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Custom Frame Geometry

Old 11-21-20, 02:23 AM
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azza_333
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Custom Frame Geometry

G'day everyone,

I am hoping to get some advise on a frame I am designing to have built for lightweight bicycle touring. I currently have a 2017 Kona Sutra frameset 54cm (https://konaworld.com/archive/2017/sutra_ltd.cfm) that I tour on, and fits me well, but it has a 72mm BB shell, and I want to run GRX with GRX for my bike tours from now on.

Below is the diagram of the frame I will have built, I used the Kona geometry as a donor, however I have made some changes.
BB Drop - I have taken the Kona's 72mm and increased to 78mm
Head Tube length - I have increased it 50mm over the Kona (on my Kona I run with 50mm of spacers under the stem, so figure I can go without the spacers, and have a bigger triangle for a frame bag, by having a longer head tube)
Seat Tube length - I have increased it by 20mm (I figure it will also give me a bigger frame triangle for a bigger frame bag)
Chain stay length - I have reduced it by 10mm to 435mm

Are any of the changes I have made not a good idea?

Thanks.
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Old 11-21-20, 03:11 AM
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LGTM! If you're going to be running panniers check that 10mm shorter chainstay isn't going to result in dreaded foot-slap. I guess see how much clearance you have on the existing Kona. And obviously tell whoever's building it what kind of tyres you're hoping to run so he can make sure they fit.
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Old 11-21-20, 06:00 AM
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Dropping the BB and shortening the chainstays will both have the effect of moving the rear tire closer to the seat tube and it doesn't look like there is a lot of seat tube clearance on the Kona. The longer seat tube and head tube are going to raise the top tube, so make sure you have enough standover clearance on the new design.
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Old 11-21-20, 09:04 AM
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I totally agree with both their suggestions! Chain stay length is so fashion and not function directed. Put back in that 10mm longer amount and after looking at the tire clearances consider another 5-10mms of length. I might have split the 50mm of stem spacer length that you added to the head tube. Maybe 30+ onto the HT and 20- of spacer? Is the fork length (A-C) the same as with the Mag 21's (and that's not the dimension given on the drawing)?

Maybe give us more info? Tires, crankset, Stem/bars WRT the seat, Seat height/setback. No mention yet of Top Tube length. Andy
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Old 11-21-20, 10:08 AM
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Originally Posted by Andrew R Stewart View Post
Put back in that 10mm longer amount and after looking at the tire clearances consider another 5-10mms of length. Is the fork length (A-C) the same as with the Mag 21's (and that's not the dimension given on the drawing)?

Maybe give us more info? Tires, crankset, Stem/bars WRT the seat, Seat height/setback. No mention yet of Top Tube length. Andy
Thanks for the response. Yeah I think I will put the CS back to 445mm. The Fork length is from the axle to the top of the crown.

Tires I will use are 700c x 35mm (will also run fenders), crankset GRX 810 172.5mm, +6 degree 90mm stem, compact drop bars. I would have to go back and measure (currently at work) the seat height but not very high compared to handle bars, maybe 1-2cm lower than the handlebars.

I haven't figured out how to calculate the TT length, that is why its not listed.
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Old 11-21-20, 08:39 PM
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Top tube length (or seat to bars) is generally considered to be a given, of course a non horizontal and miter to miter dimension is a resultant. I guess i was trying to point out the not so usual path to your numbers. Do you use an on line calculator, a 3d design program, BikeCad or other frame design software?

Tires don't seem too large, I suspect 445/450 chain stays will work well enough. I try to not use a stem as a given but allow a bit of after build up fudge to be dealt with by the stem if needed.

As I mentioned the drawing doesn't show the A-C dimension but what I would call "fork length" with the upper end being above the headset lower stack (or to the Head Tube's bottom), not the usual crown seat as it is on the steering axis.

These little differences are not issues when discussing general design styles but do have their additions to the final geometry and thus the mitering and jig set up let alone the final ride. Andy
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Old 11-22-20, 09:34 AM
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I would draw in and call out the rim ( braking track) and OD tire dimension

also a true view of the chainstay run with tire shape- might as well know right up front on chainstay clearance.

the front center, top tube dimensions are blank- if you are running fenders decide on any front overlap you are willing to tolerate.

and I suggest drawing out the ground plane to review trail- not criticizing yet, but understand what you are setting up based on other bikes you have ridden and experienced.
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Old 12-05-20, 01:48 AM
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Thanks for everyone's feed back.

I have another though. if the fork length is reduced to 400mm (was 415mm), will increasing the head tube length by 15mm keep everything else the same, or will I have to change things like the rake, trail, headtube angle, ect.

I have been thinking about it, but wont reducing the fork length and increasing the heat tube length, change the head tube angle?
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Old 12-05-20, 03:28 AM
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Originally Posted by azza_333 View Post
Thanks for everyone's feed back.

I have another though. if the fork length is reduced to 400mm (was 415mm), will increasing the head tube length by 15mm keep everything else the same, or will I have to change things like the rake, trail, headtube angle, ect.

I have been thinking about it, but wont reducing the fork length and increasing the heat tube length, change the head tube angle?
You can make the HT longer and keep it at the same angle but if course the angle (and length) of the DT will also change a bit. This is no problem though.
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Old 12-05-20, 04:06 AM
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Originally Posted by guy153 View Post
You can make the HT longer and keep it at the same angle but if course the angle (and length) of the DT will also change a bit. This is no problem though.
Wicked, thanks that was just the answer I was chasing.
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Old 12-05-20, 05:35 AM
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Originally Posted by azza_333 View Post
Wicked, thanks that was just the answer I was chasing.
Also make sure you have accounted for the headset lower stack. In your diagram it looks like it's being included in the fork length. It's usually 12mm. So you probably have to add 12mm to whatever the new fork is to get a comparable figure. It looks like the fork in your diagram is actually 403mm.

Most forks quote the "axle to crown" distance as described here: https://www.bikecad.ca/taxonomy/term/52

My favourite approach where possible is to actually make or buy the fork (and the headset) before finalizing the design for the frame so I can measure it all to be sure what I'm dealing with.
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Old 12-05-20, 05:43 AM
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Originally Posted by guy153 View Post
Also make sure you have accounted for the headset lower stack. In your diagram it looks like it's being included in the fork length. It's usually 12mm. So you probably have to add 12mm to whatever the new fork is to get a comparable figure. It looks like the fork in your diagram is actually 403mm.

Most forks quote the "axle to crown" distance as described here: https://www.bikecad.ca/taxonomy/term/52

My favourite approach where possible is to actually make or buy the fork (and the headset) before finalizing the design for the frame so I can measure it all to be sure what I'm dealing with.
It will be an integrated headset, so I am assuming that doesn't factor into the new frame.

But the old one I have based it off, KONA BIKES 2017 | ROAD | SUTRA | Sutra LTD (konaworld.com) do you think the 415mm fork length includes the 12mm lower headset, or will I have to add it to the new frames head tube length?
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Old 12-05-20, 11:00 AM
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The quoted fork length usually does not include the lower stack. But your diagram (which is your new design) does appear to include it. So just make sure you're straight on who is and who isn't including it!
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Old 12-05-20, 05:14 PM
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I have an earnest question for you all. If the OP is not doing the actual building of the frame, what difference does it make what his drawing shows? The builder will make his own drawing and that is the drawing where the details matter.
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Old 12-05-20, 08:05 PM
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As someone who has had a customer (many years ago, before I learned the value of being insured) who wanted to be a "partner" in the design I will say that both the customer and builder had better be on the same page (or drawing as in this case). After a few go rounds and lot's of effort to explain what counts and what is a resultant I ended up returning the deposit and tried to keep them as a friend. Andy
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Old 12-05-20, 08:30 PM
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Originally Posted by Andrew R Stewart View Post
As someone who has had a customer (many years ago, before I learned the value of being insured) who wanted to be a "partner" in the design I will say that both the customer and builder had better be on the same page (or drawing as in this case). After a few go rounds and lot's of effort to explain what counts and what is a resultant I ended up returning the deposit and tried to keep them as a friend. Andy
Thanks for the feedback, the frame builder is Waltly, from what I have read online, they make great stuff, but they will build what ever a customer asks for, and generally not recommend to much. Even if the customer asks for something that is a bad idea. So essentially it is my job to ensure the frame has been designed appropriately.
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Old 12-07-20, 04:26 PM
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"So essentially it is my job to ensure the frame has been designed appropriately."

Wow. I leave that sort of thing up to the ones with experience, it only makes sense. From the waltlytitanium.com website, "Pay a 30% nonrefundable deposit via PayPal or wire transfer and our draftsmen will design your frame and create CAD drawings for your approval," Perhaps I am reading it wrong by not weighting the Submit Plans section,

Having built several of my own frames I know what happens when an idea goes the wrong way when put to use!
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Old 12-07-20, 10:09 PM
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Originally Posted by TiHabanero View Post
"So essentially it is my job to ensure the frame has been designed appropriately."

Wow. I leave that sort of thing up to the ones with experience, it only makes sense. From the waltlytitanium.com website, "Pay a 30% nonrefundable deposit via PayPal or wire transfer and our draftsmen will design your frame and create CAD drawings for your approval," Perhaps I am reading it wrong by not weighting the Submit Plans section,

Having built several of my own frames I know what happens when an idea goes the wrong way when put to use!

I have parallel thoughts about a contracting frame fabricating shop VS a frame builder. I can see the business plan for a contract job shop but it places nearly no value on experience and design skill and instead elevates the low cost of delivery. Once more the separation between commodity and custom is drawn thinner. Andy
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Old 12-08-20, 04:53 AM
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I see where you're coming from but the hard part of making a frame is actually making it not designing it. Of course Trekgiantspecialized would like us to believe we're paying for all their design work, carbon layup blah blah and this makes it fine for them to produce the actual frame as cheaply as possible as if the guys making it were just grunts. There are a few parameters with a steel frame (diameters, walls, angles, CS length, etc. etc.) but the customer can make an informed choice about those especially now that he can discuss them with random people on the internet like us. What makes a custom frame special is that it's exactly what you wanted with no compromises and it should be beautifully made.
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Old 12-18-20, 03:13 AM
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Another quick question,
KONA BIKES 2017 | ROAD | SUTRA | Sutra LTD (konaworld.com)
the stack height listed on this page would that include the height upper headset bearing? thanks.
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Old 12-18-20, 05:47 AM
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The frame stack height is for the frame only. That measurement is to help you compare frames and headset stack heights vary between manufacturers.
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Old 12-18-20, 11:12 AM
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Kona has ridiculous frame sizing. It far understates the size of their bikes relative to any other brand. It really makes it hard to compare, and probably puts a lot of people on bikes that are too big for them.
I know someone that takes a 65 and he decided Kona didn't have a bike for him because the largest they make is a 62 or something like that.
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Old 12-18-20, 12:08 PM
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Welcome to the world of labels that sound like measurements. Perhaps the bike industry can't stand following common sense practices The frame sizing labels are much like the tire/rim sizing systems, only to be considered a starting point and not a claim of dimensions. Andy
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Old 12-18-20, 01:17 PM
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Originally Posted by dsaul View Post
Dropping the BB and shortening the chainstays will both have the effect of moving the rear tire closer to the seat tube and it doesn't look like there is a lot of seat tube clearance on the Kona. The longer seat tube and head tube are going to raise the top tube, so make sure you have enough standover clearance on the new design.
I second this. Standover clearance could be an issue.

How does the top tube length and reach feel currently? Do you want to try and shorten that a little as well to maybe get a more comfortable upright riding position where your weight is more rear biased?
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