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My brompton has disc brakes. I believe effective wheel diameter is 419mm (349 rim + 35mm tyre), compared to 698mm for 700c + 38mm tyre. I have 160mm rotors, which for stopping power would seem to equate to a somewhat unbelievable 267mm rotor on the larger wheel (160 * 698/419).

I live in one of the flattest parts of the UK so don't stress my brakes much. I am planning to cycle around the UK, some parts of which do have hills. My total weight (me + brompton + luggage) is likely to be around 120kg.

My understanding is that on normal big wheel bikes, 160mm rotors are okay for stopping power but something larger might be better for heat dissipation on longer descents.

The highest speed I have recorded myself is around 40mph coasting down a (5-10 minute long?) descent a couple of decades ago on my touring bike, at which point I decided I should apply some (rim-)braking to avoid getting any faster. These days, I think I'd probably not want to risk going much above 30mph on the brompton.

Re heat dissipation, does a smaller wheel affect the choice of rotor size?

I believe small wheels spin faster but that the amount of energy that needs to be dissipated is unrelated to wheel size. So, I would guess 160mm is still okay on the small wheel?

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The amount of energy to be dissipated in coming to a stop depends on the total weight, the initial speed and any slope. That's all. Riding conditions and braking technique can change the time over which you dump that energy, important on long descents, and increasing your drag can dump some of that energy into the air directly (a good thing for once).

Wheel size doesn't come into it.

On small wheels the rotor passes through the caliper faster for the same forward speed, you're right. That will produce a slight difference in braking feel, but within the normal range of modulation that we use instinctively. People with typical hand strength can essentially never use the maximum brake force they could apply with well-maintained modern brakes

A Brompton will have a couple of challenges in hard braking, compared to a longer bike with bigger wheels. The short wheelbase makes it easier for the back wheel to leave the ground so reduces the maximum usable front brake force, which is the majority of your stopping power. Putting your load weight further back helps a lot, especially compared to the large front bags you csn get for Bromptons. The small wheels can break contact more easily on a bump, slightly increasing your chances of a skid.

That all means you may be more comfortable at a lower maximum speed, at least if sudden braking might be required. You may also find that the loaded handling makes you want to moderate the speed round bends.

But I live in Bristol, where it's fairly hilly, and Bromptons are widely used here.

One thing I would take a bit of care over is pad wear and adjustment (the latter mainly on cable discs). While higher rotation speeds might slightly increase pad wear, strapping on a load and going to hillier places than normal certainly will. I carry spare pads and can have been known to change them at the roadside when I run out of adjustment.

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  • Thanks, more concerned with prolonged than hard braking but worth noting. Looks like roughly 50/50 front/rear split for my planned luggage. I already shift my body weight behind the saddle when going "fast" (25mph?) down the only hill here, in anticipation of any need to suddenly slow down. My more normal speed is 10-12mph on flats with 10-15kg of shopping.
    – jhnc
    Commented Aug 13, 2023 at 8:12
  • I'll certainly be carrying multiple sets of spare pads. I need to work on checking pad wear more regularly (hard for me to gauge if they need changing just by peering into the caliper slot and removing is tedious). I use organic because I figure replacing pads is nicer than replacing rotors (3500 miles and only now feeling any rotor wear; pads lasted 3000+ miles). Any advantage using metallic/etc on hills?
    – jhnc
    Commented Aug 13, 2023 at 8:12
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    I like metallic pads (on my tourer with cable discs). Rotor wear isn't a major factor with the right ones and metal pads. That bike has done nearly 30000 miles. The first rotors were trashed by ceramic pads (even harder than metal); that didn't do me much good either. My key sign on BB5s is that if you can't tighten the inner pad further, and you'd like to, change pads straight away. That clear mark that you're running out is preferable IMO to the self-adjusting behaviour of hydraulics, which really require pad inspection
    – Chris H
    Commented Aug 13, 2023 at 8:22
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    Prolonged braking is harder to analyse but still doesn't scale with wheel size. With organic pads and/or hydraulic brakes it's more important to avoid dragging the brakes. It's good to give them a chance to cool but it's also necessary to stay in control bearing in mind how tight, twisty, and poorly surfaced our roads can be, with the chance of a car coming up the hill fast - those factors are far more often the limit round here than the rider's ability to handle a bike at speed on the open road
    – Chris H
    Commented Aug 13, 2023 at 8:34
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    @jhnc With decent-quality rotors, metallic pads don't cause that much wear. Replacing rotors is a 5 minute task vs say 2 minutes for pads, so really, not that much difference. If it helps, metallic pads last longer, so you don't have to worry about pad wear as much. They can also tolerate higher temperatures under emergency braking, if that helps ease your mind about safety.
    – MaplePanda
    Commented Aug 18, 2023 at 3:13

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