Geometry question: chainstay length and rim vs disc

[lightweenie]

Hello,

I am playing with the idea to get a custom road frame made at some point next year. To a big extent I would like to replicate the geometry of my current rim brake bike. I am wondering if it would be possible to keep the short 405mm chainstay length if I decide to go with disc brakes and have clearance for 30mm tires. My understanding is that due to the wider hub and tires and in order for the chainstay to clear the cranks the chainstays would need to be a bit longer, is that correct?

Another thing I am thinking about is to perhaps have the bb drop increased by a couple of millimeters which would effectively also lengthen the chainstays. Would that affect the above?

Disclaimer: I will of course also take my builders input on all this, but I am asking here because: (1) I have not decided on a builder yet, and (2) I would like to have a preliminary understanding of the issue before discussing it. Also I kind of like thinking about these things and learning more about bicycle geometry.

[marciero]

That's basically it. Wide tires and short stays compete with narrow Q and chainline that we like on road bikes.

My understanding is that chainstay length is the literally the length of the chainstay from hub to bb and not the horizontal distance from hub to a point directly above the bb. If that is the case dropping the bb with the same stay length does not change anything clearance-wise. It simply rotates the crank and bb around the rear hub.

On the other hand, lowering the bb with the same length stays would move the rear wheel forward horizontally. So the horizontal length is shortened, if that's what you mean by effective length. But not enough to make a difference-less than half a millimeter for 405mm stays if you increase drop 5mm.

It seems odd not to measure stay length horizontally when it seems that that is what we are really interested in. I suspect it's because of the very negligible errors doing it along the stays, and the latter is easier to measure. Or maybe we are interested both??

Some recent designs that address the clearance issue are the dropped DS chainstay yoke on the Open and others, the 3D printed yoke on the Firefly, and flat bar yoke on bikes like Lynskey and others.

[Dracarys]

My Giant TCR Disc has 405 mm chainstays, and the shifting is perfectly smooth. So it is possible.

[prototoast]

It can absolutely be done, though depending on the materials and methods of the builder, he or she may not be able to execute it. As an example:
English cycles best road bike from NAHBS had 405 chainstays, disc brakes, and clearance for 35mm tires.

Http://www.englishcycles.com/customb...ke-nahbs-2016/

[Peter P.]

Chainstay length is irrelevant with regard to discs vs. rim brakes. Witness the latest crop of pro road bikes running discs; they still have short chainstays. To see what the shorter limits might be, check out the web sites of manufacturers currently sponsoring pro teams such as Specialized and Trek.

Dropping the BB will effectively shorten the distance between the rear wheel and the back of the seat tube, so interference can be an issue. For instance, my road bike will fit 32mm tires, but with its 8cm BB drop, I have to squeeze the tire past the seat tube when trying to fit it in the dropouts. So in your case you may have to make some adjustments to get your tire size, chainstay length, and BB drop to all work together.

Crank clearance is something I haven't commented on because I can't answer that definitively. Yes, the wider hub and shorter stays can present problems, but if you look at my first paragraph you may have your answer. Of course, many of those sponsors are building frames with curved chainstays, to provide that crank clearance. Maybe your builder will have to bend the stays to provide that crank clearance.

[false_Aest]

There's more considerations than just wheel clearance.
Crank clearance
Disc clearance (140mm vs 160mm rotors)
Heel clearance

are a few off the top of my head.

In steel CS + SS spacing probably won't be an issue if you're ok with some bent stays or a yoke. But the builder will also need to consider the STA and your saddle set-back. The wheel gap will be pretty tight with a 74* STA. Tighter if you use a FD. It get's tighter if you need a slacker STA. A bent ST or a cut-away (a la Rob English) helps.

Increasing BB drop will decrease your rear wheel gap unless you compensate with a longer CS length.

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I made a really tight geometry road bike with clearance for 30s not too long ago. It's my personal whip. My chain stays are 406mm long. With 28s or 30s the bike is good. With 25mm tires it's great and shreds. It wasn't something I expected but hey, that's the fun part of being in the early phases of building frames.

On another bike of mine (not a true 1:1 comparison - the tubing isn't the same) the HTA, STA, stack + reach are the same but the CS are 1cm longer. It rides much better with 28-32mm tires. I don't like it with 25mm tires.
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