Every so often I get some time off and we go to the mountains to hike, and have recently started taking our bikes. There’s not much in the way of purpose built offroad trails, and what there are are more for hiking, paved leading to soil, leading to rocky areas. Some are rideable. But there are mountain roads where you can bomb down at silly speeds with great views. I wouldn’t take a roadbike there as the roads can be a bit dirty, or have fallen rock obstacles to quickly skirt around (earthquake zone, this region got hammered back in 2008 and some places still not repaired).

One day we’d got 5km up, and when we were freewheeling back down I was using my brakes short & hard, sparingly, trying not to overheat (Shimano MT420 4pot with 203mm splined up front, RS Revelation boost fork) I was on and off the brakes, short, hard braking, trying not to drag my brakes but when I got down the bottom my front rotor was a bit malformed and I had to get the rotor straightening tool out.

I’ve seen some discussion about floating rotors (perhaps limited to 203mm?) resisting warping better? Are there options for 223mm? Or am I just attempting riding that’s rarely done so just have to deal with it by carrying a heavy tool with me on these rides?

I find brake pads have a significant influence on heat build up. This is compound first. Shimano get v.hot in my experience, I use UberBike Kevlar or Race Matrix and they run much cooler and cope with heat better. It was eye opening in the Alps how much difference it made – same descent, same day, two different pads after burning up Shimano ones…

I’m not sure if you can get finned pads for M420.

I also find new fluid helps – when did you last change the fluid?

I’ve no experience of bigger or different brand rotors. My M420’s, previous Magura MT4’s and Deore M615’s all have stopped my 13+ stone arse in the Alps, Scottish hills, bike park Wales etc. I’ve had occasional brakes too hot moments and a drop of power.

We had a set of MT5’s in the house – they were another League of power and dealing with heat. My son did two Alps seasons and once managed to set the paint backing of the pads smouldering(!) but they just keep working like a stick in the spokes repeatedly. The fluid did need changing more often.

To add – what brand rotor?

You could try something like Shimano ice tech rotors and finned pads?

That and the heavier the rotor the more surface area it has, and the thicker the spokes probably are so don’t be a weight weenie when picking them.

Yeah floating rotors will help, that’s why they make them. The braking surface is a ring shape – if that tries to expand, and it’s fixed to the spokes of the rotor which are cold, it’s got nowhere to go but sideways so it can buckle. A floating rotor is not directly fixed to the spokes/inner spider, so it can just get bigger and stay true. It’s more obvious on Hope rotors but I think the Shimano ones with the alloy spider also float.

Magura do a flaoting rotor one. I’m always tempted but price makes me change my mind

Thicker discs, 2mm

https://galfer.eu/bike/wp-content/uploads/sites/3/cat_bike_2021.pdf

246mm rotor? (link)

Bike is a Marin San Quentin 3 (2021) Calipers are M420 (Deore 4pot) Rotors are SM-RT64 (SLX?), levers are BL-M4100 (Deore), judging by the sales website, what’s on my bike and pics on the Shimano website.

I’m in China and can’t find any Uberbike pads for sale, or centerlock Hope rotors.

Galfer has mostly moto-x parts here, no mtb floating rotors, BUT….. one shop does have the fixed waved centerlock 203mm 8mm rotor (DB004WCL) that’s in that catalogue for £56. Without buying utterly new brakes (and I want to replace the levers for 1-finger levers) will just solely this rotor replacement for the front prevent warped rotors on massive descents?

Most of my riding is on the flat and urban drops/stairs, with the occasional visit to the mountains where, frankly, it could be considered “Alpine descents” and I know, really, I should have better brakes all round but it’s that “occasionally” bit.

Cheaper shimano bikes can often have resin pads, and they do some cheap “resin only” rotors, sou ds like you might have those. Any standard rotor and decent pad will fare better.

Don’t know what the pads are. They fade with heat though. A bit on/off when cool, but lacking in power when going downhill and dificult to grip the bars and use both fingers on the lever to get any decent stopping power when bouncing around.

The rotors state for both types of pad on the shimano website.

Will require a chance of brakes but dot 4 fluid has a higher boiling point, and sintered pads are less affected by heat.
If youre finding your rotors are getting too hot and as you say fading performance, then bigger rotors will only prolong that effect for a bit of time, and maybe not enough .

Is an 8mm thick rotor not for a motorbike?

Another shout for Galfer here. Their fixed (non floaty) discs come in a range of sizes but also widths which I think is cool.
Currently on a 2mm wide 223mm up front. Had to take it off over winter though as I just couldn’t get any heat in the brake!

Will require a chance of brakes but dot 4 fluid has a higher boiling point

Won’t make a difference, were not talking about brakes boiling.

, then bigger rotors will only prolong that effect for a bit of time, and maybe not enough .

No, bigger rotors reject more heat into their surroundings. If you design a system from scratch it’s their principal advantage (You could get more power for less weight just by having a larger hydraulic ratio).

Hope V4 vented rotors are what you want.

Magura mention that as pads wear they dont dissipate heat so well.
At about 3:20 in

“…short, hard braking, trying not to drag my brakes but when I got down the bottom my front rotor was a bit malformed and I had to get the rotor straightening tool out.”

Heat sink

At 15:30

No, bigger rotors reject more heat into their surroundings.

We’re talking 20mm bigger. I honestly dont believe 20 mm is going to completely dissipate the heat to such a degree that the problem will go away.

“ We’re talking 20mm bigger. I honestly dont believe 20 mm is going to completely dissipate the heat to such a degree that the problem will go away.”

10% more leverage, 10% more area, 10% more rotational speed vs the still air, 10% more thermal mass, etc, it all adds up.

If the current rotors can only dissipate 2000W at their max temperature before buckling and the bigger rotors can do 2200W, but the maximum braking you’re demanding stays below 2100W average, then the bigger rotors will work whilst the smaller ones won’t. But realistically that 10% increase in diameter will gain more than 10% max brake power.

What he said —-^

It’s also more than just the brake track increasing in area, the spokes are generally wider, and the whole disk thicker.

Just picking shimano rt66 as an example, the 160mm rotor is 113g, the 203mm is 227g.

So 33% more leverage for immediate stopping power. But >33% more* area to dissipate heat and 100% more mass to absorb it before it can be dissipated.

10% more leverage, 10% more area, 10% more rotational speed vs the still air, 10% more thermal mass, etc, it all adds up.

If the current rotors can only dissipate 2000W at their max temperature before buckling and the bigger rotors can do 2200W, but the maximum braking you’re demanding stays below 2100W average, then the bigger rotors will work whilst the smaller ones won’t. But realistically that 10% increase in diameter will gain more than 10% max brake power.

Righto 🙂

Hope V4 vented rotors are what you want.

Hope V4 rotors are 3mm thick and will only work with Hope V4 calipers

Ive got them on my E-MTB and they haven’t let me down yet, bike weighs 24kg + im 95 kg so that’s 119kg they are stopping, also running the purple Hope E-bike brake pads (made by Galfer)