What size rotors do you recommend?

[Noctiluxx]

I'm building a 2019 Cervelo R5 Disk with Dura Ace 9170 drivetrain. Complete R5 disk models come with 160mm F&R. I'm anywhere from 148-152 lbs and do lots of climbing out here in Southern California. Is going 160mm front and rear overkill?

[joejack951]

Similar weight here. I went with 160F/140R and can't see how I'd benefit from a larger rear disc. I could probably get away with 140F/R but that's uncommon enough that I decided to not bother.

[LiquidCooled]

At your weight, 140mm are probably enough. According to Shimano, the weight difference is 18 grams per rotor:

Average weight
118 g (160 mm), 100 g (140 mm)

(from https://bike.shimano.com/en-EU/product ... RT900.html)

I’d just wear out the 160mm rotors and then when the time comes to replace, go with 140s.

Edit: Alternatively, you could go with a 160/140 arrangement as mentioned above. Just buy a 140 rotor for your rear and keep the stock 160 rear as a replacement for the front when it wears out.

[HShimada]

I would take the extra 18 grams to be honest. It's not just about stopping power but also less risk of overheating with a bigger size disc AFAIK. I'm 143lbs don't have a disc bike yet but I'll be getting a 160 front & back

[IrrelevantD]

I'd say just go 160 if it will fit.
Aside from a slight difference in weight, I don't know that there are any real disadvantages of going with 160mm in the rear. All the weight will be close to the hub, so it's not going to be as much of a rotational weight penalty (won't affect acceleration). If you've got a lot of hills and will be dragging your rear brakes a lot, you're going to have more leverage and lower heat buildup with the larger rotor, so benefits there.

[okimy]

I weigh about 65kg (145 lbs) and ride 160 front and rear. The 160 rear is overkill, if you do an emergency stop, you will likely lock up on the rear. When braking for corners there's no problem though. I'd stick to 160 front as I like the braking feel and the stopping power. Not quite the 180mm on my MTB but not far off either.

When my discs eventually wear out I'll go 160 front 140 rear.

[RedbullFiXX]

I preferred the feel of 140/140, but @ 165lbs, the 140 fr would fade during extreme braking on steep decents here in SoCal
You may be ok with 140/140 at your weight

My cross bike is 140/140, and that is more than enough off-road

[Bigger Gear]

I have a setup with 160 f/'r and honestly if I was doing it again I would have a 140 rear. 70% of the braking is done with the front brake.

[joejack951]

I have a setup with 160 f/'r and honestly if I was doing it again I would have a 140 rear. 70% of the braking is done with the front brake.

In my case, I rode for many years on a commuter bike I built using a rim brake CX frame but with a disc fork. Even while heavier (~75 kg) and riding a heavier bike plus a pannier (probably 15 kg total compared to 7.5 kg for my current full disc bike) I managed to get by quite comfortably braking nearly 100% of the time using only the front 160mm rotor. My rear brake track looks sparkling new after ~40k km. Hence why I assumed and was correct in thinking that I'd be fine with a 140mm rotor. I actually use the rear brake now that it's a disc but I know I could stop just fine with only the front.

[emotive]

One disadvantage of 160 rear is that it is more difficult to fit into the frame when swapping wheels. Perhaps not a consideration on an R5 but it is on a gravel bike with multiple wheel sets.

Another consideration is that larger discs are easier to bend while in storage or during transportation. Again more an issue with multiple wheel sets than with a road bike on a single wheel set, but you can bend a disc just by accidentally leaning against it.

Last consideration is aesthetics. I like the look of smaller discs.

I have 140/140 on my road bike and 160/140 on my gravel bike. I did Italian Alps with 8kg of luggage on the gravel and At 75kg I never wanted for more stopping power even on 30km descents.

[IrrelevantD]

One disadvantage of 160 rear is that it is more difficult to fit into the frame when swapping wheels. Perhaps not a consideration on an R5 but it is on a gravel bike with multiple wheel sets.

I have more of a problem getting my RD out of the way of the cassette than with my rotor. Shimano RXs have some pretty tight springs.

[Bigger Gear]

One disadvantage of 160 rear is that it is more difficult to fit into the frame when swapping wheels. Perhaps not a consideration on an R5 but it is on a gravel bike with multiple wheel sets.

I have more of a problem getting my RD out of the way of the cassette than with my rotor. Shimano RXs have some pretty tight springs.

The additional wrap of the Shimano Shadow derailleurs (9100, 8000, GRX) also makes wheel removal a bit more difficult.

[IrrelevantD]

One disadvantage of 160 rear is that it is more difficult to fit into the frame when swapping wheels. Perhaps not a consideration on an R5 but it is on a gravel bike with multiple wheel sets.

I have more of a problem getting my RD out of the way of the cassette than with my rotor. Shimano RXs have some pretty tight springs.

The additional wrap of the Shimano Shadow derailleurs (9100, 8000, GRX) also makes wheel removal a bit more difficult.

Yea, the RX just seems a bit even more so than the 8000, but then I'm also dealing with GS vs SS cages. And wow... I just completely derailed this thread. Someone get this back on toppic.

[LiquidCooled]

My TCR came with 140mm f/r. I switched to 160/140 f/r, and then to 160/160, which I still have. I never noticed any difference in how easy or hard it was to put on a wheel, front or rear, due to rotor size. Not saying there isn’t a difference, but if there was one, it was marginal.

[DaveS]

Rear braking needs are far less than the front. Should be no need for 160s.