CLARIFICATION: While the Terminal Velocity of a Touring Brompton (T-Bag/Large Borough+20Kg luggage) is similar to having no luggage, the braking force required to slowdown (and the associated rim heat generated) will obviously be higher with a Touring Brompton. Brake force and rim temperature shown in the video were computed for a Touring setup.

This channel is a gold mine. These are not Brompton videos, but general science and physics that everyone riding a bike should pay attention to. The Brompton part just adds for a good exemple of physics variables changing and how the models follow.

Holy moly your videos are of a different league than the usual crap I watch on YT! What a fantastic explanation!

I've reached 56 km/h terminal speed, on a long slope I travel very often last year and know very well in northern part of China(Jinan city, Shandong Province). Mine is a six-speed s-bar Brompton.

To prevent overheating my rims on long descents from Alpine mountain passes I do fully stop the bike and apply a wet cloth/rag to the rim (be careful not to burn your fingers!) to help them cool down quicker. I do that after every half a dozen to a dozen hairpin bends (every two to three hundred meters of elevation loss) (depending on the gradient of course) If there's a fairly flat section I ride that with wet rims without breaking so that the evaporating water cools down the rims quicker. Spritzing them with my water bottle has the same effect as wiping them with a moist rag but uses a lot more water, hence the wet rag. I noticed that the rear wheel heats up much quicker than the front wheel, probably because there are more obstructions to the airflow (frame, my legs, rear fender). (May be a funnel could help with directing more air onto the rim to help with cooling) I blew my tube once because I raced a motorbike down a mountain... the rim was unbelievably hot afterwards and it's scary but I could stop alright. On Großglockner I saw full sized bikes with rim brakes blow their tubes too. (Dipping the wheels in ice meltwater besides the road helps too if the brook can be accessed easily/safely but I'm always worried about abrasive stuff in glacial meltwater) (Usually the maximum speed is determined by the road, i.e. I could go faster if there wasn't a bend in the road. But there is so I need to brake...)

That's a great piece of work - thanks. Great science and clear graphics. I must slow down on the hills of West Cork!

professional analysis on the physics😍😍

Purely anecdotally, I recall descending from the Cat and Fiddle towards Macclesfield some years ago on a Brompton equipped with old-style single pivot brakes; I shall never forget the look on the face of the driver of the oncoming car I met near the bottom! It would seem from your helpful comments that black aero rims would be an advantage.

Great analysis. Thanks for sharing.

Very educational and clear! And... thank you so much for using the metric system!! 🙏

The only nerdy content I love listening to. Always great and very informative. Thank you!

Brilliant explanation as always 👌

I'm always learning with your videos. I have two Bromptons, one for commuting and general traveling, another one for long distance tours. I've done 215k on a single day on my Brompton. Many of the strategies I apply using my Brompton were adopted after watching your vids. Awesome work, I appreciate it a lot 👍🏼🤜🏽💥🤛🏽

You are really just so wonderful at explaining all things bike, thank you.

Amazing piece of content! I have never thought about the rim temperature. However Downhilling the brompton at top speed is not my thing. The bike feels comfortable to 30km/h to me. Airbreaking is good advice as it is effective at top speeds

Loved this. I love digging into the science of things and also recently bought a Brompton. I doubt I'll be pushing the limits of braking in the streets of Chicago, but it was really interesting to understand the math of it.

Extremely informative. Saw 1st hand riding buddies falling due to constant breaking riding downhill, but i personally never experience it. Great video

when returning from the mountain, I make a braking parachute out of the tent😛

Wonderful. I am always leary of too much speed downhill when touring fully loaded on my Brompton. On my latest tour on Isle of Skye I reached 34.6 mph (55.6 km/h) with 10 kg front and 10 kg rear of extra weight. The bike was solid with absolutely no wobble. I pulsed my brakes (equally front and rear) and just enjoyed the ride. The ambient temp was approximately 19 C and the road was smooth tarmac. Thanks for the excellent analysis.

So impressed with your videos!! Science and Bromptons…a wonderful combination! You’re providing fantastic knowledge to someone like me that wants/needs to learn! Thanks again 😊

Masterpiece again. What you're doing is so great, thanks. My personal feedback : 62 km/h descending on the long slope arriving on the airport in the Shetlands islands, Scotland. M6L with my 78 kgs on it. I think I had no luggage then, as far as I can remember.

Day 1 Alpe DHuez I met someone who saw someone go over a hairpin having suffered brake fade at high temperature. They were very lucky to survive. Advice was the same. Release brakes to increase air cooling, brake evenly and hard when you need to. Don’t just trail them! I was on a full size roadie. Checking periodically by hand probably a smart move on a Brompton - if too hot to touch, stop and enjoy the view.

Too many uphills here in Korea, thanks for good tips~!

Professors of Brompton!

As a Dane in a flat country (hardly 200 m above the water at our highest point!) we do have a rocky island in the Baltic, Bornholm! And on this we have a few serpentine roads which are very steep! I couldn't ride up even with 6 gears, but had to walk much of the way up. And besides that, we have "cattle prevention bars" crossing the road at several places on the one and have to walk over or around them, with a self closing gate! Going down again I had my serious doubts, but even braking down from extreme speeds to pass the bars, my Brompton brakes never let me down!!! Finn. Denmark

Great video. I have had a quite bad experience that I think is worth sharing. I once did a descent from Chiang Dao to Ching Mei in northern Thailand. I was riding my titanium Brompton, that is equipped with one of the best v-brakes you can get. The bike was only lightly loaded, with a Brompton bag mounted in the front carrier bracket. My guess is that I was doing 60 km/hour, the road conditions was perfect, but suddenly the bike started shaking. I couldn't do anything to stop the shaking because the front bag that caused the shaking is mounted directly on the frame. If it had been mounted on front rack I would have been able to counteract the vibrations with the handlebar. The brakes was useless, If I had applied them I would be send flying. This is probably the most scary thing I ever experience bicycling for 40+ years.

So when is Brompton going to make the move to disc breaks?

Hello to you both and thanks for creating such useful and informative videos. I just have a very quick question. which cycle mirror do you use?

When I go downhill, I use the front and rear brakes alternately for no more than 5 seconds each time to avoid overheating.

Shortly after a longish descent on a steep and slightly bumpy route, I did have the spokes on a Brompton front wheel unravel - it was very alarming. It may have been a badly built wheel, and I may have been unlucky, but it makes me somewhat cautious in the speed I go and the kinds of shocks or vibrations that I may be creating on these little rims.

I have had inner tubes pop a few times on descents, two at pretty much the same point on different years on a charity ride on a 1:6 hill using Kojak tyres. Never happened when using Brompton Green tyres. I did note that it happened where the tube was touching the rim where the yellow Brompton rim tape was not centrally position. I have replaced the B tape with normal bike adhesive rim tape since. Not sure if this has helped. This year I had a failed in a similar manner but using Brompton green tyres and again the yellow rim tape was slightly off centre where it failed so now have adhesive rim tape on both wheels. This happened as I came to a holt after a similar hill but on the flatter part.

Rim wear is also of concern when using a Brompton. I killed at least 2 rims in my 10 years using it so far. The first one within a year using a downhill path to work.

Is it better to alternate between front and rear brakes throughout a descent, as opposed to braking with both intermittently?

Chris from Chris by bike made a video that relates to this. She actually reduces the pressure especially in her front tire for extremely steep descents. She blew a tire out coming down from Christ the Redeemer in Rio

Out of curiosity, why did you use psi instead of bars for tire pressure? Is psi more common in Canada?

Does applying small pressure in the brakes constantly to prevent acceleration cause less heat than full braking at top speed? I feel you could just use the brakes for a slow descent and there would be little to no heat generated, this problem would be completely avoided. Can someone explain?

I've gone up to nearly 60mph on my road bikes, but 37mph is the fastest I've descended on my Brompton (S3E upgraded to S5E), for fear of overheating (and not that stable to start out with).

0:11 the road sign says 'steep uphill', not downhill in Korean. Pardon me to point it out.

So if I go do that Montana downslope, I might be able to finally hit 100kph. Fastest speed I've gone on flat ground here is 76.6kph. Downslope is supposedly 96.6 but I think Strava was glitching out, since that was on my MTB, which shouldn't be able to go even near that considering the gear ratio means I top out at 50. There's no way I could have done that. Singapore by the way, your example of high air pressure. The 76.6 is on my road bike, I've done it with 2 different bikes, downslope on my old one and on flat ground with my new one because some illegally tuned e bike overtook me, he was probably going 60 and I don't believe they deserve the speed when they didn't train for it or even pay for it, therefore I must show them the error of their ways. Unless they're going 80 on those things that are more like electric motorbikes, then they paid enough for it . I heard that there are now disc brake compatible parts for Bromptons, probably safer to switch to those if anyone plans on going down a large slope. Even comparing road bikes, my old one with rim brakes really cannot compare to my new one with disc brakes. Then again my old road bike had pretty low end components. It was really terrifying going downslope on the old one. I believe Bromptons come with better brakes. But speaking of the luggage to add extra weight, what if the luggage came with an aerodynamic fairing of sorts, detachable so you can fold and reattaches around the folded form when you want to push the bike around. Maybe with air ducts for the disc brakes. I think the limiting point for that would be the wheel size. Those small wheels aren't the most stable when they hit bumps.

i wouldn't want to do 40k on a Brompton, let alone 50 or 60.

That's why I wish bromptons had a disc mount, there's no downside to having one as far as I can tell.

Lower psi? I've ridden in the 20's w/ cheap BMX tires Since my bikes doesn't have suspension Zero snake bites too Learned that would make regen braking useless

What if you have brake discs

For me the fastest I’ve ever gone on a brompton is about 35mph down hill 😅

4:49 Lack of fenders? A bicycle without fenders isn't fit to ride...

Solution: disc brakes

how bad would this be on 20 inch rims? i hit 40mph on average on my ride to the train station and gone almost 50 on some other hills on 26 inch rims and was thinking of getting a dahon mu to get up the hill easily and then take it on the train or bus its hilly everywhere round here too should i convert to disks maybe?