curved S3 seat stays, cantilevers and braking power

[ztah...@uwalumni.com]

Greetings,

I'm new to this group but have .  Thanks all for sharing your knowledge!

I know this subject has already been covered to some degree on this group, but I have a situation that I'm hoping to resolve:

I'm working with my local builder on a prototype low-trail randonneur based on the new 54-559 Compass tire with cantilever brakes.  I wanted somewhat slender seat stays for aesthetic reasons, so I requested 16mm single-taper stays.  My builder chose true temper S3, which I believe to be responsible for an alarming lack of braking power.  The stays flex quite a bit during braking.  I should also note that the builder added s-bends to the seat stays, which is a definitive style element for his brand.

My questions for the group are: 1) can I get acceptable braking power from 16mm stays and 2) could the bends be facilitating the excessive flex... almost like slack in a rope?  Not the greatest analogy, but intuition suggests a super light, curvy seatstay may be more likely to deflect independently before distributing braking forces to more rigid adjacent frame members.  

I presume the answer is to #1 is yes, but with a thicker tube wall.  I've had production bikes with similarly slender tubing and cantilevers which performed great. (couple Rivendells and a Rawland).  I didn't actually measure those bikes' stays with a caliper, however.

Your thoughts are appreciated!

Zach Ahrens

[Jon Norstog]

Zach, the toothpaste is out of the tube now.  A good move would be to get one of those braces that go over the studs

http://singletrackworld.com/forum/topic/v-brake-boosters-which-one

Good luck!''

jn

"Thursday"

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[David Bohm]

Zach,

 

We cannot see what this all looks like but it is unlikely that the stays are your issue.

 

Consider that although S3 stays are quite thin compared to some that making a tube thicker only makes it stiffer in a linear fashion.  I.e. make it twice as thick/make it twice as stiff.   You S3 stays are only about 25% less thick than others.

 

There are 12mm-14-16-19mm stays.   Your builder chose the largest diameter that would look decent.  19’s are out of place on a rando.  (although this isn’t a rando with 54mm tires…it’s a mountain bike, call it what you will)  Increasing the diameter is a cubed function (3 or 4 power depending on torsion or bending) A 16 is quite a bit stiffer than a 14.  

 

A thicker stay may have been in order but it is unlikely that he entire issue is stays.  You should troubleshoot elseware to find your issue.  It may just be cable routing, brake levers, spongy housing, improper pads or pad placement, improper stud mounting or any number of other things.

 

Hope that helps.

 

Dave Bohm

Bohemian

[Greg Christensen]

Hello,

Not sure that this helps, but I used to ride a Giant ATX 990 aluminum frame bike.  It originally came with v-brakes.  I upgraded to Avid Arch Rivals and with those brakes the seats would flex a lot when you braked hard.  The frame never broke, and I never had any problems.  You would only notice it if you watched the stays and pulled the brake lever hard at the same time.

Greg

On Thu, Apr 28, 2016 at 2:11 PM, <ztah...@uwalumni.com> wrote:

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Greg Christensen
South Dakota School of Mines and Technology
501 East Saint Joseph Street
Rapid City SD 57701

[Colin Pinney]

These three bikes linked below all have S3 16mm seat stays that have been S-bent by me and I have had no problem locking up the rear wheel with Tektro CR720 canti brakes and the mystical Koolstop Salmon brakes.  I have had Paul canti brakes on the orange bike and I forget what the owner is running now but all work.  The orange and the Silver bike road 200 miles from Fort Collins to Steamboat on mainly dirt roads and ~20,000 feet of climbing and decending, over a 9000 foot pass in December on a 100 mile ride, and many miles of simple single track.  I have never had an issue locking up the rear of the bike in dry conditions.  The only issue I have seen with the seat stays was when my buddy missed the memo that everyone rides with panniers up front now-a-days and had 2 rear bags and the back-end noticeably swayed when he pedaled hard.

https://www.flickr.com/photos/kelpie_cycles/23452310190/in/dateposted-public/

https://www.flickr.com/photos/kelpie_cycles/26207032011/in/dateposted-public/

https://www.flickr.com/photos/kelpie_cycles/25416478852/in/dateposted-public/

Good luck figuring it out!

Colin Pinney

970-217-5569

Bikefo...@gmail.com

photos at https://www.flickr.com/photos/kelpie_cycles

              https://instagram.com/kelpie_cycles/

[John Caletti]

The S bends are not an issue.

As the other’s have commented, a 16mm stay is a common use and a good selection. You said you are working on a prototype, so maybe you are looking at changes for the next one(s)?

You could go to a thicker stay or a larger diameter stay - and of course, as the stay gets stiffer, it flexes less, and braking should improve a little. You’ll also add a little weight and probably decrease ride quality a little. So, trade-offs, like many of the tubing and geometry selections on the bike. There is more than one “right” answer  - but you will have to see what’s the best set of parameters in frame design and material selection to meet your prioritized goals. 

Cheers,

John Caletti

Caletti Cycles

www.caletticycles.com

[Alex Wetmore]

It's a rear brake, so it's really not that important.  Under aggressive braking the weight will transfer to the front wheel and the rear will lock up even with flexy stays.

I hate canti bikes that have too-flexible fork blades that prevent the brakes from working well.  I don't care so much about what's going on in the rear end.

I did use 19mm stays on the orange rando bike shown in these photos:

https://janheine.wordpress.com/2015/06/09/compass-tire-prototypes-really-big-tires/

and

https://janheine.wordpress.com/2015/04/04/the-enduro-allroad-bike/

There is a good side profile here:

https://janheine.files.wordpress.com/2015/03/gifford_rattrap.jpg

They are big, but I don't think they are out of line with the >50mm tires.  The brown bike built for 650B has 16mm stays.  Neither are S-bend.

alex

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From: frameb...@googlegroups.com <frameb...@googlegroups.com> on behalf of John Caletti <j...@caletticycles.com>
Sent: Thursday, April 28, 2016 7:43:58 PM
To: Framebuilders; ztah...@uwalumni.com
Subject: Re: [Frame] curved S3 seat stays, cantilevers and braking power

 

[Jon Norstog]

To the list:

I have for a long time used 5/8" x .035" aircraft tubing for seatstays on mountain and road bikes.  This is a size just under 16mm, and .09mm wall thickness.  My approach to avoiding stay flex is to make sleeves out of 3/4" x .058" tubing, about 2.5" long and braze them to the stays as bases for the canti studs.  This gives you a pretty stiff braking action with cantis or V-nrakes.  On bikes that are already built up, I make a patch, kind of like a water bottle mount reinforcing "diamond," from appropriate-sized tubing.   Seems to help.

Good luck!

jn

"Thursday"

[Jim G]

I used 16mm stays on my 650B rando frame.  I have V-brakes on that bike (just to be weird).  No problems with braking.

What brakes and pads are you using?

-Jim G

[Andy Newlands]

Howdy,

Here is another way to stiffen up the rear triangle:  http://www.strawberrybicycle.com/gallery/view_photo.php?set_albumName=Store&id=rambler1

Cheers,

Andy N.

www.strawberrybicycle.com

www.torchandfile.com

www.oregonframebuilders.org

[ztah...@uwalumni.com]

Thanks for all the input everyone.  I don't have photos of my bike, but it's partly inspired by the bike Alex posted and has very similar geometry (thanks Alex!).  I do tend to rely more on the front brake, but it would be nice to have more confidence in the rear.

Dave: could you be more specific about cable routing and/or brake stud placement?  I've never had this issue with any cantilevers before, and I think the setup is good.   There are always things to learn, of course!

To answer Jim's question:  the brakes are Shimano CX70.  I chose them because the pads clear the stays when the straddle cable is disengaged (convenient with the fat tires)... and because I had them on hand.  Pads are stock.

I'll start with what seems the simplest solution (kool stop pads) then try a brake booster, high profile cantilever and linear pull brake in that order.  On the bright side, I really like the way the bike responds to pedaling so far and look forward to testing with some weight on the lowriders and front rack.

Thanks again folks.

- Z

[satanas]

Can't comment on the seatstays, but FWIW I had CX70s last year in Europe touring and had severe brake fade and noise with the standard pads a couple of times; almost ran off the road on one or two descents as a result. Changing to Koolstop pads fixed everything - night and day. YMMV.

Later,
Stephen