seen aluminum rear triangles break on mountain bikes after switching from 160 to 180mm. While steel should be more resistant to those forces, they are applied to a different area with a bigger leverage. And the fork failing isn't anything you want to witness - there is no warning, you plummet to ground with your face first.

[@dariuslankarian3282]

@@wayslow i agree but with steel it would bend significantly before breaking carbon and aluminum have no give they snap when under heavy load.

[@101francis101]

I think the heat transfer is the most important thing - I was descending a 15 percent climb reaching about 45mph freewheeling and the 140mm faded on me, very close to ending up in the ditch, only managed to cut my speed in half before they had nothing left. I don’t know if 160 would have eliminated that altogether but it would have helped.

[@gen4drummer]

8:49 My BMC came OEM with 160F and 140R Shimano Ultegra. I recently (yesterday) upgraded the rear to a 160 rotor due to brake fade on steep descents. I had a scare in overshooting a hairpin when my rear lost its grip. Other than descending I have no issue with the stopping dependability of the 140. Time will tell.

[@vipergtsrgt1]

Most of that test looked pretty good, but that 5.6m stop was suspicious. The rear wheel was in the air before the front wheel had passed the sewer cover, so I think the brakes may have been applied too early. Still a cool video.

[@billincolumbia]

Nice stoppies, Alex! Use a nice big rotor on front. Dig a 180 or 203 out of the MTB parts bin. Then, you can go 140 on the rear. Every disc brake vehicle has a similar disparity in front vs rear rotor size and capability.

[@Rahulsom]

There are two independent rotors, each offering up to 14% improvement. If both brakes take on an equal force, that should give you about a 29% improvement. (1.14*1.14 = 1.29) If all the stopping force came from one brake, you would be stuck with a 14% improvement. So the 22% improvement is within the range. I think trying out 140/160 and 160/140 in addition to 140/140 and 160/160 will answer the question of how much stopping force comes from each brake.

[@tomgreenall3948]

breaking performance is determined by the area of the disk that is swept by the pad(s) not just rotor diameter. Assuming a 10 mm2 break pad for each rotor the 160 has 2100 mm2 more swept area per side than the 140. A larger break pad/caliper( think quad piston calipers) on the smaller disk would have the same effect but at a weight penalty.

[@PoulHansenDK]

As you can see on the video and this is also confirmed on motorbikes, the rearbrake is almost useless in an emergency braking as all the weight is on the front. On my racing motorcycle I had 2 x200mm on the front and 1x120 mm on the back. You save a bit of weight on the back disc.

[@flurgerbla7609]

Lots of people joking about 200mm rotors, but it would actually be interesting to see where the diminishing returns are for rotor size on road bikes

[@SuperGamingeek]

I was hoping you guys would have further explained the 160 front/140 rear setup. Cars and motorcycles run this setup because it helps with brake bias. You want your front brake to be stronger so that with equal brake force applied at the calipers (generally people don't squeeze one brake any harder than the other) your rear brake has much less of a chance of locking up given the weight distribution on a bike. I started running 160/140 and my back wheel has been locking up a lot less. Just purely anecdotally.

[@KaminKevCrew]

I think there are four main arguments for the 160mm rotor, one of which was discussed in the video (heat dissipation). Another argument is that a 160mm rotor has a larger swept area (the area that the pad runs over the disc) and so should wear out slightly slower than a 140. The other two reasons I can come up with for a 160 are directly related to the stopping distances achieved in the video: 1) By having 14% more torque on the 160mm rotor, that means that you’ll reach maximum breaking on the front wheel that much faster in terms of lever stroke. It takes less time to travel less distance, and since you can only pull your fingers so quickly, the more torque you have on a given wheel, the more quickly you’ll be able to lock up the wheels - just because you don’t have to pull the lever as far to do so, assuming that the pads and rotors have the same clearance. 2) This kind of relates to above, but I think the other reason the 160mm rotor stopped faster is that it requires less force overall to go into the lever in order to reach maximum braking. Fingers are incredibly sensitive, especially when it comes to lower levels of force. I think that the 160mm rotor will give you more feel because you don’t have to pull as hard, so you can ramp right up to the edge of grip/control more easily, and you can more easily modulate the brakes. For instance, I can much more quickly and easily squeeze something with about 50% of the force I’m capable of than I can with 80% or 100% force. I have a much lower cycle time with lower exertion, so I would more easily be able to handle the fine tuning needed to stop on a 160mm rotor as quickly as possible. A bit of a digression: for most of those emergency stops, the back wheel was lifting off the ground. That means that 100% of the braking force was going through the front wheel as soon as the rear lifted off the ground. I’d bet you’d be able to stop just as fast with a 140mm in back as you could with the 160 front and back because of this.

[@willwhite1987]

08:00 This is the main point: rotor heat accumulation and dissipation. The 160mm rotor has more surface area and has higher relative speed vs wind for heat dissipation, and more weight for heat accumulation. This will help tremendously while descending big climbs.

[@chris_tyson]

Surely we need a comparison now with 160mm on the front and 140mm on the back!

[@robertredziak6461]

I believe difference will be insignificant as during hard braking rear wheel barely or even doesn’t touch the road.

[@chris_tyson]

@@robertredziak6461 undoubtedly, but it would be interesting to put numbers to it as guidance to equipping/optimising bike set-up

[@robertredziak6461]

@@chris_tyson I think you may go for such mixed setup except some strange cases like mine: an oversized, almost 20 stones big MAMIL ;)

[@natdlareg]

Can you do a video for brake pads too? Which is best for what use (wet, dry, rider weight, etc)?

[@poxcr]

What about comparing how they perform in all other circumstances except emergency stops? Long fast descents, for example? What about discussing how brake action feels, it's not always about raw performance.

[@chrisj5443]

You can see Ollie gently reminding viewers there are some not-rigorously-controlled variables in this experiment, which actually serves to point out the importance of the rider's braking technique in this.

[@minimcewen]

Mixed. 160 front, 140 rear---better modulation. Braking only works with the wheel on the ground. Like with cars/motorbikes if you have too much bite in the rears it will upset the balance and handling.

[@MrTOON]

It's good that Alex did this, if hank would do this, he would make salto's every run

[@dudeonbike800]

Yes. Archimedes said it best about being able to lift the earth with a long enough lever. Larger rotors have greater leverage. More mass & surface area as well, thus better cooling. And this is why tandems run 180-200mm rotors in front.

[@Seppster58]

Great video guys. And to the point of new bikes with larger rotors on the front, I just looked at my new Orbea and it has a 160 on the front and a 140 on the back. Go figure. Thanks for pointing that out!! 👍🙏

[@paolocapozzi927]

another very important aspect is that larger rotors cool off a little bit faster. Not important in the UK I assume, but try and descend the 48 hairpins of the Stelvio in 35°C and you know what I'm talking about ;)

[@MOTORRAD_ONLINE]

Major outcome: you can reduce braking distance by 4m if you just practice 5 minutes!

[@kokongjava]

I thought you'll end up trying 180mm to 203mm rotors. For Science of course.

[@Digi20]

that is not possible for flatmount road bikes. 160 is max. there are some gravel bikes/forks appearing that accept 180mm in the front though.

[@artt3231]

Ideal set up being 160 front and 140 rear. Just like motorbike have smaller rotors rear side as 66% of braking comes from the front. Makes me smile these 160/160 standard set up from the factory. 😅

[@nicolasb5899]

Personally bigger rotors are more of a heat dissipation thing than absolute power for me. As a heavier rider who likes to decent fast, I’d love to run a 180mm front if my fork would allow. I wish you guys would test brake fade down a long decent, but still a very interesting video.

[@sandgroper1970]

I have also seen the professional teams use mountain bike rotors because supposedly they are better at dissipating heat than the ones normally used in road bikes

[@martinkrivicka497]

I'm just about to order 160mm for my front to replace 140mm, so the timing of this video is just about perfect 👍

[@spikev5842]

check the warranty on your forks

[@seanirving808]

Possibly an effect of Alex having more control over the braking force with the lower force needed at the lever? Better modulation of the brake so could hold the stoppie better

[@spikev5842]

I don't think the physics supports that there will be lower force at the brake lever if it needs to deal with increased torque

[@robertkerrigan4276]

Now try the same with 25 vs 28 mm tyres as well as tubeless, running recommended lower pressures, and tubed tyres at the recommended higher pressures.

[@csendiii]

Those emergency brakings.... I was wondering, how the hell would I clip out without flying off the bike, if I would brake like that. Chapeau!

[@mjv1967]

140mm likely brake similar to (relatively) rim brakes. That is a test I would like to see. I always found them weak on a mountain bike.

[@Hill_Walker]

I agree with Oli on the reliability of the data. Wouldn't be hard to do preliminary tests to dial in the method. Followed by the measured tests. An n of 10 for each disc would be much better, allowing for outliers to be removed.

[@ConfuzedCactus]

You’ll notice that most of the 140 attempts had the back wheel skidding. I think that because the 160’s provide a larger braking force, they move the centre of mass further forward than the 140’s, increasing the loading on the front tyre subsequently producing additional grip adding to the effectiveness of the 160mm discs (evident by the lifting of the rear wheel under breaking). This might explain the additional 0.8m reduction in braking distance you observed. The 160mm will dissipate heat faster too, it won’t have impacted this test but it would be a consideration in the real world. I ride 160mm for this reason (and because they were already on the bike) 😉.

[@stevewindisch2882]

Seems like traction would make more difference than rotor diameter. One of the stops with 140mm he was balancing on his front tire, which tells me the rotor was plenty powerful for that stop. I think the unevenly wet road surface and the inexact start to the braking zone had more impact on the stopping distance than anything.

[@sam_s_]

The larger rotor is not for stoping in a shorter distance. A larger rotor will do the following: -Allow you to stop without squeezing the lever as hard. -Allow better breaking modulation. -Resist fade and overheating on long descents.

[@mriguy3202]

YES! And a 140 mm rotor gives far better real world braking performance than any rim brakes. Please, GCN, bring in an engineer or science background person to advise you on the tests you run, because this one was so unscientific...by that I mean riddled with errors. For example, if your original data points are 8.7, 8.2 and 11, you have very noisy data. One way to fix that is to make, say, 10 braking runs. You need to bring in someone to tell you when your tests are invalid and miss the point. Carbon rims can fail when they overheat with rim brakes on long downhill runs, especially with heavy riders. So how about a test that measures the temperature of the rims on a long downhill, maybe laying on a 30 lb weight on the bike? Or three tests, one with rider alone, one with rider + 20 lbs and one with rider + 40 lbs? And also measure the temperature of the 180, 160, and 140 mm disks in the same conditions? And then how about measuring stopping effectiveness once you have heated up the rotors/rims? A disk brake rotor can also overheat and fail, but it will probably not break in to pieces and kill you that way. You guys recently tested aero effectiveness on a couple bikes. That test also was entertaining but really flawed. It could have been done so much better by putting the bikes at the top of a long gentle hill and coasting down in a controlled riding position. How fast were you going at the bottom? That speed difference can be mathematically correlated to watts....and that test also could be run using a non-aero bike. Many smart aero experts claim that riding position is far more important than the shape of the frame tubes; why not test that? Maybe you can have a 'Science Guy" (or Lady) to comment on your road tests in the videos. Just a couple minutes, not many equations, to explain the theory of what works.

[@olennmonkey1010]

Nice to see that you use the metric system :) !

[@letsgo_inc]

Run the biggest rotors you can. You can never have too much braking power provided you can modulate it. The weight penalty is slight for the added power and heat capacity.

[@LeonEvans_Guyver1]

Why on earth did you avoid (deliberately or not) the whole issue of rider weight?!?! Strap 20kg on your back via a backpack to simulate a typical 90kg rider and run the same tests. surely the most important factor in sizing rotors is how much you weigh as the rider!!

[@obikedog]

I'd certainly not put a 160 on the rear but for the front I'm considering it.

[@31.8mm]

did that same exact pads has done the bedding in process for both rotor size before test?

[@metamurph]

Rim brakes of course :-) I never bike in the wet. but my MTB 180 front...140 rear

[@Ikkei-4444]

Exactly why I run double 160 for my race bikes

[@kamucho]

It's been so dam long since I've seen/used/thought about a trundle wheel. Absolute scenes, absolute truth. Can I get a GCN trundle wheel of truth in the GCN shop?

[@davepratt9909]

Ollie, way to push objectivity to reduce variables and make Alex do a second run with the 140s!

[@pej0s]

you could also measure the temperature of the brake discs after braking

[@DamnitsAdriel]

Changing the rotors and also the brake pads too.. Since both of them had grove marks..

[@alutious]

i can see how the front rotor should be 160, since the rear wheel was lifting. For swapability I use 160. For my mtb i want cooling of a big rotor and a nice feel/modulation on a nice lever and caliper, so i use a 4 pot 203mm setup on my mtb. Offroad i have a 180 on my gravel bike's front. i have zero 140mm rotors. Nice test. I liked the emergency stop test that was done earlier, i think rim vs. disk, that was a real good one.