I understand your point, but I hate waste. If there is a way to make wheels last much longer, I do it. On my road bike at 7,000 miles a year, it was 3 year life due to fatigue cracks at the spoke holes. After 2 sets, I switched to double socketed rims, and that has never happened again.

I never wore out the sidewalls on my road bike rims, but I rode it in fairly flat land. Now, I ride in a place with quite steep hills, frequent and long braking, and it wears the (406) rim sidewalls quite a bit, significant concavity on the rims after 9 years, even with frequent pad and rim cleaning, and not riding in the rain. Yes, that's a good long time, but a new set of wheels is not cheap; Why should I replace if I can avoid? I think the only reason the rims have not worn thru yet is because they are cheap single wall OEM Dahon rims, so the sidewalls are probably as thick as the center section. But I know they are not long for this world.

I have wondered about that. One one hand, a 20" rim has a lot less mass with which to dissipate heat versus a 26" or 700c. But on the other hand, the ground braking thrust produces less wheel torque, so shouldn't need to squeeze the brake as hard. Makes sense, as the total "work", i.e., the braking energy, is force x displacement, so it should even out, but again, 20" is less mass to take the same energy.

I was told years ago at a bike shop that disc brakes not only save the sidewalls, they reduce rim spoke stress (counterintuitively, as the spokes have to pull hard in braking), because aluminum rims have a high coefficient of thermal expansion, and heating them up overstresses spokes. This is true; When my front 406 rim started to show more concavity on the sidewalls, I tried using just the rear brake, and at the bottom of a 200 foot descent, *ding*, a rear spoke popped, drive side of course. That reason, as well as sidewall wear, and better braking in the wet, is why I want discs. The only advantage for me of rim brakes, is that I had an opportunity to buy a lifetime supply of cartridge style pads cheap, and disc pads will most certainly cost me more. I like the cartridge pads for ease of removing them for cleaning, however they were designed for road calipers, so I just needed to buy bolts long enough to accommodate the convex/concave washers for V-brakes, to use these. I know rubbery stuff ages, so I keep the stock in a cool dry place.

9 months, 5500 miles , fairly hilly and quite a bit of grit on the roads. I was 20 pounds lighter then , could be worse now ?

Mine was 10,000 miles on the particular bike, with plenty of grit and winter riding but no serious hills. I.e., maybe getting close there.

The energy to dissipate depend of the total mass (bike+cyclist+luggage) and deceleration, its independent of the wheel size.

For a small rim size (like for a small disc size), the rim become warmer than with a bigger one. But for the rim; heat goes to the tire and inner tire that also become warmer which can be damaged in case of extreme braking like in case of long downhill.
Warmer weather will increase the risks since the heat dissipation depend of the difference between rim temêrature and ambient temperature.

Yes, when I said mass, I meant rim mass to accept the braking heat. I would assume other masses were constant.

Heat into the tires and tubes is a factor. As I mentioned, aluminum has a high coefficient of thermal expansion, so the rim will expand in circumference and diameter and yank on the spokes. But also, aluminum begins to lose strength and temperatures a lot lower than most people would guess. But yes, I think tubes and tires will be in trouble first. Found online:

"A major concern in the design of land-based and marine aluminum structures is fire safety. This concern is exacerbated for aluminum alloys due to property degradation which occurs at temperatures as low as 150°C with a 50% yield strength reduction at ~275°C (Langhelle and Amdahl 2001)." So yeah, I agree with you, I think tubes will be in trouble before the aluminum. A think the aluminum also galls and transfers bits of aluminum into pads at elevated heat, those bits are more of a frequent problem to me than road grit, I have to pry out those shiny bits with a knife tip when cleaning the pads.

But in general...
(Mass)(Specific Heat)(Delta-T) = Heat Transfered

Like riding a bike, some things you never forget. :)

Oh, by the way, aluminum has about twice the specific heat of iron, so can store a good amount of heat. This has great implications in comparing aluminum and iron cookware for searing, iron having the advantage (for storing heat) in 3X the mass for a given thickness, but aluminum having the advantage of 2X the specific heat, means:
- for the same thickness, the aluminum pan has 2/3 the heat storage capacity of the iron one
- if the aluminum pan is 1.5 times as thick as the cast iron, equal heat storage capacity (but aluminum having far better conductivity)
- if the aluminum pan is 3X the thickness of the cast iron, equal mass, you really have a great aluminum pan, as that has 2X the heat storage of the cast iron one, and immensely better conductivity. It can also be seasoned, just like cast iron.

The above matters little over a restaurant industrial-strength burner, where even a thin steel wok works fine. But at home, with much weaker burners, key to getting a good sear is a pan that can be preheated to store a lot of heat, so as the food renders water and cools the pan by evaporation, a heavy pan can keep up much better than a light one, evaporating the water and searing the food, rather than cooling and boiling the food.

And...it's here.

Video:

- The shifters are mounted backwards from every other bike in the world, for petesake. And you can't just reverse them, as then they will be upside down.
- The gear range is too high, about 30-120 gear inches. I think a low of 20 is best, but for the same cassette range, that would put the high at 80, and a bit higher there is warranted. That is most easily accomplished by making the cogs bigger by 50%, and I'm sure with modern hyperglide, they could go a bit less than that on the high cog, giving a bit wider range for the cluster. The question then is whether the Brompton derailleur would work, or if it requires a road derailleur with more incline to the pantograph linkage, I'm sure any road derailleur would work, though might need a longer cage for the fold. By the way, the gearing I'm suggesting is what I have on my 20", 21-85 gear inches. But having a Brompton that would do the same would be nice. A standard Dahon has about a 32" low, that's not low enough in my opinion, unless you climb out of the saddle on all hills. Off the top of my head, instead of cassette 11-18 (11-13-15-18?), I'm thinking a cassette 14/15-18-22-27. EDIT: See below, a helpful suggestion from Splithub that I didn't think of due to slow and limited mind today.

I'm not a current fan of IGHs, but I've also said, in for a pence, in for a pound, so I think no disadvantage if they had just gone to a wide range IGH. Except... running the IGH in 2 (direct drive 1:00 ratio) while using the 4 derailleur gears, is more efficient, less friction losses.

...maybe one day you will discover the magic world of smaller chainring

Oh... thank you. My mind is slow today, and, I'm used to the limitation of my 50/34 double, which can't mount a smaller ring (and a triple would have been difficult on my bike). So yeah! Just put a smaller chaining on the Dahon 12! In the video, that looks like a 52 or 53 on a 5x130mm BCD. So for low enough, like a 33 or 34T, that's too small to fit on 130 BCD, you'd need to swap out the crank, I prefer 5x110mm BCD, 50/34 is common, and you could leave on the 50 as a chainguard, or find a chainguard in that size. The only issue there is, that low, the high gear will be 80 inches, that's a bit too low, so I would advocate just a tiny bit wider range on the cassette, if possible.

Like for the other transmissions, 4s derailleur, 6s BWR+derailleur, Brompton offer the choice between several sizes of chainring including a smaller 44t.

A long cage derailleur cannot maintain the chain tensionned when the rear triangle is folded (this is valid for the Brompton and also for the Birdy, the Birdy Touring is factory equipped with a long cage Deore rear derailleur)), this has been tried many times.
There is a need for an additional chain tensionner whatever the length of the derailleur cage.
The Ltwoo Fold B special derailleur has an integrated long tension arm similar to the one of the Brompton tensionner.

(above) Good knowledge, thanks!

It looks like a decent spread actually. One thing is missing though, considering how they're touting it as a hill climber - DISC BRAKES! I also agree that the low end isn't low enough, but a smaller chainring would easily solve this problem. The high end loss would be inconsequential for me personally.

https://cimg1.ibsrv.net/gimg/bikefor...757e14cff1.png

"similar" is quite a bit of a stretch. I was able to accommodate 3 chainrings with the Brompton tensioner, and with the Ltwoo, I would have a loose chain for the folded bike slapping around.

I agree with all. Yeah, just needs (I think) a 33 or 34 chainring to get down to 20 gear inches. But for that, needs a different crank, no bigger than 110 BCD. Discs... I don't know if that SA hub is available for discs(?) But, I've considered if just a front disc would be enough for long descents, if Bromptons use a 100mm OLD front hub (my Dahon is 74mm fork, so would need a 20", 100mm disc fork, I've yet to find one aftermarket to retrofit).

This graph is erroneous.

the 4s ans 12s are supposed to have the same 50t chainring and the IGH has a 1:1 ratio on its second/mid gear, meaning that with the same cogs 4s and 12s on second hub gear should have the same gear inches while on the graph, the gear inches of the 4s are longer than the gear inches of the 12s on its second hub gear!

Sturmey-Archer offers disc brake versions (6 bolt) for their standard IGHs, but, and it's a big but, Brompton uses an exclusive-to-them super wide range three-speed hub on their 6-speed models AND this new 12-speed model claims to use yet another different exclusive wide range hub.

So, no disc brakes for you.

Just, as usual, wishful thinking... :p
Then you had better call Brompton ASAP, since THEY'RE the ones who created the chart that YOU'RE stating is "ERRONIOUS". Watch the video (apparently you DIDN'T???), give em' a call.
I want my 8 speed IGH and disc brakes!!! :cry: :roflmao2:

I have seen the video before and knew very well that the chart is extracted from this video published by Brompton and there is something erroneous on the chart, either the drawing or the statement that the chart show the gearing of the 4s and 12s with the same chainring size.
The 4 sprockets are the same on the 4s and on the 12s and the BWR hub has a 1:1 ratio on its second gear, so with the same chainring size, the gear inches must be the same for the 4s and 12s on its IGH second gear.

Then please post the CORRECT information, since the manufacturer has made the mistake of creating a video with erroneous information. If that's what Brompton posted, THAT'S the specs as far as gear inches is concerned... Honestly I'd love for you to show us where they screwed up. Bromton are so backwards.
ufactureracturer

Brompton sez the BWR and the IGH for the 12-speed are 'different inside and out'. They're certainly different outside:

https://cimg0.ibsrv.net/gimg/bikefor...5188cd6a9c.png

What if 2nd gear for the 12-speed IGH isn't 1:1?

Naw, that's crazy talk. :)

Like for the other transmissions, 2s, 4s, 6s, there are several sizes of chainring for the 12s between which the customer can choose while ordering its bike. the error can be the text saying that both the 4s and 12s gear inches shown are with the 50t chainring.

What we can see from the IGH of the 12s is that the spacing between the gears seems the same as for the BWR.

There was another video posted by a German Brompton reseller in which a gear inch table provided, according to the reseller, by Brompton in which a gear inches table is shown but these gear inches for the 12s are completely wrong:

https://cimg2.ibsrv.net/gimg/bikefor...671675600a.jpg

That's the key question. Otherwise Jipe is right, logic as flawless as Holmes.

Guys, one step at a time. After this 12s bike is established as the flagship - the "bike-to-have", and sales numbers have passed the peak, the next stage is ignited - disc brakes. The new Sunrace designed hub shell gives it away. Fascinating product - not the bike - the way the consumers are conditioned.

As much as I'd like to say that you're correct, Brompton won't put disc brakes on their bikes anytime soon. They're famous for NOT giving the people what they want. I personally like their bikes, but their elitist attitude kills it for me.