I've got Shimano BR-M575 brakes front and back on my Genesis with 160mm rotors. One of the rotors is a bit warped and looking for a replacement part I see I can get a 203mm Rotor and a different adapter for just a few quid more than just replacing the 160mm Rotor.
There's no clearance issue and I guess it only adds a few grams
So is there a good reason not to go bigger? seems like a no Brainer to me and bigger is always better right 🙂
Clearance and what the bike/fork is rated for.
If all that's good, then crack on.
Minor things are that bigger rotor are easier to bend on rocks and stuff, slightly heavier and more expensive, and having different sized rotors front and rear means you need two spares instead of one. Minor concerns. I run 200 front, 160 rear on my bikes.
Kinda depends on the Genesis
You know you can just bend rotors back into shape really easily don't you?
I'm not sure I buy it, but this article argues that it's better to have a smaller rotor on the front: https://enduro-mtb.com/en/rotor-size-myth/
Also in winter/cold/wet conditions, smaller rotors 'should' warm up faster than bigger rotors.
You know you can just bend rotors back into shape really easily don’t you?
Not if you mangle them on a rock.
I’m not sure I buy it, but this article argues that it’s better to have a smaller rotor on the front:
That article is nonsense.
Depends
All IMO after much reading around the topic so a slightly informed layman
A disc brake relies on getting hot for best performance. "Cold polishing" exists when you never get them up to operating temps. so if yo are a gentle braker and / or ride in flatland then the smallest you can fit. If you are a hard braker and regularly get the brakes really hot then maybe go bigger. All my bikes run the minimum size and never found wanting.
I believe that a lot of the issues folk have with lack of power and squeel are down to brakes than never get hot enough
No problems if it fits. There are a few frames that have problems, but they’re usually well documented so if yours was one you’d know. I’m a big fan of being over braked rather than under braked for sure.
You don't need rotors to be perfectly straight, just straight enough, and it is fine to just do it by eye. I have done this countless times, I couldn't afford to replace rotors every time they went out of true!
I’m a big fan of being over braked rather than under braked for sure.
As TJ mentions above there is some nuance to this. You want a bit of heat for better performance, but also need to regulate that heat so they dont get too hot and glaze the pads or worse.
How many times have you seen someone on t'internet talk about upgrading their brakes/rotor size because the stock ones didn't give them enough. Then they still come back and find them underwhelming?
I've seen people say hope E4s with 180 rotors are not man enough for trail centres!
Don't forget to add a click or two on your rebound damping to take account of the change in unsprung weight.
Might speed up the onset of the dreaded creaking CSU. Alternatively it might not. Second sentence is for the inevitable STW reply.
+1 for TJs point.
I've generally run smaller rotors on my singlespeeds as the pads ultimately last significantly longer.
My thinking is that pads bed in when they're hot. And then that surface is worn away. If you don't get them hot enough again before that hard surface layer is worn away then its not replenished and you end up with rapid wear.
The summer bikes get bigger rotors because
-the pads aren't being abraded by wet grit.
-the trails are dry and gripppy so that power is actually usefull.
We used to get literally thousands of miles out of pads on the tandem which was used off road - IMO because they regularly got hot enough to blue the discs.
Size doesn't matter; it's what you do with it that counts...
+2 on TJ's point re getting enough temp into the pads.
203f/180r on my MTBs as that feels right to me. They get hot enough to discolour the rotors and sizzle, but never lack power. Had brake fade once only, ever: front brake on a fast, steep, long alpine descent, i.e. 0.01% of my riding.
180f/160r on my commuter with organic pads. Braking is more gentle/infrequent and I was glazing pads often with larger rotors and sintered pads. 160 would likely be enough on the front but my fork mount won't allow smaller than 180.
On TJ's point you can go too far, I've just today taken delivery of a 160mm rotor for the rear of my fat bike as the 140 was getting too hot slowing the beast down and I think I've glazed the pads 😁
I fitted the 140 thinking I'd never get it fast enough to need anything bigger, wrong!
Well eitherway I might give it a go at some point, but in the mean time if i can straighten the old one I suppose I could do with a better tyre on the front first, that should probabaly be the priority.
It's actually an Alfine 8 geared Hub that one and I'm 94Kg before i put anything on so I'll probably get the heat into them 🙂
I've got the same brakes (hope v4 4 pots) front and rear on both my bikes, but 180/203 on the FS that is ridden more in summer, and faster over rougher and steeper terrain, and 180/180 on the hardtail which gets used more in winter and is generally ridden a little slower.
The 180 discs were getting hot enough to stop honking at a mellow local trail centre at the weekend.
None 220 front and rear remember skids arnt just for kids
I think smaller rotors were still a throwback to when MTBs were modelled on road bikes and weight was the main priority. People seem to be realising that saving a hundred grams to end up with worse braking performance is stupidity. I was 200mm f&r but as I needed new rotors I've gone 220mm front.
All my bikes run the minimum size and never found wanting.
You need to up your game TJ 😉
I think smaller rotors were still a throwback to when MTBs were modelled on road bikes and weight was the main priority.
Yeah, I remember when forums were full of arguments that disk brakes were unnecessary and rim brakes were lighter and gave better modulation, plus the silly advice like running 140 mm aluminium rotors with only three mounting bolts to save weight. Still, those people were probably running 1.8" tyres at 40 psi and didn't have enough grip to do much braking anyway.
Something can be sufficient for what you do but it doesn't mean it can't be improved upon. My rim brakes on my road bike bring me to a halt so in that respect they do their job but I'd rather discs if I'm descending an alp.
I miss my IOID. That's all! I ran 180 front and 160 rear on mine. Worked fine with the standard (very good) Shimano brakes.
The Alfine just worked. Loved that bike. Not quite sure why I sold it!
I've 160/160 on my Gravel bike, 180/180 on my HT and 203/203 on my FS.
The power they generate feels appropriate to their usage and bear in mind I live in a hilly area. When I lived down south, I ran 180/140 on my HT.
You can never have too much brake, but for the Genesis I'd probably go 180 rather than 203 at the front.
I've pretty much always bought rotors to suit that terrain and speed. Enduro has 200/180 and xc bike 160front and rear. I will say that I don't see any reason not to have decent brakes though. Code, Saints, E4 whatever, but just buy the best you can afford.
Never have any issue getting my brakes hot enough to work and in fact my Maguras work from absolutely stone cold, so not sure that's an issue I can relate to. I wouldn't run less than 200 / 200 on an enduro / aggressive trail bike and I have 223 / 223 on the E-Bike. My local is pretty steep though.
took a 203mm off the rear of my FS in favour of a 180.
Its an orange, so i found that it was causing savage brake jack every time i looked at it.
Other than that you need to bear in mind what the frame/fork is rated for (probably no bigger than 160 on a road or pavement bike)
and are you at the limit of the tyres? no point in being able to lock up the wheels easier on a pavement bike, itll just make you unstable.
off road though, with a good contact patch and dirt to hook into, more is betterer.
took a 203mm off the rear of my FS in favour of a 180.
Its an orange, so i found that it was causing savage brake jack every time i looked at it.
Did it actually stiffen less under braking with the smaller rotor?
I feel it did, though thats totally annecdotal/subjective.
Even the new budget 32mm RS forks (Sektor, etc) are now rated to a 220mm rotor so I don’t really think there’s much to worry about. It’ll all come down to how heavy and fast you are really. I’m currently 100kg (COVID + dadbod) and have a hardtail, but at the moment I tend to ride at FOD on the trails there so still go pretty fast and upgraded to 180/180 (as I had the rotors and adapters already), if I need to I’ll chuck a 200mm on the front in the summer.
If you are lighter or ride tight, janky tech at relatively slow speeds you might need less breaking power.
But I still can’t see many reasons not to run a biggish rotor for most things.
I feel it did, though thats totally annecdotal/subjective.
I have 180mm rotors on the back on my Oranges. I've not gone bigger because I had a load of discs to use up.
Maybe I'll just keep them like that forever now.
I’m not sure I buy it, but this article argues that it’s better to have a smaller rotor on the front:
The first time I saw it, I thought it was tosh.
But doing some thinking while actually riding, I have wondered if bigger rear discs would make sense for steep trails.
But doing some thinking while actually riding, I have wondered if bigger rear discs would make sense for steep trails.
Theres probably some limit with modulation versus all out power that makes sense in some situations but not others.
Plus I guess there is the argument that you use the rear brake as a general speed retarder, and the front to fine tune your speed in reponse to the immediate upcoming obstacle.
I honestly dont think I'm this good, I generally use both together and then adjust/modulate to find the traction limit.
But if anyone reading this is thinking "dont use the front brake on steep trails you'll go OTB" then they need to learn to ride their bike with its 2 brakes as is; before adding to their rotor size or number of pistons.
But doing some thinking while actually riding, I have wondered if bigger rear discs would make sense for steep trails.
I don't think so. The article was about brake dragging on steep descents. If you're racing down stuff so fast and steep that you're burning out your rear rotors, you need big front rotors too. When you really need to stop, it's your front rotor that does the work. Putting a big one on the back is fine, but putting a small one on the front is silly.
I live in Yorkshire. 203mm discs always fitted front and rear if the bike or fork will take it. Gravel bike has 180mm front and rear.
Why not fit bigger discs? It’s never going to be worse and it’s barely any extra weight.
Why not fit bigger discs? It’s never going to be worse and it’s barely any extra weight.
As above - If the terrain you are riding in and / or your riding style means you never get them hot enough you either get "cold polishing" of the pads leading to squeal and poor performance or you get rapid wear
As an anecdote - I used to have a BMW motorcyle with big brembo discs. I never used the brakes hard. They were OK. I lent the bike to a pal who is much harder on brakes than me - when I rode it again they worked much better. My gentle braking style had "cold polished" the pads, His hard riding style had conditioned the pads to perfect braking.
Best is the correct size for the riding you do so they are more often in the correct heat range
I really don't think that's an issue, if they're uncontaminated and bedded in properly then they'll work just fine. Perhaps they might wear a little faster but the main thing is to be buying the right compound for where you ride.
It really is a thing. This article describes it well referring to it as "mechanical polishing"
Glazing
Glazing can be identified as a hard, glassy surface on the friction pad. There are two common types of glazing. The first is “mechanical” which occurs typically at low temperatures and operating pressures. The second is “thermal” which occurs due to excessive operating temperatures; those exceeding 350° F
https://www.montalvo.com/article-library/in-depth-look-brake-noise/
That's very theoretical for MTB rotors just 20mm bigger though.
I doubt there's a significant difference in the heat they reach.
I have seen this happen both on motorbikes and MTBs
All I know is that I don’t get brake fade any more with 203 rotors, I did with 180 rotors. If you are only ever braking lightly enough to gently polish the rotors then did you need to brake on the first place?? 😂
This article describes it well referring to it as “mechanical polishing”
It also suggests that it's a problem for constant slip application on conveyor belts rather that stop start applications like cars. i.e. Don't drag your brakes.
I prefer a smaller rear disk, as when it’s too grabby, it’s harder to feather/control.
Manuals and wheelie type manoeuvres are more controllable like this I find.
Front, I just need a big anchor with full power.
Surely the answer to the possible problem of bigger rotors not working as well is to brake properly rather than bumble around dragging the brakes half on all the time.
