Cycling UK Forum

Incidentally and, again, in case it's not obvious, the Brompton set-up is unusual regarding the derailleur arrangement. Normally, a derailleur would have the secondary function of taking up slack chain. That creates a certain tolerance to changes of chainring (and sprocket) size, limited by the capacity of the derailleur. On a Brompton, the derailleur is a strange little push-me-pull-you cage thing under the chain stay, and the long arm is a chain tensioner with much less capacity to absorb changes in chain length - its biggest role is stopping the chain going slack during a fold. So, you'll probably need to shorten your chain. There's a Brompton fact sheet (PDF) with information, and it also shows that chain widths are a little more variable than I stated.

What I've said so far doesn't take much account of the latest Brompton models from the last year or two, which include some models with more of a recognisable derailleur system. I haven't caught up with the changes. Someone may comment on those.

Nearholmer wrote: ↑14 Apr 2024, 6:46pm...because coming down them at the speeds where air resistance really starts to help would involve crashing at the first bend.

That is known to be an infallible technique for preventing brakes from overheating.

That's a pretty confusing thread! The straight answer is, I believe, that the Brompton chainset is 130 BCD. That imposes a minimum size of 38 teeth. There are plenty of rings from many manufacturers to fit, although Brompton rings generally come with a plastic guard attached. If that matters to you, you could either buy the Brompton 44T ring, or buy an independent one and look on eBay for a guard.

I think pjclinch knows this but, for other readers, BCD is the bolt circle diameter, which means what it says - the diameter of a circle drawn through the five (in most cases) bolts that hold the chainring onto the crank. BCD varies between chainsets, but any make of ring for a 5-bolt 130 BCD crank should fit the Brompton. Since the teeth must necessarily be on a circle outside the BCD, then a chainset with a 110 BCD will obviously accept smaller rings than does the standard Brompton one. It's possible of course to fit a different crank with a smaller BCD, but that adds to cost.

As pjclinch will also know, the chainring needs to be narrow enough to fit the chain. As derailleur bikes have progressed from five to (now) 12 or 13 sprockets, chains and chainrings have got narrower. However, the standard Brompton chain is 3/32", which is a size dating back to five-speed sprockets, so anything except a 1/8" ring intended for single-speed use is going to work.

Edit: For a correction on chains, see my next post below.

And I believe that an element of randomisation is programmed in, so that AI does not always produce exactly the same answer to the same question. Might be similar though, would need to see some examples!

Psamathe wrote: ↑11 Apr 2024, 1:19pmMy understanding of physics (claiming no expertise) is that the amount of energy to be dissipated is just total mass and height (assuming gravity is constant). Things like rotor size, material, etc., wind, speed, etc. will affect how that energy is dissipated. eg 100 Kg dropping 1m altitude will lose 981 joules potential energy. Assuming start and end speeds the same, that means there is 981 joules will turn into heat irrespective of what sort of braking system you have.

I think the last few posts have been in jest. Obviously, the type of brake that you have does not affect the physics. The way that air resistance increases with the square of speed (and so, in some circumstances and on some hills, it might be effective to go faster in order to get more total effect from the air resistance) has already been covered.

I agree, I have no bikes with rotors either. This thread is an interesting example of some cyclists beginning to assume that discs are the only kind worth mentioning. I was looking at a bike in my LBS with rod brakes recently. That would give some an apoplectic fit, I imagine

Although I admit that it would not be my first choice for descending an alp.

I can see why not! I guess ultimately it's about what intelligence is, without wanting to get into a lengthy philosophical debate. But AI sounds much more scary than clever programming, however scary some programming (that turned out not to be so clever) may have proven to be!

Jdsk wrote: ↑10 Apr 2024, 4:46pm"The British Nationality Act as a logic program":
https://web.stanford.edu/class/cs227/Re ... ityAct.pdf

Going back to my theme of defining what is AI and what is just clever computing/programming, what in your view makes that example AI? Especially given that it largely uses an older programming language that would not normally be considered to meet that criterion?

The legislation thing sounds interesting

Bear in mind that what they are doing is using a "training set" of information and then using predictive techniques to figure out what you might consider a good answer (on the basis of your question). If someone feeds them duff information in the training set, they'll feed it on to you. Nor will they, as far as I know, spot contradictory information in the training set. Certainly AI has no understanding, in any sense that a human might recognise, of the information that it is processing.

drossall wrote: ↑8 Apr 2024, 11:51am

plancashire wrote: ↑8 Apr 2024, 10:31amMore physics: if you move slowly the descent takes longer and heat has more time to dissipate.

Interesting, because I once read an argument that over-caution, leading to over-braking and hence additional heat, can be more dangerous. I suspect there's some truth in both, but your argument makes perfect sense.

Good news for me because, the older I get, the more cautious.

I was thinking about this overnight. I suppose the argument would be this. Let's assume that cautious and exuberant riders both take bends in a similar manner, on a switchback downhill. However, the cautious riders brake down the straights, whereas the exuberant allow themselves to run free, relying on air resistance to moderate their speeds. Given decent straights, they'll quite quickly reach a terminal velocity and stay at it. So the cautious are doing braking where the exuberant do not. Then, the question becomes whether the kinetic energy dissipated as heat by the cautious on the straights exceeds the extra braking needed by the exuberant to scrub off maybe 20-30 mph into corners.

I can see how it might, however much I might instinctively be cautious.

You're welcome. I've corrected bars to stem - the one I have is a stem mount. On a Brompton, not that that's what you're asking, it needs to be positioned vertically just so in order to avoid the fouling I mentioned but, with a bit of experimentation, works well.

plancashire wrote: ↑8 Apr 2024, 10:31amMore physics: if you move slowly the descent takes longer and heat has more time to dissipate.

Interesting, because I once read an argument that over-caution, leading to over-braking and hence additional heat, can be more dangerous. I suspect there's some truth in both, but your argument makes perfect sense.

Good news for me because, the older I get, the more cautious.

When I had a Dahon, I found it simpler to put a Rixen and Kaul bracket on my stem. They are widely available and there are variants with locks, for different bar diameters, etc. Gives access to a wide range of bags.

Now I've got a Brompton, I still use it, but facing towards me because it doesn't then interfere with the fold. My Brompton bag is large, so this gives me the option of a small bar bag when I just want to carry a wallet and keys or whatever.

There are adaptors, but it would help a lot if you told us what bike you have!

First question would be what type of brake, I'd have thought? Some brakes can be adjusted to give better or worse braking (strictly, all brakes can, but some are more sensitive to it, in less obvious ways).

I agree, I was always urged to pump brakes a bit, so that you have an idea of how much you have in reserve. But that's hard to do when you're hanging on for dear life. Like you, I'm relatively heavy, and frankly these days I'd rather ride up a 1:5 than down.