Framebuilders - First time builder: geometry help please!
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06-23-12, 08:58 PM
I'm planning to solder my first frame in the coming months. I have been reading extensively about it and have purchased a few good how-to books for noobies. I'm currently completing my CAD mockup for the roadbike I'd like to build but have some questions that I figured someone on the forum might be able to answer.
A few quick notes about the bike in the works:
-I'm building with lugs and will be silver soldering, so I have relatively fixed joint angles.
-I'm going to be using Llewellyn's Manorina lugset that allows for a 6* TT slope.
-I have weird proportions, with very long legs and a relatively short torso that necessitates a top tube of about 53-54 cm (depending on ST*).
So here goes:
1. Is it unreasonable to think that I can bend lugs to accommodate angles that are less than 1 degree off? For example: I need a lower HT lug with a 59.1 degree angle--it will come as purchased with a 60 degree angle.
2. The current front-center distance is 56.54 cm. This feels a bit low to me, as it seems that most road frames have around 58+ cm. Should I be concerned about this? Will the frame be unrideable with that little front-center? A little toe overlap is okay with me.
I have posted two images of the geometry sheet for the bike to be built and would love any geo input, especially regarding the two questions above.
Thanks so much!
SF Bay, California
Andrew R Stewart
06-24-12, 04:39 AM
Darius- Your geometry link didn't open for me so I'll respond to your written questions for now. Lugs should be able to accomidate a degree of change without too much effort. The deeper sockets will be a bit more a challenge but doable. However I'd recommend that you start to get profecient with brass as it will fill the potential gaps (that come from angle manipulation) better. The front center length is very short. Depending on crank arm and foot dimensions expect around 2+cm of overlap, this is a lot. Worse if you use fenders. As a rider that track stands a lot while in traffic and uses fenders on my commuting bike, I wouldn't tollorate this much overlap. I don't know what your head angle, rake, tire radius are so i can't recommend here but a slacker steering geometry will help extend the front center. Please keep us in the loop as to your progress and have fun, Andy.
06-24-12, 06:13 AM
I think I fixed his link-- works for me now anyway.
Trade some stem length for front-center and you will be happier.
One degree of change in lugs is about the normal amount of slop. Much more, and you'll have to bend.
06-24-12, 10:37 AM
Thanks. The link should work now if you're interested. I think I will extend the TT/DT a bit and shorten my stem to squeak some more front center out of the bike. I've noticed a lot of bikes with a front center distance of around 58-61 cm--is this normal? And if I were to use fenders/wider tires for my road bike, is there a value you think would be ideal? I will definitely keep the posts coming as I know there will be many more hiccups with the build to come. Thanks again.
You could also increase the rake to 50-52 mm. Would be better for a 72 deg head angle.
Btw a lot of people (including myself at some point) seem to say they have long legs and short torso. Not saying it isn't true but a second opinion could help. I appreciate being a bit stretched out.
06-24-12, 07:32 PM
Darius, I've got similar body proportions; I'm 6' tall, with a leg length of 36", so half of my overall height is in my legs. I really can't make stock frames fit me comfortably because I need a 60cm to 61cm seat tube, but a relatively short top tube of 55cm to 56cm. Compounding the seat tube to top tube ratio issue is the inevitable problem of reduced body flexibility that comes with getting older (I'm 70).
I'm building a new road bike for next year's AIDS/LifeCycle ride using Dazza's 6° sloping top tube Manorina lugs to raise the handlebars without looking too goofy. You're on the right track.
Andrew R Stewart
06-24-12, 07:36 PM
Darius- The plans did link up, thanks to who ever "fixed" it. Now I can offer some better observations based on the plans as they are in the smaller but more complete bike version.
Front Center- Reads as 58cm. 58.5 is my min for no overlap with 170 arms and no fenders. Maybe add 1.5cm to allow 172.5 and fenders.
Reach- I see a lot of seat/bars difference in height. This does impact the feel of reach to the bars as wel as actual horizontal dimension. What is the 7.4cm based on?
Seat angle- You gain about 1cm per degree of change here. Could this go forward and the seat be set further back on it's rails?
Head End- the same 1 degree gets about 1cm of front center holds (with a rake change adding a little bit more if the trail is maintained) (I do like the 6.1cm trail, some prefeer a shorter trail, I don't)
Top tube- As mentioned an exchange between the stem and TT length is the most straight forward way to increase Fc with out any other changes.
Chain Stay- Not asked about but my question is that if you do have a short torso and then need a short reach to the bars (which BTW might be more flexibility ditermined then body dimension ditermined) the you'll have more rearward a weight bias. So maybe a touch longer stays to keep the center of gravity ballance better. Just a thought here.
I assume you have been fitted on more then one bike and recorded those numbers then applied this knowlege to your new design. The two images on the link don't match. Is one a first version and the other a revision? Andy.
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