First Custom Frame, Please Critique and Advise

gearcycler · January 23, 2025, 10:44pm

Hi all,

I’m getting an all-round/rando whatever frame for an unsual position: Big and long head + neck, longer torso, shorter leg with an even shorter femur compare to tibia. I would like your advices on Geo and (a bit) Tubings please!

1. Geo: My biggest concern is Front Rear Weight Distribution
[Previously]
JAN 29 UPDATE:** Now I seat further back. Red line is MyVeloFit’s estimated center of mass but I think this is not right, i feel like my COM is much more in line with the BB is not front of it

Current Front / Rear ratio is 562/430 which is ~ 1.3. It will only decrease as I may want to seat more upright or at least rearward, and I’m not sure a 435CS is enough for 650x43 + fender. To compensate I seat a bit back which is a more natural position for me I feel like as well, 73STA

My general need is a bike that rides comfortably but still lively, quite upright.
2kg saddle bag and 5kg front load (for ease of use since I dont want too much rear weight or having to deal with a second rack). Since it’s primarily front load, I’m aiming for 50 trail (hope it’s not “no man’s land”), achieved by 71.5 HTA and 60 rake for now. (I want the front wheel further out)
New geo 72.7STA is an experiment, it will be 72.5 or 73 actually

Resulting trail is 50, so semi-low because I’m a bit scared to fully commit to low trail for now.

2. Tubings:
TT: 455 long (maybe I will increase for more butts): 25.4OD 8-5-8 with 90mm toal butts left…
DT: 555 long, 28.6OD 8-5-8 with with 155 butts left, should I have the downtube shifter boss on the thick butt?
ST: 28.6 with a 1.0 or 1.2 bulge probably doesnt matter, should I have the FD clamp on the thick butts?
SS: I can get away with 14mm thick but I guess 16mm cant make for a noticeably less compliant ride
CS: just have to zig zag around the rings and fender a bit, hope the bends wont be. too sudden

Updated:

That’s it, and though difficult I would want in the middle where I can easily adjust it to be more upright or more agressive just based on moving stems, bars and saddles. As well as, changing postures throughout the ride. Look to achieve that in my second fit session (plan for now is more upright and seat back further).

Look forwards to your insights, thanks all!

DEVLINCC · January 23, 2025, 11:19pm

With short femurs that seems like a slightly slacker seat tube angle than I would have thought. Going a little steeper there would move the seat forward, open your hips, similar to sitting more upright in some respects and will move the front center out. Where is the top of the tibia in relation to pedal?

I’ve never ridden a low trail bike with weight so take my next comment with that in mind. Possibly go for a longer trail that will add a fair bit of stability, though I would imagine the amount of weight you end up loading on the bike would have a big change in feel.

gearcycler · January 23, 2025, 11:54pm

Thanks for the insight Sean! I read so too but for the purpose of experimentation, drew the bike with a 40mm setback (those 2 sketches) instead of the 30mm that my fitter advised (screenshot of me on the fit bike)

And bottom

Maybe after all, my COG is not that bad but I am not sure if it’s that simple to determine honestly

DEVLINCC · January 25, 2025, 10:34am

IT’s always hard to be honest without seeing it all in front of you. I would move the saddle to where your fitter had you, add 4mm to your trail for your chosen tyre size and look at using a 10mm shorter stem. See where that lands everything for you. This would be a set up that would need the front loaded. It will be a little nervous handling without the weight. To me 5kg is not a big load and if is in a bar bag it’s fairly close to teh steering axis. If it going to be on a rack then it will have more affect and you’ll need to adjust your trail to suit.

gearcycler · January 25, 2025, 3:48pm

Thank you! I have been curious about how the various front load mounting points would handle differently…

I would have a Nitto front rack to the support the bag then a decaleur or stick it to the bar on top additionally…

So instead of opting for a sub 50 or even 40 trail, I guess your thinking is left a more average trail but quicken the handling by using a 70mm stem instead?

One other thing i was think of doing is maybe bend the fork to 55 rake for a higher low-mid 50 trail instead, then bend it further if i need an even lower trail. Would that be sub-optimal?

DEVLINCC · January 26, 2025, 3:17am

Ahh…my experience with loaded touring geo’s is limited so I could be talking shit. Hopefully others will chime in here and correct me but I’ll have a crack at it… If you are going a rack then I’d probably stay low 50’s and realise that it needs a load to ride nice. The stem doesn’t really affect steering when changing by 10mm. Its more to increase the front centre so you have a better balanced bike. So 10mm in the stem and 10mm in the more forward saddle position will give you roughly 20mm more front centre and you’ll have a better front to rear balance.

bushtrucker · January 28, 2025, 1:13am

What sort of terrain do you mean by “all-road”? If you plan to ride on rough or loose surfaces then a predominantly front loaded bike will feel sketchy. This is not where low trail bikes excel and I definitely wouldn’t go as low as 50mm of trail for that use. On smooth gravel and pavement you will be fine though, especially with the relatively small loads you’ve described.

As far as general design feedback goes I would aim for more neutral handling — around 70mm of trail — and even weight distribution. I would definitely take Sean’s advise and design around a shorter stem and longer front center too. The short front-center and long stem thing is really just a holdover from road racing where you want to be as physically close to the rider in font of you for aerodymanics. A longer bike will be more stable and more comfortable on big rides. I’d potentially push the chainstays out a touch for the same reason. I have a 60mm stem on my gravel/all-road bike and it handles well. That same bike has a 71deg HTA and 47mm offset fork. I actually wish it was a touch slacker.

STA is very much a personal preference so hard to comment but I’ve found with a more upright position a steeper STA will open up the hip angle and be a lot more comfortable. If you’ve been riding a bike with a traditional slacker STA this new position might take some getting used to. In that case I would design around an inline post and then you can always chuck an offset post in if you feel like you’re too far forward.

Oh and i think you’re right that rear end compliance is negligible in the scheme of things. 16mm stays are fine if that’s the builders preference. Ya rear tyre will provide 90% of the ride feel. If you want a more comfortable bike you’re better off goin up a size of rubber than down a size of seat stay. Like some brands might say a new model is 20% more compliant but that’s just marketing. The absolute values are still negligible in relative terms. That’s 100% a personal hot take but I’m yet to come across any hard evidence that disproves is.

gearcycler · January 28, 2025, 5:50am

Thanks Sean! In your experience, what front/rear ratio range works well for a…really small bike?

And…I actually completed my 2nd part of the fitting just now. I ended up seating further back and raised the handlebar up more. But the seating back 13mm might be a bit radical so I will puch in the numbers for 6-7mm further to be in the middle.

With a longer upper body, it removes some weight from my arm but I will keep the hip angle thing in mind @bushtrucker

And 15-20% of. he time I ride gravel it will both mixed dust and clean stone at the same time. My concerns is that tarmac where I am from is very bumpy

Anyhow, biggest questions for me now are probably:

Deciding numbers to get the “middle” for everything, so future adjustment would be. easier. I will have to figure out how my posture would change for slightly more upright or slightly less, and where the different respective touch points would be?
Maybe semi-dipping with trail? Start safer with a 53 rake ~ 57 trail then if it’s not too suboptimal bend the fork further to 60 rake ~ 50 trail?