Rotor sizes

[Dangals]

Looking at upgrading my 2 pot XT's to some 4 pot goodness but unsure on rotor sizes.

Was originally looking at going all out and putting 223 front/back but was advised that my frame doesn't officially support anything bigger that 203, which I assume is probably because they haven't stressed tested it to support the bigger rotor and won't guarantee it.

Has anyone fitted larger rotors than intended and what sort of issues would it likely cause, stress on the frame or is it a clearance issue?

Curious for people's thoughts

 

[beeb]

What size rotors are you running now? Going from 180 to 203 will be a fair jump in braking power. Just going 2 pot to 4 pot, I dunno - depends a bit on the 4 pots you choose.

 

[Nambra]

Check your fork specs - they often have a maximum rotor size; typically 203mm or so on longer travel single crown forks, but might be limited to 180mm on XC forks or those with thinner stanchion diameters.

The maximum rear size is usually limited by frame clearance, but the bigger the rotor the more likely you'll be locking up the rear when you grab the brake.

 

[Dangals]

What size rotors are you running now? Going from 180 to 203 will be a fair jump in braking power. Just going 2 pot to 4 pot, I dunno - depends a bit on the 4 pots you choose.

Running 203 front and 180 rear - combined with 2 pots just feel underwhelming on long descents. This was further emphasised with some recent posts indicating people pushing for larger rotors etc.

Check your fork specs - they often have a maximum rotor size; typically 203mm or so on longer travel single crown forks, but might be limited to 180mm on XC forks or those with thinner stanchion diameters.

The maximum rear size is usually limited by frame clearance, but the bigger the rotor the more likely you'll be locking up the rear when you grab the brake.

Forks are 2020 Fox 36 Factory with 180 post mount - fox have updated all their material online for the 2021 range and that is stated as supporting up to 230mm rotors so just assumed it should be ok. If someone has a link to anything to say otherwise I'd love to see it as google has not revealed much so far.

If it is a clearance thing that makes sense I guess just didn't think the increase in pressure on the mount between a 203 and 223 would be that much to pressure the frame to damage.

 

[Oddjob]

XT 4 pots with 203mm rotors and good pads (Shimano sintered, Uberbike racematrix) are more than enough for almost anyone. I'm heavy and a chronic brake dragger and the only place this combo has come up short is at the end of a very long run at Maydena.

 

[beeb]

In my experience, bigger rotors will give more power but keep a similar power delivery - ie: if the brake's power delivery is progressive ("has good modulation"), it'll be still progressive - if it was aggressive it'll be still be aggressive, just with more outright power. You can also get more power by changing to certain 4 pot system, but getting to try before you buy can be tricky.

It would be tempting to try just the rotors first as the VFM is a bit friendlier, but maybe just double-check compatibility of rotor with the fork. The 2021 Fox 36 have new lowers from memory (curvy arch type now), and the max rotor allowed may differ on the earlier models.

I'm with @Oddjob on this though and unless you're riding world cup downhill or running a very heavy e-Bike, I can't imagine any situation where you'd need a 220mm+ rotor on the rear. Maybe on the front if you love doing stoppies, but probably 4-pots are order of the day.

 

[Dangals]

just double-check compatibility of rotor with the fork.

I just found something on a resellers site that shows max size of 203....seems that is what I am stuck with unless I want to void my warranty.

Going to go 203 4 pot front / back and see how it goes.

 

[nexusfish]

I just found something on a resellers site that shows max size of 203....seems that is what I am stuck with unless I want to void my warranty.

Going to go 203 4 pot front / back and see how it goes.

Get metallic pads. That will make a very big difference.

 

[Tubbsy]

Some interesting thoughts on the topic here:

https://enduro-mtb.com/en/rotor-size-myth/

 

[moorey]

Some interesting thoughts on the topic here:

https://enduro-mtb.com/en/rotor-size-myth/

I have tried this a few times...and it really didn’t work for me. With shimano, there’s a lot better feeling of control with a large rotor up front. Bigger on the back, and I just seemed to lock up more.
That said, the article wasn’t really about a 75kg Gumby not riding hard. That said, I definitely brake a lot more than an elite DH rider.

 

[Dangals]

Some interesting thoughts on the topic here:

https://enduro-mtb.com/en/rotor-size-myth/

yeah that did sway me to consider the 220/223 on the rear as well but looks like unless I want to void my fork/frame warranty I need to stick with 203 max front/back - so just going to upgrade to a decent 4 pot system and see how it goes

 

[The Duckmeister]

I disagree with their assertion that resin/organic pads have better modulation. The soft compound bites harder than metal, but has a narrower operating band because it starts to cook and glaze (fade) under heavy load. Metal bites more gently due to the hardness, but really dials in under more lever pressure + system heat, so you get a much more controllable brake.

 

[coarselanguagw]

I upgraded my Shimano 4 pot MT520 calipers to SLX while still keeping the same levers. I noticed an improvement and can recommend the SLX which is cheaper than the XT.

 

[Ultra Lord]

That said, I definitely brake a lot more than an elite DH rider.

One thing the pro’s have on us mortals however is how bloody hard they pull those skid sticks. They migh not tug on em often, but when they do. Ooft.

 

[Cardy George]

One thing the pro’s have on us mortals however is how bloody hard they pull those skid sticks. They migh not tug on em often, but when they do. Ooft.

Technique and timing go a long way too. Pin the brakes when your weight is pushing the bike into the ground, not when it's floating over loose shit.

 

[Tubbsy]

I have tried this a few times...and it really didn’t work for me. With shimano, there’s a lot better feeling of control with a large rotor up front. Bigger on the back, and I just seemed to lock up more.
That said, the article wasn’t really about a 75kg Gumby not riding hard. That said, I definitely brake a lot more than an elite DH rider.

Yeah, I don't have an opinion either way as I haven't experimented much.

For me at my size running Hopes at 180 at the front felt a little underbraked, now with 200 f and 180 r balance feels right. A sustained descent like at Thredbo might well make the case for a 200 at the back but I don't think my frame supports that.

 

[moorey]

Back in the day, my 222 with floating brake would only accept 160 rear max, on a DH bike....with pretty average brakes.....I was a lot faster then, I wonder if there is a correlation.....