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needs some help with a small frame

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needs some help with a small frame

Old 01-03-15, 08:59 PM
  #1  
veloguy1
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needs some help with a small frame

Hi folks,
I am starting my first frame. It is for my friend who is 5'1.5". I wanted to build a city commuter(the already has a Specialized 700c frame) build around 26" rims, but with road geometry. I have a workable drawing but BikeCAD and others just make my head spin. We are using lugs so I can only play with angles so much.
Anyone with experience in this area?

Thanks!
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Old 01-03-15, 09:22 PM
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Andrew R Stewart 
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I've built a number of small frames for my late wife and a close friend, both were 5'1" to 5'2" tall. Some were Terry style with a 520 ft and 622 rear wheel. A couple have been dual 571 wheels.

You are right to think about the lugs being a challenge. Specificly the DT lug and the sockets on the shell will shift a few degrees from the 700c norm that every lug or shell I've seen are assumed to be for.

But the lugs and sockets can be simply bent to fit. Or you can cut a slice in the lug to allow the angle to spread then fill it in with brass. On my bikes I used Omar's bending bars (on the main tube sockets, thick walled tubes for the stays) and a lot of blacksmithing then I brazed the DT lug and shell sockets with brass to insure that any gaps weren't a problem. Took a bit of clean up and thinning filing to look good.

This and other reasons are why I've decided to do more with fillet brazing. A lot less prep, any angle can be done equally easily, a different look then the common lugged or TIGed frames that abound. Andy.
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Old 01-04-15, 05:56 AM
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I'm a short guy too and I had a VERY custom frame made for me by Paul Hillbrick in Sydney Australia. I asked Paul to build it with lugs and he did so as he's an experienced builder. When I do it again I will drop the requirement for lugs. Lugs are great but I was just making things hard for the builder.

I haven't RANTED about frame design for small people for YEARS. Anyway, my two bobs worth is start with a SHORT set of cranks. 150mm max, 140mm or 135mm might be nice. Set the bottom bracket low for the shorter cranks. You will then find that the required seat tube angle will be VERY relaxed, say 69 to 71 degrees and you will need to pull the front centre distance right back. Um, OK, you will probably need smaller wheels. My custom has 650c wheels or you could go 24" mountain which you can get good quality Schwable road tires for.

Still with me? About now lugs are out the window.

Why would you do this anyway? You do this to put a small rider in the same riding position as a taller rider. We are building a bike to truly suit the rider rather than messing around with a compromise to fit the rider to the bike.

Anthony

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Old 01-04-15, 09:32 AM
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Originally Posted by AnthonyG View Post
I'm a short guy too and I had a VERY custom frame made for me by Paul Hillbrick in Sydney Australia. I asked Paul to build it with lugs and he did so as he's an experienced builder. When I do it again I will drop the requirement for lugs. Lugs are great but I was just making things hard for the builder.

I haven't RANTED about frame design for small people for YEARS. Anyway, my two bobs worth is start with a SHORT set of cranks. 150mm max, 140mm or 135mm might be nice. Set the bottom bracket low for the shorter cranks. You will then find that the required seat tube angle will be VERY relaxed, say 69 to 71 degrees and you will need to pull the front centre distance right back. Um, OK, you will probably need smaller wheels. My custom has 650c wheels or you could go 24" mountain which you can get good quality Schwable road tires for.

Still with me? About now lugs are out the window.

Why would you do this anyway? You do this to put a small rider in the same riding position as a taller rider. We are building a bike to truly suit the rider rather than messing around with a compromise to fit the rider to the bike.

Anthony
Anthony- Why is this (in bold) a requirement? I do feel that the body needs to be centered (weight wise) over the pedals but as one pedal moves rearward (the one in front because of the short cranks) the rear pedal moves forward, so their center of balance is still the BB. If anything I've found that with shorter thighs and feet that a slightly steeper seat tube is a possibility.

Also fitting really short cranks is a wonderful concept if both the availability of them covered a wide selection of brands as well as the rider being comfy with that choice of leg movement. IME many riders are already use to 170mm arms as that's the standard in most current small adult road bikes (for many unfortunate reasons). I've known customers (at the retail shop) who don't like even a 5 or 10mm arm length change (let alone a 30mm one). True there are places for such a short arm (or even two different length arms on the bike) but these cases, for adults, are very uncommon and not usually found as a solution (that works well).

I do agree to place the BB drop such that whatever arm length produces the desired cornering capability (for road bikes). Andy.
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Old 01-04-15, 11:10 AM
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That is very helpful. I am using lugs because it is a first build. I am using 26" wheels instead of 650c for heavier city tires that will be easier to fit than 700. Also 165mm cranks, I wanted smaller but 150 was much more expensive. Anyway, we have the box of parts. But with your two advice, I think I am ready to finish my drawing and start cutting metal.
Thanks!
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Old 01-04-15, 01:31 PM
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Originally Posted by Andrew R Stewart View Post
Anthony- Why is this (in bold) a requirement? I do feel that the body needs to be centered (weight wise) over the pedals but as one pedal moves rearward (the one in front because of the short cranks) the rear pedal moves forward, so their center of balance is still the BB. If anything I've found that with shorter thighs and feet that a slightly steeper seat tube is a possibility.

Also fitting really short cranks is a wonderful concept if both the availability of them covered a wide selection of brands as well as the rider being comfy with that choice of leg movement. IME many riders are already use to 170mm arms as that's the standard in most current small adult road bikes (for many unfortunate reasons). I've known customers (at the retail shop) who don't like even a 5 or 10mm arm length change (let alone a 30mm one). True there are places for such a short arm (or even two different length arms on the bike) but these cases, for adults, are very uncommon and not usually found as a solution (that works well).

I do agree to place the BB drop such that whatever arm length produces the desired cornering capability (for road bikes). Andy.
My guess is if using cranks in the 140 mm range or less, the seat tube angle may need to kick back for pedal to knee positioning. A guess.
For a rider 5'-1"... 150-155 mm cranks have been useful to me this was for an offspring and leg to torso length ratios are different often.
I do agree that if you drop the bottom bracket as allowed by shorter cranks, all the better for the bike's handling.
If using 571 rims I would design to have a wide range of tire sizes to be used. I went with 650c as it gave a road bike look, like Dad's machine. I used a stamped bottom bracket shell as it allowed for easier angle manipulation. Short point stamped lugs for the same reasons.
I would draw it out on paper too if bikecad is not working for you. I use a typical CAD program and have the parts at hand to measure off from, no guessing that way.
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Old 01-04-15, 04:04 PM
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Yes the relaxed seat tube angle is for getting Knee Over Pedal Spindle in the ball park with short cranks. While I don't think that a strict KOPS is required its certainly helps in getting in the ball park. Bike manufacturers use steep seat tube angles for short people, A, because its a dodgy way to shorten the top tube specifications without really shortening the front centre distance and B, because when you are using cranks that are too long for your legs then it does help to stay on top of them as it opens the leg angles. The number 1 downside of this is that riding with such a steep seat tube angle (and corresponding lack of saddle set back) is that a phenomenal amount of weight is placed on your hands/wrist/arms which makes riding a bike VERY uncomfortable.

Yes, short cranks are more expensive yet in the scheme of things they are cheap compared to the cost of the frame. Short cranks for people with short legs greatly reduces the leg movements which reduces leg rub. It takes the stress off the knees and when you combine this with the relaxed seat tube angles required for KOPS then the riders weight is taken off their hands/wrists/arms making the whole riding experience MUCH more enjoyable for the small rider.

When I first started thinking about and working with these issues I thought that I had come up with something new. Then I came across some OLD small bikes. All of a sudden there everything was, short cranks, low BB's, relaxed seat tube angles and short front centre distance. So, nothing new under the sun. We USED to know how to build bikes for small riders. The trouble is that this knowledge was lost in order to make bikes cheaper by using less different sized parts for different sized riders and less variations in frame designs to accommodate different size riders.

Anthony
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Old 01-04-15, 05:23 PM
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Originally Posted by AnthonyG View Post
Yes the relaxed seat tube angle is for getting Knee Over Pedal Spindle in the ball park with short cranks.
I believe short people usually have proportionally smaller feet. Therefore, to maintain the Knee Over Pedal Spindle principle you would need the same seat tube angle. I just imagine everything to be shorter (limbs, tubes, cranks ...) with the same angles.
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Old 01-04-15, 06:20 PM
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Originally Posted by waterlaz View Post
I believe short people usually have proportionally smaller feet. Therefore, to maintain the Knee Over Pedal Spindle principle you would need the same seat tube angle. I just imagine everything to be shorter (limbs, tubes, cranks ...) with the same angles.
I thought that it would be the same myself initially but it just doesn't seem to work out. I believe that its to do with saddle placement but I haven't sat down and drawn it all. I think the issue is saddle setback and the desirability of maintaining a reasonably constant saddle setback through different frame sizes. Since the nose of the saddle is in front of the post, and most saddles are of similar size then the seat tube angle needs to be relaxed to maintain the saddle setback distance over a shorter distance. An example would be that on very small frames even with say a 73 degree seat tube angle the nose of the saddle ends up in front of the BB. Crazy. 75 and 76 degree seat tube angles on small frames puts the nose of the saddle way in front of the BB.

That's what I think is going on anyway.

Anthony
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Old 01-04-15, 08:32 PM
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Ohh, I've forgotten to explicitly add, wheels small enough to fit the design. A fundamental flaw of current bike design for small riders is that companies are being cheap and lazy by using 700c wheels on small frames which totally compromises the design. The centre front distance simply can't be brought back far enough with 700c wheels so they bring the seat forwards instead.

The frame geometry needs to be worked out properly first and THEN you figure out what wheel sizes will fit. Wheels first is not good design.

Anthony
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Old 01-04-15, 09:07 PM
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I might suggest that the fit geometry is designed first, then the frame geometry with an eye to the components that will make the fit work. Good bike design is a back and forth process but the fit should establish as much as possible.

In an ideal world there would be a range of wheel, crank, bar, stem, levers sizes with equal quality and performance. But that's not the case so compromise is a part of real world design. Where we call the threshold of acceptable compromise is different for each of us as well as every builder/manufacturer. Andy.
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Old 01-05-15, 12:35 AM
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Sure, to me fit geometry and frame geometry are pretty much the same although I know that there is some leeway. When I was stating frame geometry I really meant fit geometry.

Lack of suitable cranks isn't a stumbling block for me. I've had cranks shortened. Its getting harder to find suitable cranks to start from with hollow arms and big scollops all the fashion yet there are options. There are smaller wheel options available such as 650c and 507 (24" mountain) or even several 20" sizes.

Sure, I'm a bit cranky on the subject from time to time. I used to race bicycles when I was young and despite the bike being custom made, supposedly to fit me it didn't fit. The seat tube angle was WAY to steep and it really held me back.

Big bike companies don't care. They make the customers fit the bike in a half way fashion. I just wish that there was more knowledge and action on the matter from small custom builders. The knowledge seems to have been lost and without the knowledge there is no demand. Small people suffer on bicycles without really knowing why and without good alternatives.

Anthony
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