Custom titanium frame questions...

Discuss light weight issues concerning road bikes & parts.
Marin
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by Marin

DUB BBs are already shipping. BSA DUB is quite light too - I hope they got it right. I'm currently modding Shimano BBs for all my Sram cranks...

Red ist still the lightest crankset out there except for Clavicula, I hope it stays that way for the DUB version

by Weenie


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MayhemSWE
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Location: Stockholm, Sweden

by MayhemSWE

2lo8 wrote:
Wed Jun 06, 2018 9:37 pm
I think the big question for DUB is if the 1mm difference lets them get big enough bearings for BB86 and other 24mm specific PF variants.
If one is to believe Sram they claim the problem with 30 mm axles in BB86 shells wasn't so much the tiny ball bearings but that there was no room over for decent seals. Supposedly the extra room afforded by a 29 mm axle is used for better seals so probably the bearings themselves will be very similar to say Rotor UBB for BB86.

NickJHP
Posts: 124
Joined: Wed Feb 14, 2018 2:22 am

by NickJHP

Berzin1 wrote:
Sun Jun 03, 2018 7:02 pm
Hello everyone. I'm looking into a custom ti frame, and am thinking of something along the lines of the old Litespeeds with the oversized downtube and curved seat stays.
Why not get a Lynskey. Same people who started Litespeed, and they still do the same sort of frame - curved seatstays, large DT, threaded BB, breezer dropouts. e.g. https://lynskeyperformance.com/r150-road-frame/.

Berzin1
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by Berzin1

NickJHP-I am trying to sort out some of the finer details, because the frame I have in mind is going to be custom geometry, with no sloping top tube. I detest sloping top tubes.

As for builders, it's a toss up between Litespeed and Lynsky. Haven't decided yet.


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zank
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by zank

Yes, but that is only for the 68 mm variant.

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Calnago
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by Calnago

Yes, and it’s still dependent on a third party solution as opposed to Campy making their own T47 cups. When you’re talking about a situation where the only solution available is one being provided by a very small independent company, it raises concern over what happens in the long term, or even short term, if that company ceases operations for whatever reason.
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Nixster
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Joined: Wed Oct 04, 2017 3:30 pm

by Nixster

Well that's Bottom Brackets thoroughly discussed :D

Now, what about tube sizes, more specifically, should tube diameters be as follows:

Down tube>top tube>seat tube?

Or something else? And if the top tube is oval?

Interested in opinions either functional or aesthetic or both!

2lo8
Posts: 473
Joined: Thu Mar 31, 2016 10:32 am

by 2lo8

DT>=ST>=TT
DT>TT

The biggest reason for an oversize seat tube is to have something to miter an oversize top tube to, without ovalizing the top tube in the wrong way. Vertically ovalized top tubes are a bodge trying to fit an oversize top tube with a standard seat tube. It makes the top tube vertically stiff, laterally compliant, the opposite of what most people want from a top tube. There's no real advantage to an oversize seatpost either, there's a limit to how thin you can make them because they have to deal with calmping forces. You only need their stiffness for stuff like folding bikes with massive exposed seat tubes.

Downtubes should be bigger than top tubes because that's what a century of trial and error says works. There gets to be the point where there is no weight savings from a bigger downtube, only large stiffness gains with some weight penalty because the tubes can only get so thin. If you're the kind of person that thinks a standard frame is stiff enough, then the point there is no more weight savings from bigger and thinner is going to be the upper bounds of downtube diameter for you. If you want maximum stiffness, then you'll probably try to get the largest downtube you can miter to the headtube and BB shell.
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pdlpsher1
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by pdlpsher1

I don’t agree that the seat tube should be bigger than the top tube. The ovalization of the top tube at the top tube to seat tube junction is so little that it’s moot. Secondly I strongly believe that the seat post should be 27.2mm for comfort. So a 1.25” seat tube will be ideal. The top tube could be 1.375” and the down tube 1.500”. If the frame size is larger than, say 56cm, then perhaps bigger DT and TT could be used. Of all the tubes on a bike, the chainstays have the greatest effect on perceived lateral stiffness and power transfer. The tubes in the main triangle need to be adequately stiff but not overly stiff for the best riding experience.


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Nixster
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Joined: Wed Oct 04, 2017 3:30 pm

by Nixster

Interesting. I guess my assumption was that the ST should be wider than or equal to the TT to allow a neat joint. Ovalising the TT vertically seems counter-intuitive but I guess the number of millimetres is small (or fractions of an inch if you're that way inclined).

I agree on the seat post diameter, can't see the point of more than 27.2 personally and if additional lateral stiffness is needed then ovalise the ST at the BB instead? Using 34.9mm ST results in shims (or an insert) which is less than elegant IMHO.

I am considering a 40mm DT, 31.8mm ST and horizontally ovalised TT at 32x28mm, so from the side the TT will look thinner than the ST. I don't see many if any frames like this, (J Laverack is the exception) there may be a good reason? :noidea:

Bike will end up as roughly a classic 52 size, give or take, and I weigh around 60kg. My Lynskey is 44DT/35TT/35ST and seems overbuilt to me (I call it The Tank) although its plenty stiff enough. I would likely keep the same 25x19 chainstays however.

As ever grateful for the collective WW wisdom!

pdlpsher1
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by pdlpsher1

34.9mm ST won’t look nice with the 27.2mm seatpost. A 31.8mm ST is perfect. Here’s a pic of a 31.8mm ST joined to a 34.9mm TT. The TT is ovalized but you could hardly tell unless you measure it with a digital caliper.

Image

Here’s a pic of a 38.1mm DT joined to a 44mm HT. The DT is slightly ovalized. Once again it’s hard to tell but the ovalization is there. There’s no need for a more dramatic ovalization. The TT is not ovalized at the HT.

Image

Here’s a pic of the DT ovalized at the BB shell. This ovalization is a bit more dramatic. The ST is not ovalized.

Image

For your weight and frame size the Lynskey is way over-built. You will do just fine with 1.5” DT, 1.25” ST, and a 1.375” TT.


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2lo8
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by 2lo8

pdlpsher1 wrote:
Fri Jun 15, 2018 8:39 am
I don’t agree that the seat tube should be bigger than the top tube. The ovalization of the top tube at the top tube to seat tube junction is so little that it’s moot. Secondly I strongly believe that the seat post should be 27.2mm for comfort. So a 1.25” seat tube will be ideal. The top tube could be 1.375” and the down tube 1.500”. If the frame size is larger than, say 56cm, then perhaps bigger DT and TT could be used. Of all the tubes on a bike, the chainstays have the greatest effect on perceived lateral stiffness and power transfer. The tubes in the main triangle need to be adequately stiff but not overly stiff for the best riding experience.


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I am saying you that you shouldn't increase the ST diameter if the TT doesn't call for it, and should generally stop increasing top tube diameter once you hit the OD of the ST, not that you should increase ST diameter, unless you have a very good reason for it. If you have to ovalize a top tube, it should really only be ovalized for the miter. Often times tube diameters are increased for basically no reason, only because bigger shaped tubes look more modern, or some supposed benefit from stiffness to weight ratios, etc, even if a smaller diameter lighter tube would still be more than adequate. Especially in regards to the top tube. You could make a bike frame without a top tube with a sufficiently strong downtube. There's always going to be niches where rules of thumb don't apply. Frames that might benefit from ovalized top tubes are very large ones, not just medium-large, which are also the ones that also start suffering from looking more like a trapezoid than a triangle when it comes to in-plane stiffness.
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2lo8
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by 2lo8

pdlpsher1 wrote:
Fri Jun 15, 2018 6:17 pm
Here’s a pic of a 38.1mm DT joined to a 44mm HT. The DT is slightly ovalized. Once again it’s hard to tell but the ovalization is there. There’s no need for a more dramatic ovalization. The TT is not ovalized at the HT.
I'd guess the only reason the DT is ovalized at the head tube is because the tube is a bi-oval, not because the frame needs any additional in-plane stiffness from the DT at the HT. The benefit of a bi-oval comes from increased lateral stiffness at the BB because the BB shell is not triangulated and trying to flex the tubes it is attached to. A round-oval tube would probably be more optimal for a DT than a bi-oval. Bi-ovals come from steel frames with external headset cups and needing to miter to smaller headtubes. A TT that is ovalized only for the miter is really mostly a round tube. My issue with shaped top tubes is really more like teardrop shaped top tubes and fully oval top tubes, less so about locally shaped tubes just so a miter can meet. I still don't think they're needed in most cases though.

edit: My opinions are biased from caring more about steel which tends to use skinnier tubes, and 28.6mm is still an OS TT, and not really caring too much about titanium.
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Your one-stop source for information and reviews on cheap eBay bike junk.

by Weenie


pdlpsher1
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by pdlpsher1

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Last edited by pdlpsher1 on Fri Jun 15, 2018 10:34 pm, edited 1 time in total.

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