childcarepro

I wanted to bounce this idea off the community: is the Brompton A-line the ideal base Brompton to mod to an ultralight folder?

There are endless lightweight parts available for Bromptons no matter which model you start with. Your options as base bikes to mod are (highest to lowest cost):
1. T-line Urban - $5K, 17.5 lbs (4 speed derailleur) - not an option d/t price
2. P-line - $3K, 21.8 lbs (4 speed derailleur) - also not an option d/t price
3. C-line Urban - $1.55K, 24.8 lbs (2 speed derailleur - no point paying for an internal gear hub only to throw it away for weight savings)
4. A-line - $1.05K, 23.1 lbs (3-speed IGH)
5. Used M6L - $1K, 26 lbs (6 speed)

Obviously, for $2K more than the A-line, the P-line is a very nice bike, with plenty of potential for dropping all the way down to 15 lbs by replacing nearly everything with titanium and carbon fiber parts, though at that point you could be very close to the $5K price point of the T-line.

Seems to me that short of the P-line (and T-line, of course), the bike that starts out closest to sub-20 lbs is the A-line, possibly achieving 20 lbs simply by removing the IGH in favor of a 3-4 gear derailleur setup [YouTuber Twisted Wheels claims the one he removed from a used M6L weighed 3.5 lbs (1.5 kg)]. Maybe more recent 3-speed IGH's are a lot lighter?

Am I missing something here? Obviously the C-line Urban is much easier to mod from a 2-speed derailleur to a 4-speed. But why is it 1.7 lbs heavier than the A-line despite no 1.5 kg internal gear hub? I know it has fenders, folding left pedal, light and dynamo, front luggage block… where is all the weight coming from, and should I just stick with the A-line?

Of course, the C-line – and ONLY the Explore model, at $1755 – offers 7 colors. I thought it would be easy to wrap the A-line's white with some other color, but that would add $800-1200. So unless you want a white bike, maybe the C-line Explore is the better "deal" due to the cost of paint/wrapping!

Thanks in advance for your wisdom.

EDIT 2022-06-14. I thought I should list what I've discovered so far so people don't have to scroll thru the whole thread:

Structural Parts:
Starting Weight: 23.1 lb (10478 g)
Titanium front fork: –275g
Titanium rear triangle: –350g
Titanium stem: –288g
Titanium handlebar: -131g
Titanium seat post: –170g
TOTAL: –1214 g (9264 g / 20.42 lb) Cost: $1050 for A-line + $1079 for parts

Wheels & tires:
• remove Sturmey-Archer internal gear hub + wheel: -1415 g
• remove front stock wheel: –650 g
• add Silverock 3-speed 14H/21H alloy wheels: +800 g
TOTAL: –1265g = 7999 g / 17.63 lb
Cost: +$250 for new wheels, –$200 for selling old Sturmey-Archer wheel = $50) = $2179 total + drivetrain parts

Drivetrain: (still working on these numbers)
• add 4 cogs + spacers: XXX g (?)
• add derailleur / tensioner: XXX g (?)
• switch to 4 speed shifter from 3 speed Brompton: –XXX g (?)
• lightweight aluminum crank: –XXX g (?)
• dual chainring: +XXX g (?)

tcs

FWIW, Sturmey-Archer lists the short axle + no spacer version of the bare SRF3 hub @ 970g.

This would be replaced by a freehub, a 3/4 cog cassette and some sort of a derailleur. It'll be lighter, but I don't think you'll be saving 1.5kg.

childcarepro

Thanks, tcs!

Yeah that sounded too good to be true. Still, might be able to come in at possibly 1 to 1.5 lb savings? Anyway, that'll probably be on my bike. My wife might like her internal gear hub just fine, and she doesn't like me fussing with mods. Haha

mlau

Hey Childcarepro,

I sorta understand what you're asking, as I was thinking about it myself with the A-line.
I'm not sure how much lighter a C-line urban (M2L) would be than an A-line (M3).

IMHO, if you want a truly lightweight build, PM Raxel.
Some Brompton clones are probably lighter than anything available from Brompton.

That being said, I'm not that handy or smart.
I bought an A-line for my fiance with no regrets.

My main complaints with the stock Brompton are remedied anyways with the A-line: cheezy roller wheels, I want a honey brooks saddle, the stock pedals are sorta heavy, I can't stand the bell. Sorta ironic that the best bike for my needs is one of the cheapest.

mlau

Oh yeah, looking at google:

Sturmey Archer hub: 970 g
Brompton 2 speed hub: 312 g

So about 658 g savings?

Jipe

A light build based on a steel frame Brompton will never be as light as one based on a steel-titanium frame (P-line) or a full titanium frame (T-line) due to the higher weight of the frame!

The weight savings applied to the steel frame bike can also be applied to a light weight frame resulting in a lower total weight.

Even the expensive T-line uses some heavy parts like the whole braking system and saddle that can be replaced to reduce the weight.

childcarepro

The C line urban is 1.7 lb heavier than the A line (24.8 vs 23.1 lbs) despite the lack of the internal gear hub. It does add fenders, front luggage block, folding pedal, and other stuff I would probably rip off and replace with lighter or better gear.

Thanks, I've been reading through his threads on his titanium clone build. He is my primary inspiration for considering a scratch build. But just looking at all the parts I need and how many options for each… getting cold feet. Haha

I'm sure you're just being humble! I'm handy enough to tackle anything I put my mind to. I haven't built a bike before, but I've built lots of other things. My wife hates my projects, so I might get her an A-line and get myself some titanium parts and futz with it as she rolls her eyes. LOL We'll see.

I've subscribed to your "Pimp my A-line" thread and follow your progress!

Thanks for that. I was hoping to save even more by going with lighter hub, derailleur and tensioner. But I'm not so sure now, since I do love the clean look of internal gear hubs, plus I just learned about derailleur maintenance. Internal gear hubs are zero maintenance. Might just stick with the Sturmey Archer and save weight elsewhere!

Anyway, the point of all this weight shaving is so I can ADD ~6 lbs: I’ve pre-ordered the gen 3 (I think) Swytch Air kit. We currently have Electra Townie Balloon 7i's (7-speed internal gear hub) that are unridable on hilly terrain, which detracts from the enjoyability when you can only ride a few flat routes. They're also 37 lbs each despite aluminum alloy frames, and are a bear to load and unload on our bike rack. The thought of adding a 6-8 lb e-bike conversion kit made me fall right out of love with the Townies. That's when I decided on tiny folders instead. Plus, we're seriously considering moving to a high-rise condo, and a folder would allow us to save two trips to the bike storage every time we ride!

childcarepro

Thank you for the reality check!

I was looking at a MiniMODs X3 w/ 4th 21T or 25T cog + lighter tensioner. It would offer a significantly lower gear for hill climbing, but would require a LOT of work (sourcing parts, futzing w/ the 4th cog, etc.) plus the maintenance a derailleur system requires. I would have to save at least ~450g for this to be worth the work.

Something to put on the back burner for now, as I really do love the look of the maintenance free internal gear hub. But 970g, while not quite 1.5 kg (maybe Sturmey-Archers have gotten lighter since the one removed by Twisted Wheels), is very low hanging fruit to a weight saver!

mlau

Personally, I think the A line would serve you well. FWIW, I have a Swytch kit on my M6R. I ordered the air upgrade for my bike, so that I can better use it with my front basket (and maybe fold better).

childcarepro

I'm super pumped about the Swytch. My wife keeps saying we don't need e-bikes because we ride for pleasure and for exercise. But our hilly rides have been horrible (Electra Townie 7i Balloon) and keeps us from riding more than we would. So we would only use it on the uphills.

But that makes me think… if I only need the Swytch for hills, maybe I don't need the 7-8 lb Swytch at all with a different setup. I might just need a 3 or 4-speed derailleur setup + manual dual chainring for steep hills. Not just that, but also changing from a 37-lb cruiser w/ 7-speed internal gear hub (6% drivetrain loss, I've heard), 60mm wide balloon tires at 40 psi (very high rolling resistance) to a 20-lb Brompton w/ 3x2 speed derailleur and 35mm wide tires at 100 psi (very low rolling resistance).

I guess if the bike change and gearing upgrades work out on hills, in addition to a couple of rear wheels w/ Sturmey-Archer BWR hubs, I'll also have a couple of Swytch kits to sell! That oughta make my wife happy!

Jipe

A wider tire at a lower pressure doesn't necessarily mean higher rolling resistance.

It depend of the type of tire.

childcarepro

They’re Schwalbe Fat Frank 26’s. I’m pretty sure they have higher rolling resistance at 40 psi than a Continental Contact Urban at 100 PSI, but I could be wrong.

Thanks for making me learn! TIL wide tires have lower rolling resistance than a narrow tire at the same PSI due to less flattening/deformation. Who knew? Not me!

Jipe

Yes, with these tires, they have a higher rolling resistance. But if you use some wide road tires like the Big Apple, Big Ben... they roll very well.

Now the Continental Contact Urban have an exceptionally low rolling resistance and you could mount them also on your 26" bike, they exists in many different wheel size and width.

childcarepro

Updated original post with actual proposed parts replacement. Still a work in progress, but at least the shape of an ultralight Brompton is forming.

mlau

Riding on my fiance's bike is pretty fun.
It shows me just how much I need to maintain my own bike!
I'll need to take apart and lube the rear wheel, and adjust the derailleur.

This bike rolls pretty nice!

Looking forward to the last bits to complete her bike: some m5 screws, and some EazyWheels. Both coming in today.