Kirk Frameworks

[anon.]

Hi Dave,

Apologies in advance if this question was addressed up thread and I missed it (I'll admit to skimming some of the 13pgs)...

I've been thinking about chainstays a lot lately, and I know you have some custom ones drawn for you by reynolds. I'm just curious about the thought process that went into the design, what shortcomings you were hoping to overcome, what the final specs for the stays wound up being (wall thickness, dimensions, alloy, etc), and if you'd consider them to be a success.

I'm not sure if I've seen a close up of them or not, but from the photos they look quite substantial at the dropout.

Thanks!
Michael

[Dave Kirk]

Hi Dave,

Apologies in advance if this question was addressed up thread and I missed it (I'll admit to skimming some of the 13pgs)...

I've been thinking about chainstays a lot lately, and I know you have some custom ones drawn for you by reynolds. I'm just curious about the thought process that went into the design, what shortcomings you were hoping to overcome, what the final specs for the stays wound up being (wall thickness, dimensions, alloy, etc), and if you'd consider them to be a success.

I'm not sure if I've seen a close up of them or not, but from the photos they look quite substantial at the dropout.

Thanks!
Michael

Hey,

Thanks for bringing up one of my favorite frame related topics.

I feel very strongly that the chainstays are spine of the bike as all forces generated at the pedal have to go through them to get to the rear wheel - in fact it's the only way they can get to the rear wheel. I'm also a strong proponent of round c-stays as I feel they give the best combination of structural traits.

With that in mind I wanted a c-stay that met my desire to give the bike lots of jump and snap when getting out of the saddle and to deal with big gear mashing. I wanted to go as large in diameter as was practical without having to resort to large dents for chainring and tire clearance. That meant that the BB end would be 22.2 mm in diameter. I wanted the stays to be tapered so that they would spread the forces out over the length of the stay and no focus it all in one spot but I wanted the small end to still be pretty large. After playing with some different sizes and measuring the load/deflection ratio I ended up with a stay that was 15 mm at the small end.

The wall thickness is interesting. It's butted so that the small end is thinner than the large which keeps the weight down and helps further distribute the load along the stay. The exact numbers I'm keeping under my hat. It was a lot of work to get it where I wanted and don't want to give it away. I suppose you could buy a frame and cut it up to get the wall.............. :)

The alloy is Reynolds 631. This was chosen because it allowed for the butting and manipulation I wanted done on the stay and still be strong enough regardless of the use or weight of the rider.

I do consider them to be a success. In fact I wouldn't change a thing at this point - they are just as I like. The newish Triple F dropout is optimized to work with this stay diameter to give a large interface as well and blend smoothly into one another.

Thanks for reading/skimming and of course for the question.

Dave

[anon.]

Dave - thanks a bunch for the detailed response! I'm happy to hear that the stays have worked out so well for you.

I'm curious about the prototyping/development phase... did you start out with straight gauge 7/8" 4130 and go from there or did you work with the reynolds folks on a series of one-offs before deciding on the final spec?

Also, are you using the stays for everything now or just road bikes? I know you mentioned that you went with 7/8"/22.2mm for clearance (i'd guess lugged bb shell availability would also play a part in it...), but I can't imagine you'd be able to fit much more than a 25mm tire without dimpling. Are the JK Cross frames getting stock stays or are you experimenting with your own s-bends on your custom stays?

yrs
Michael

[Dave Kirk]

Dave - thanks a bunch for the detailed response! I'm happy to hear that the stays have worked out so well for you.

I'm curious about the prototyping/development phase... did you start out with straight gauge 7/8" 4130 and go from there or did you work with the reynolds folks on a series of one-offs before deciding on the final spec?

Also, are you using the stays for everything now or just road bikes? I know you mentioned that you went with 7/8"/22.2mm for clearance (i'd guess lugged bb shell availability would also play a part in it...), but I can't imagine you'd be able to fit much more than a 25mm tire without dimpling. Are the JK Cross frames getting stock stays or are you experimenting with your own s-bends on your custom stays?

yrs
Michael

Good Morning,

Thanks again for another good question.

The design/development phase was pretty simple. One can do the math and see how the different designs (diameters/walls/tapers) will react to a given load. Combine that with doing load/deflection tests with existing stock stays to confirm the math and you come up with a very solid idea of how different designs will compare to one another. Then I looked at it from a very simple and pragmatic viewpoint - I wanted the big end to fit a lugged shell and I wanted them to be round so that meant 22.2 dia. The small end size was chosen by picking what I wanted and what could be made and what could be stuck to a dropout. I also didn't want there to be heel strike issues one can get with very large stays so it came down to what the ideal diameter would be. I then asked how close Reynolds could come to that and they could come really close with tooling on hand and this is how I came to have the 15 mm small end. I had a small run made and used them on my own bikes to test them and was very pleased with the results. I then finalized the tooling design with Reynolds and they have been making them for me ever since.

I use the same stay on my JKS road and JKC cross bikes and I do so without any bends. By the time you make the stays a good length (42.5-43.0ish) you move the tire back far enough that all one needs to fit in a good sized cross tire is some very small dimples to allow for plenty of tire room. My JKS road bike uses the same stay and it will fit a 25 tire without any issue at all without dimples and if someone wants to run a 28 then I just need to put the smallest hint of dimples in there and we are good to go. The dimples are so shallow that I'm seeing next to no difference in load/deflection tests so the bike retain the snap I want them to have.

It's been a fun process and I look forward to working on other stuff that I think will benefit my bikes. I only do so when there is extra time/money in hand so that I never need to take a loan. I do not like owing anyone for anything so it all has to happen in due time. I very much enjoy the design and testing aspect of the biz - it was my job for a very long time and I gave it up when I left Serotta. I'm now enjoying being able to do more of that. I feel that by designing your own stuff that you can end up with a design that evolves over time and best reflects how I think the bike should ride and handle. Most of the stock stuff offered is fine but I do feel that there are gains to be had by spending the time and money to optimize the design. Since most tubes are pretty similar and basic geometry is fairly well established it's easy to end up with a bike that is for the most part just like the next builder aside from the logo and paint schemes. So product differentiation often comes down to one of perception and not one of a true ride difference. I work hard to try to make my bikes ride different and better and that is the really fun part that keeps me at the bench.

Thanks again for the good questions.

Dave

[suhacycles]

Hi Dave,

Thanks for your insights... great stuff!

So how important would you say seat tube diameter/stiffness is in relation to chainstays and the down tube? Garro makes a good argument for sticking with 28.6 seat tubes but there has to be some 'loss' in the BB moving sideways and twisting downward or is that something that can be compensated for by using a larger diameter down tube and/chainstays?

Also in your testing were you able to measure deflection or make any observations of various chainstays, such as oval compared to round or ROR?

Thanks,
Hansen

[Dave Kirk]

Hi Dave,

Thanks for your insights... great stuff!

So how important would you say seat tube diameter/stiffness is in relation to chainstays and the down tube? Garro makes a good argument for sticking with 28.6 seat tubes but there has to be some 'loss' in the BB moving sideways and twisting downward or is that something that can be compensated for by using a larger diameter down tube and/chainstays?

Also in your testing were you able to measure deflection or make any observations of various chainstays, such as oval compared to round or ROR?

Thanks,
Hansen

Another very good question - thank you.

The seat tube and down tube both play a role in making the BB stable and providing a good platform to push against and I think you can keep the BB from moving around by throwing a good bit of diameter at either or both those of those tubes - that said it's the chainstays that get the job done. If I sound like a broken record I apologize but the c-stays are the tubes that transfer the power to the rear wheel............. in fact they are the only two tubes that CAN transfer any significant power from the BB to the rear wheel. As the people at "Slingshot" have proven you don't even need to have a down tube to have a stiff BB and efficient drivetrain. The seat tube on a slingshot is monster but there is no DT and they have a seriously stiff BB.

It's been a long time since I did lots of comparison testes on all different types of stays and I don't recall any of the numbers but a round stay always ends up being laterally stiffer than an oval or ROR due to it's greater lateral cross section. If you make the oval stay 22.2 mm wide it's about the same stiffness laterally as a 22.2 round stay but it is of course heavier. The oval stay is stiffer vertically but since they are part of a triangle that doesn't really affect anything.

UPS is here and I need to sign.

later,

dave

[Dave Kirk]

Karin and I wish a Merry Christmas and Happy Holidays to you all. Thanks for a good year.

Dave

[Dave Kirk]

Happy New Year to all from the crew in Bozeman.

Dave

[alexstar]

hi Dave, have you hashed out pricing for the "JKS X" frames? I am... interested.

Thanks,
Alex

[Dave Kirk]

hi Dave, have you hashed out pricing for the "JKS X" frames? I am... interested.

Thanks,
Alex

Hey Alex,

Thanks for the interest.

At this point final pricing has not been set. I'm waiting on final price of the seat post I'm having made to go with the JKS X. Once that is in I'll be able to commit to a price. That said, I think you can come pretty close by taking the JKS price and adding that of a top shelf carbon post and you will have a good idea.

Let me know if you have more questions.

Dave

[Cycles d'Autremont]

Hi Dave,
Great thread. My question is about saddle position and fit in general(sorry if this opens up a whole can of worms). I read your Kirk Tip #2 and really liked the idea, but was wondering how you can bring this into a fit studio, or even a conversation with the customer if you aren't doing a full blow fitting. I went to Serotta for their fit school and felt that a lot was missing and there focus on KOPS was a little frustrating. Obviously you are well versed in this method being a designer of the size cycle, but my question is how you can determine saddle position with the customer in an isolated situation as well as clients of varying physical fitness, seeing as most cyclist don't have abs of steel, but I think that this would vary the results. Also how to you determine the balance of the bike (wheel location) for a given rider/bike?
Thanks.

Hubert H. d'Autremont
ps I grew up outside Alder, MT beautiful country, I'll give you a shout next time I'm out that way.

[Mike T.]

Dave, I stumbled into this fine site and forum a few days ago and have since spent many hours reading what TK and yourself have to say. You sure are an eloquent pair! In your many posts in this thread (and I certainly haven't read all of them yet) a couple of your comments stand out -

• I come back to it I sit down and start removing anything that doesn't need to be there. Design for me can often be a reductive process.
  
• IMO good design leaves nothing left to be removed.

I collect quotations. Let me share with you one of my favorites. This you will like -

"A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away.” - Antoine de Saint-Exupery.

[Dave Kirk]

Hi Dave,
Great thread. My question is about saddle position and fit in general(sorry if this opens up a whole can of worms). I read your Kirk Tip #2 and really liked the idea, but was wondering how you can bring this into a fit studio, or even a conversation with the customer if you aren't doing a full blow fitting. I went to Serotta for their fit school and felt that a lot was missing and there focus on KOPS was a little frustrating. Obviously you are well versed in this method being a designer of the size cycle, but my question is how you can determine saddle position with the customer in an isolated situation as well as clients of varying physical fitness, seeing as most cyclist don't have abs of steel, but I think that this would vary the results. Also how to you determine the balance of the bike (wheel location) for a given rider/bike?
Thanks.

Hubert H. d'Autremont
ps I grew up outside Alder, MT beautiful country, I'll give you a shout next time I'm out that way.

Good Morning,

You've asked a good question and one that feels challenging to answer in writing....... over a beer with lots of hand gestures would be easier but I'll give it a shot.

I set the fore/aft position of the saddle for a given rider based on their body measurements. Since we are all a fairly simple series of levers one can determine where the hips should be relative to the feet by looking at the length of those levers. The hard part is that it does not factor in core strength or upper body mass. I try to get a feeling of the rider's upper body mass but shy of having everyone come to my house to see how long they can hold a plank it's very difficult to get a realistic idea of the rider's core strength...... and this core strength (or lack thereof) is the elephant in the room. I feel I need to design the bike as best I can with the idea that the rider's core is reasonably strong to allow them to progress as riders. The small fine tuning of the fore/aft position can only be done once the bike is together and on the road. I design so that the saddle is in the middle of the rails and this allows for fine tuning of the position to get that ideal balance.

It's ironic in a way - strong cycling requires a strong core yet does little to produce one. It makes our hearts and legs strong but the core not so much and a strong core is the thing that is needed to have a stable platform one can push on the pedals from.

If that doesn't give the answer you were looking for come on over and we'll go for a ride and then have a good beer and use lots of hand gestures.

Thanks again,

dave

[Dave Kirk]

Dave, I stumbled into this fine site and forum a few days ago and have since spent many hours reading what TK and yourself have to say. You sure are an eloquent pair! In your many posts in this thread (and I certainly haven't read all of them yet) a couple of your comments stand out -

• I come back to it I sit down and start removing anything that doesn't need to be there. Design for me can often be a reductive process.
  
• IMO good design leaves nothing left to be removed.

I collect quotations. Let me share with you one of my favorites. This you will like -

"A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away.” - Antoine de Saint-Exupery.

Thanks for the note. I'm glad you've been enjoying the reading. My words bore me but I like reading TK's posts............

I think good design is one of those things that "we know it when we see it". Good design looks right and is simple and elegant and not overwrought. It is of course easy to see but hard to do but it is something I strive toward. There are certain things that really stick out to me as good design and while they are often pretty to look at they sometimes aren't in a classic sense. Here are a few things that float my design boat.















I could go on all day. The thing all the above have in common IMO is that the form follows the function and that they are simple and to the point. I imagine some saying "wha?" to the Moulton but when you think about it's aim and how elegantly it gets there and how well it works I think you might agree.

Time to get to the bench.

Thanks again,

dave

[Mike T.]

Thanks for the note. I'm glad you've been enjoying the reading. My words bore me but I like reading TK's posts............

Dave, don't underestimate the value of your prolific writings.

[Jason Musgrave]

Maybe I'm overly sensitive to it because I lived there, but it seems as though you've sent several frames to Taiwan lately (or Asia in general). Is it coincidental or do you have some sort of connection over there?

[Dave Kirk]

Maybe I'm overly sensitive to it because I lived there, but it seems as though you've sent several frames to Taiwan lately (or Asia in general). Is it coincidental or do you have some sort of connection over there?

You lived in Taiwan? Very cool. What was that like? What brought you there?

You are right. I've shipped a good number of bikes to Taiwan and have a good number in the queue that will follow them. It all started with one guy buying one and liking the bike and the experience and he told a friend who then told a friend and so on. So I have no direct connect there, just a few happy customers. It's fun dealing with the folks there. There is a language gap at times but they are so versatile that we work through it without issue. I love getting photos from there.

Thanks for asking,

Dave

[Jason Musgrave]

Yup. Parents were missionaries when I was a kid. Spent 6 years there (left when I was 11). Somewhere in my brain I'm a fluent Mandarin speaker--spent 10 days there in 2003 (Taipei Bike Show) and the last couple of days I could understand any conversation. I've been wanting to go back for a year to relearn the language which would make me pretty marketable in this day and age.

[Dave Kirk]

Yup. Parents were missionaries when I was a kid. Spent 6 years there (left when I was 11). Somewhere in my brain I'm a fluent Mandarin speaker--spent 10 days there in 2003 (Taipei Bike Show) and the last couple of days I could understand any conversation. I've been wanting to go back for a year to relearn the language which would make me pretty marketable in this day and age.

That is very cool. I am not well traveled and barely speak one language let alone another. Did you enjoy your time there?

dave

[Dazza]

Taiwan, = good food!
My short trip there was a buzz