Designing a Custom Fork for 650b Conversion

[Adam Kilgas]

I've picked up a 2016 Lynskey Urbano frame (no fork), with the idea of building it up as a 650b, low trail-ish machine. I've been lurking 'round these parts for a while now, dreaming of my own 650b adventure bike, but it hasn't happened yet...until now, hopefully.

My biggest uncertainty, and where I'm seeking advice, is the fork; specifically just how much trail I should shoot for. My plan is to have a fork custom made. I'm hoping folk more experienced with this than I would be able to look at the geometry chart and give me a few pointers. I have a short list of builders in mind for the fork, but I don't really want to pester anyone until I'm more sure of what I want...

Obviously, the frame is designed for 700c, and I'd like the fork to retain the ability for me to switch back to that size should the mood strike. But I don't know if there's a sweet spot trail-wise that I should shoot for, one that would work well for both wheel sizes.

As a secondary question, does anyone know how well brushed stainless steel matches brushed titanium..?

The Urbano details and geometry chart:

https://lynskeyperformance.com/urbano-frame-only/

Thank you everyone!

[William Lindsay]

One idea would be to simply match the fork that Lynskey recommends for that frame. If you were to do that, what what would you be missing in terms of desired ride quality?

Bill Lindsay
El Cerrito Ca

[Justin Hughes]

Low trail. 

[Justin Hughes]

Have you put a rear 650b in there? The Urbanskey (no DS plate) would not fit a 43mm RnR for me so it would've been too tight for a Babyshoe Pass or the like. Basically, the design is such that you don't really gain much width going with the smaller wheel size. Doesn't mean there still aren't valid reasons for going 650b. Fender clearance is one. 

Fork offset would depend on the size you bought (HTA). But the ballpark of what you're looking for would be a fork with an A-C of ~396 and 62-65mm of offset to achieve a trail of 40mm with 42-584 tires. Trail value would increase by 5-6mm by using a 700c versus 650b in a given width. The builder could use a standard steerer for 1 1/8" head tube and you could fit that to the frame with the appropriate headset. You could also use an AngleSet headset and go from "low-trail" 650b to "mid-trail" 700c. Interesting options. 

[Adam Kilgas]

Sorry, I did forget to mention it's a size Large frame. And yes, biggest reason for a custom fork is to go low trail.

Measuring 320mm from the rear dropouts, I get right at 60mm of width between the chainstays, so I'm hoping that would be more than enough for some Compass Switchback Hills, maybe even with fenders. Lynskey claims a 700c clearance of 42mm, and though the chainstays do narrow up a bit close to the bridge, it still seems like you could fit more than that in there. Maybe they played it safe and assumed people would be using 42mm knobbies...

I noticed the Ocean Air Rambler in the larger sizes is listed with 40mm of trail... That's with 700c wheels, so it would seem relatively safe to go with the same, since that would put the 650b trail in the mid 30's. Obviously, there are a lot of other variables, and that's what worries me... For one, I'm going to assume that the Lynskey is a much stiffer frame than the Rambler (the downtube is nearly 1-3/4"!), or most custom built 659b machines, for that matter. I simply don't know how much of a factor that actually is, whether or not it will really effect anything.

[Paul Sherman]

Comparing tube diameters across different frame materials is not a very good way to evaluate a frame's stiffness. Titanium needs larger and/or thicker tubes just to match the stiffness of a traditionally proportioned steel bike. That being said, the Lynskey is almost definitely going to be stiffer than the Rambler. That doesn't make it better or worse, just different.

In terms of trail measurements I think you're dead on, although this is again a matter of personal preference. Something in the mid-30s to 40 range (for wide 650b tires) seems ideal if you're going to be carrying a variety of loads on the front of your bike. This is the range that most semi-production and many custom low trail bikes fall into.

Finally, it's important to choose a builder that understands what you want and has built this kind of fork before. I can't really stress this bit enough.

Paul
Pasadena, Ca

[Justin Hughes]

Seems that Lynskey more or less figured out what most people interested in the Urbanskey/Urbano wanted it to be. 

[Adam Kilgas]

The strange thing with that new frame (to me) is the slack head tube... 71 degrees in my size, Large. Even with the high 50mm offset fork, trail is still in the upper 60's. Though I suppose the target audience isn't necessarily on the low-trail-with-a-front-load wagon, so this makes sense.

[Justin Hughes]

My guess is it has a lot to do with accommodating a 700x42 tire with drop bars while minimizing TCO

[Adam Kilgas]

One other question I forgot to include... The frame had a tapered head tube, but I'm unsure if this really provide an advantage or if it's just mostly marketing. Either way, I'm not sure I like the look of a straight steerer in a tapered head tube - it looks kind of cobbled together to me.

Anyone know of any real advantages? I would guess it would help with low trail shimmy, but the stiffness of the frame will probably take care of that anyways.

[Eric Keller]

On Wed, Sep 28, 2016 at 4:15 PM, Adam Kilgas
<adam.rach...@gmail.com> wrote:
> Anyone know of any real advantages?

tapered head tube allows you to buy a currently made carbon fork,
almost all of which require a 44mm head tube. They do look funny with
most steel forks.
Eric Keller
Boalsburg, Pennsylvania

[Nick Favicchio]

I think the slacker ht angle has to do with the terrain the bike is designed for. Rougher terrain often means slacker ht angle. And more important to avoid tco.

Don't cross frames often run slacker head tubes? And mtn bikes... SUPER slack these days.

[Chris Cullum]

On Sep 28, 2016 13:45, "Adam Kilgas" <adam.rach...@gmail.com> wrote:
>
> The strange thing with that new frame (to me) is the slack head tube... 71 degrees in my size, Large.  Even with the high 50mm offset fork, trail is still in the upper 60's.  Though I suppose the target audience isn't necessarily on the low-trail-with-a-front-load wagon, so this makes sense.
>

If you get a custom fork made for it you could spec a shorter A-C measurement and that would steepen the head angle (and the seat angle too, of course).

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[mitch....@gmail.com]

On Wednesday, September 28, 2016 at 11:45:15 AM UTC-6, Adam Kilgas wrote:

The strange thing with that new frame (to me) is the slack head tube... 71 degrees in my size, Large.  Even with the high 50mm offset fork, trail is still in the upper 60's.  Though I suppose the target audience isn't necessarily on the low-trail-with-a-front-load wagon, so this makes sense.

That's pretty standard for gravel grinders which usually have slacker head angle than the road bike version and have long trail.

--Mitch  

[Justin Hughes]

It does more than change the STA and HTA. I'm not a frame builder, but I suspect a builder who was building a fork for a frame he/she didn't build would be inclined to keep the A-C around which the frame was designed. Changing the A-C to tweak the HTA just doesn't strike me as good sense. As I stated above, an Angleset is an applicable tool for that purpose in this case. 

As for the tapered head tube, it's not simply marketing. Whether or not the advantages of a larger diameter steerer apply to you all depends. And besides Ritchey, there aren't many high quality full carbon disc forks available with straight 1 1/8" steerers. Likely zero with a thru axle. 

A steel fork with a straight 1 1/8" steerer in a tapered head tube doesn't have to look funny at the crown depending on what the builder uses and how he finishes it. But, thin, steel blades so close to super oversized titanium tubes is something you just kinda have to make peace with. 

I sure wish a version of the Ritchey WCS or even a tapered Whisky fork was available with 65mm offset. Not holding my breath on that one. 

[Darrell Goodwin]

Adam,

I have the same Urbano, but in size XL.  According to the geometry charts, both L and XL should have a head tube angle of 72.8 degrees.  I have fitted 700c and 650b wheels on mine.  I used Grand Bois Hetre 42mm tires and they clear just fine. I have the frame with the plate-style chainstay on the drive side, I assume yours is the same.  I've attached some photos of the bike and the underside of the chainstays to show tire clearance.

I chose to go thru-axle on mine, so while that does add some nice lateral rigidity, it does limit the fork choices. I'm using a Taiwanese full carbon tapered steerer cross fork for thru axles with around 45mm rake.  The ride with the Hetres is OK, but I would definitely prefer low trail, having previously owned a Rawland Stag which was low trail.  I would think a fork with rake in the 65mm-70mm range would be about right, but there are online calculators that can factor in the headtube angle and original axle-to-crown measurement to define the parameters for a new fork with the desired trail.  

If you find a good builder to create such a fork, let me know.  I'd be interested in having one built as well, but mine would need to be for 15mm thru-axle.

Darrell

[Adam Kilgas]

Darrell, thanks for the pictures!

Yeah, I'm leaning toward 15mm thru axle for the front, though I'm not entirely sold on it yet. Most builders, and especially ones who do mountain bikes, offer it as an option it seems.

[Adam Kilgas]

Here's an interesting option I came across last night:

http://www.rencycles.com/parts/titanium-tapered-disc-fork

The 50mm rake with 650b wheels would put trail in the low 50's, which would match my current favorite-handling bike: a singlespeed with a 74 degree head angle, 45 degree rake, and 700 x 35 tires. I cobbled that bike together with clearance deals and garage bin parts, one of which is a set randoneur handlebars off of an '84 Schwinn Traveler measuring 40cm at the bar ends.

I don't know which factor plays the strongest part: the fat tires, narrow bars, or lower-than-I'm-used-to trail, or if it's a magical combination of all three (or even something else entirely), but I prefer the handling and comfort of that bike to any other I've rode so far...both unladen and with a small rack and Wald basket on the front. I've always wondered what it would ride like with true low trail figures.

But anywho, I digress... That REN Cycles fork has really piqued my interest.

[Mark Bulgier]

Adam Kilgas wrote:

> Here's an interesting option I came across last night:

> http://www.rencycles.com/parts/titanium-tapered-disc-fork

Interesting that it is made by TiCycles, but if you look at forks on the TiCycles webpage, that style is not shown.  They only list custom Ti forks, costing over double what the Rencycles one costs.  I wonder what the deal is -- maybe Rencycles commissioned a run of forks large enough to get favorable pricing?  I am just guessing.  I used to work for TiCycles (20 years ago) but I haven't asked Dave about this fork.

I can vouch for TiCycles as a legit company with excellent quality.  I have sawed into lots of brands of titanium frames (repairs, S&S couplers upgrade etc.), and found shoddy workmanship and/or inadequate backpurge (which leads to contaminated, brittle welds) even on some hero brands.  But TiCycles welds I have seen the insides of were always near perfect. That's down to fanatical cleanliness and adequate inert gas (argon) purging to keep the atmosphere away from the Ti while it's hot. 

BTW (getting off topic here), purging is a tricky subject due to swirling of the gasses, eddies and "backwaters" that don't get all the atmosphere flushed out by argon flowing in.  It takes a lot of thought and experimentation to do it right, but the one thing many manufacturers skimp on is time.  At TiCycles we always let the purge run for an adequate amount of time before starting to weld.  I think a lot of builders just clip the purge fitting to the hose and then start welding, in too much of a hurry to wait for the purge.  Time is money, and usually, no one will ever see the contamination on the inside of the frame/fork.  Plus the argon costs money.  So there is incentive to skimp, but Dave always impressed me with his integrity and desire for the inside of the frame to look as good as the outside.

Speaking of looks, Dave does a single-pass weld that is technically superior to a double-pass.  The second pass makes those ultra-perfect welds that look too good to have been made by a human.  Dave's welds sometimes show "human traits", but they are welded for maximum strength and fatigue endurance, not for looking a certain way.  Sometimes his welds look almost too perfect, but that's just from lots of hours at the welding table, not from going back over the welds a second time.  My experience with his welding is from 20 years ago but I assume he has only gotten better since then.  There are very few guys who can match him for amount of experience, in the world of bikes anyway.  Maybe some nuclear or aerospace guys have laid more miles of weld bead, but there are factors peculiar to bicycles that make me trust a bike welder over a generic pipe welder, for making a bike fork.

Mark Bulgier
Seattle

[Michael Mann]

The latest issue of Adventure Cyclist has an article about TiCycles in which Dave explains his relationship with Ren - they are Chinese but he has a direct connection, knows the builders, and it sounds like he oversees the work carefully.

Mike

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Michael Mann

[Greg Walton]

The Ren Cycles is Ti Cycles production line, made "overseas" to hit a price point.  The website does a masterful job of walking the line without ever mentioning off shore production http://www.rencycles.com/about/.  Mark knows Dave much better than I do, but I've spoken with Dave a couple of times and he has a hyper focus on quality, so I think it's great that he's shepherding more affordable frames.

I'm not sure about the fork, it may be made in Portland, thus the Ti Cycles branding.  Mark, what do you think of titanium forks?

Greg

Seattle

On Fri, Sep 30, 2016 at 10:24 AM, Mark Bulgier <ma...@bulgier.net> wrote:

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[Mark Bulgier]

Thanks to Mike Mann and Greg [no last name given] for cluing me in about Ren.  My guess is, probably a good value.  No way to really know about the quality inside where it counts in terms of durability except riding them and seeing how it goes.  For a fork, due to the high stakes should it fail, I might want to wait a year and see if the early adopters still like it.

That segmented design with the little cross-ways tubes at the "crown" is particularly difficult to purge properly.  I have seen a few that broke, both in steel and in Ti (see photo). 

I would trust a unicrown design more, but maybe TiCycles and their Chinese partners have worked out a way to make these just as reliable.  I would probably want details on what (if any) fatigue testing they have done.  Even with good test results though, you don't know if your fork was as well purged as the one they did the test on.

Greg asked me what I think of Ti forks.  I have to admit I have never ridden on one and I probably never will, I'm too much of a retrogrouch.  I just don't see the advantage over steel, which I understand and like and can afford.  I like a curved rake, and a fork that I can customize (braze-ons etc.), straighten if it bends, add more rake later if I want lower trail, drill holes for running lighting wires internally... Oh and I don't mind rust very much and don't care much about saving that last quarter pound of weight.  So I'm not a suitable customer for Ti.

Since I'm not very interested in disc brakes and would rather quit riding than switch to through axle, I am totally unqualified to comment on this thread at all!
☺

-Mark

[Justin Hughes]

Better not ever give them a try then. I did and. . . . . . . . . for 'em. 

[Brad]

What are you going to use it for?  If your not going to put a load on the front over the wheel, then you have more options.
Jan's testing indicates that a centered low rider pannier works even with a high trail or mid trail fork because the weight is neutral on the steering.
But hauling stuff around and having easy access to it works better with a front bag and a low trail fork.
If the head tube angle is 71º 68 mm of offset will get you into a reasonable low trail area based on 584 wheels with a 38mm tire.  http://yojimg.net/bike/web_tools/trailcalc.php
If you are going custom, a straight blade with that offset will look interesting.  You won't have the curved spring, but it should flex nonetheless.
I would have mid fork threaded holes for attaching low rider racks and two eyelets on the dropouts.

On Wednesday, September 28, 2016 at 12:32:21 AM UTC-4, Adam Kilgas wrote:

[Brad]

You know, if your willing to drop the bottom bracket slightly, you can make the head tube angle steeper by how the fork is built.

[Adam Kilgas]

My current plan is indeed to use a front bag on a small rack. The head angle of my frame is listed as 72.8; the 71 degrees is Lynskey's new "purpose built" all road/gravel grinder we were discussing.

The lowest trail bike I've ridden is my single speed, in the low 50's, and I love how it handles, even with front rack, basket, and a light <5 lb load. If I can get this new bike to handle at least as well, I'll be thrilled with it.

[Andrew Squirrel]

This is literally the most retrogrouch thing I've ever read on the internet, well done!

"Since I'm not very interested in disc brakes and would rather quit riding than switch to through axle..."

[Mark Bulgier]

I wrote (with tongue in cheek and a smiley):

>> "Since I'm not very interested in disc brakes and would rather quit riding than switch to through axle..."

Andrew Squirrel replied:

> This is literally the most retrogrouch thing I've ever read on the internet, well done!

Glad you liked it!  I'm not quite as retrogrouchy as that quote made me sound, but yeah I'm still pretty retrogrouchy.

Truth be told, my MTB-XC has hydro disks and I'm glad it does.  That bike has old-fashioned QR wheels – it's too old to have TA, that was pretty much only a downhill or "freeride" kinda thing back when I bought it.  I'm not likely to upgrade any time soon, quite content with QR for now, but I wouldn't rule out TA someday, if I outlive this current MTB.  The large selection of QR wheels at my house is a factor though.  I like all my bikes to interchange wheels as much as possible, so introducing a new technology like TA could be expensive.

-Mark

[Ryan Watson]

I don't even know what a "Through Axle" is, other than it's something related to disk brakes. To me, TA means "Traction Avant" :-)

I don't think I'm a grouch, though. Maybe "retroblivious" ?

Ryan

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[Adam Kilgas]

Something I've been curious about for a while... Would anyone be able to post the geometry for the L'avecaise "versatile performance" bike mentioned here?

https://janheine.wordpress.com/2012/08/07/a-versatile-performance-bike/

My subscription started with the very next issue of BQ, and I never picked that one up... I've always been curious about that bike because of Jan's description of the geometry:

"The bike’s geometry is optimized for unloaded riding, but it will handle fine with a front load."