Size up or size down? Geometry geek out.

[bikeboy1]

The bikegeocalc is a great tool that I have used a few times for myself and also clubmates looking for new rides.
Like a few have mentioned above with stack and reach being pretty much the first factors to consider followed by STA and ETT data.
When I purchased my V2R back in 18 stack and reach came pretty handy other wise it was always TT,STA and headtube lengths before those two figures became mainstream for me.

[ryanisinallofus]

Originally Posted by jtakeda

Size down.

It’s much easier to get a longer stem and change stack. Much harder to shorten a top tube

Or lengthen it

[wackyjack]

Can I revive the thread a little bit?

I have an idea of key measurements for my custom frame, but - since it's custom - can go either 90mm stem (riding it currently on a gravel frame with majority of time spent in endurance road) or 100mm stem and smaller frame: obviously with cascading effects on top tube (weight) and wheelbase (stability).
Plan is to have a comfortable endurance focused (up to 100 miles in one sitting) frame tailored for long solo rides, not too twitchy, not competitive. I know my measurements after a bike fit, but would appreciate opinion on which way to go: either longer F-C/wheelbase/top tube + shorter stem or shorter frame and longer stem.

[9tubes]

If it's a custom frame it is the job of the framebuilder to advise you.

If the framebuilder is a great craftsman but not the best fitter then hire a fitter.

If the builder is a great fitter but not a great craftsman then fire the framebuilder.

[wackyjack]

Originally Posted by 9tubes

If it's a custom frame it is the job of the framebuilder to advise you.

If the framebuilder is a great craftsman but not the best fitter then hire a fitter.

If the builder is a great fitter but not a great craftsman then fire the framebuilder.

Fit-wise everything is spot on in both configurations, but the builder is not a great fitter honestly. I'd say that not a fitter at all, it's just a shop/factory where I can commission a custom frame (as much custom as they can do, which is the case with other frame builders too afaik).
Somewhere on internet I read that there is a reason why road bikes are designed with 100+ mm stems, while I'm not positive that 10mm reduction would be a drastic one. I'm more doubtful about stability of the bike for long-distance rides (aka longer wheelbase), hence, the question.
I'm trying to find a direction for thoughts in this dilemma.

[fourflys]

Originally Posted by wackyjack

Somewhere on internet I read that there is a reason why road bikes are designed with 100+ mm stems,

I think that may be a holdover from a more "vintage" time.. not sure how much that is true with how many of today's frames are designed.. I could certianly be wrong, but I think is has a lot to do with head tube angle.. maybe the steeper the head tube, the more important the 100+mm length?

but as it's been said already, that's really the domain of the builder.. honestly, if you are talking to a builder who doesn't have a good idea either way on which way to go, you might want to check a couple other builders.. eRichie is famous for taking your body measurements and figuring out what the angles and lengths need to be.. my feeling is that's what you mainly pay custom builder for.. lot's of folks can lay down a beautiful weld.. not saying you have to spend Sachs money to get that as I think many others will be able to give good advice/guidance (Rock Lobster, Sycip, ??)

just my .02..

[whatshubdoc]

Originally Posted by wackyjack

Can I revive the thread a little bit?

I have an idea of key measurements for my custom frame, but - since it's custom - can go either 90mm stem (riding it currently on a gravel frame with majority of time spent in endurance road) or 100mm stem and smaller frame: obviously with cascading effects on top tube (weight) and wheelbase (stability).
Plan is to have a comfortable endurance focused (up to 100 miles in one sitting) frame tailored for long solo rides, not too twitchy, not competitive. I know my measurements after a bike fit, but would appreciate opinion on which way to go: either longer F-C/wheelbase/top tube + shorter stem or shorter frame and longer stem.

I believe that depends on the frame geometry - specifically down to HTA/Rake/Trail.

I have:

1. A "standard" road bike with 73.5 HTA, 43mm rake, that yields ~56mm trail with a 25mm tire. I ride this one with a 100mm stem. This bike also has 407mm chain stays, thus it's got a more race focused ethos. A shorter stem might make it twitchier than I might like by unweighting the front end too much.

2. An "endurance" road bike with 73 HTA, 45mm rake, that yields ~58mm trail with a 28mm tire. This one is a little longer in the top tube, and I fit it with a 90mm stem. (This one recently sold as I did not like how it felt under power/sprinting. I also did not enjoy its wheelbase after riding my #1 road bike.)

3. A "gravel" bike with 71 HTA, 50mm rake, and yields ~69mm trail with a 38mm tire. This one I ride with a 100mm stem, but in another life I would've sized up and gone with a 90mm stem. Why? Because the slacker HTA can accommodate a quicker steering arc than the other two bikes mentioned above. However, the "down side" is now that I might only be limited in going down 10mm (to an 80mm) stem as I age, or in a season where I might be inflexible. If I rode it with 100mm off the get-go, as I do now, then I have 20mm of room to go down as my back starts to crap out on me.

So, that being said:
1. What kind of bike is this custom bike? ie. what is the HTA/rake/trail? How do you like your bikes to feel/steer on the front end?
2. How long do you plan on keeping this thing?

My opinion is to go for something that can give you 100-110mm stem length. A lot of builders seem to use this as their recommendation. I think it gives the best mix of handling and fit, while providing enough room for adjustability. The other thing they might do is build the frame such that you'll have 10-20mm of spacers under the stem when the bike is assembled. I have not personally had a frame commissioned for myself, but have spoken to those who have.

[wackyjack]

Happy to share my specs:
HTA: 72 degrees
Fork A-C: 382mm (Columbus Futura Gravel)
Fork rake: 47mm
Trail: 63mm with 38mm (max measured) tire. With 28mm Vittorias for fast(-ish) club rides trail goes down to 60mm.

I'm looking for an endurance bike as I love long days in the saddle, hence the HTA and tire choice.

I did a fair amount of research on custom build bikes for similar purposes and noticed that trail is usually in high 50's or very low 60's. Also, wheelbase is usually a shy over 1 meter –*in my case (drawing) it's 1014mm.

I have a gut feel that I'm bang on with angles and measurements, while wanted to hear opinions. I can post the entire geo here if that helps.

[wackyjack]

Forgot to say: I'm planning to keep this thing for long time, potentially as a single bike (well, except a city rig for commuting, but that's a different story) over next 5-10 years or even more – depends on how my body will age and for how long I will be flexible enough to ride it.
Once I submit the drawing to fabrication I might start a thread on how I designed it, what considerations went in, how I selected components and (to some extent) tubing.

[whatshubdoc]

TBH I've ridden 90mm stems for so long on a road bike that I'm kinda tired of it. Knowing that I'm at the edge of the fit window for the bike, kinda sucks.

If the bike is primarily meant for the road, go with the 100mm. You'll get a little more weight over the front; this will help when things get tired and ugly. Instead of having to weigh down the front end consciously on a descent, the stem length will do that for you.

*Disclaimer: I used to race ~20 years back and have always preferred quicker steering bikes.

** As an afterthought, why don't you post the full geo so we can see how the front center:rear center ratio looks.

[jimoots]

Should be a conversation with the framebuilder - reach and stack are not independent.

That is, because the head tube is on an angle, if you extend reach with a stem then you gain stack.

Conversely, if you extend reach with the top tube you don't lose 'frame' stack, but you don't gain it when you add a stem (i.e. you end up using a shorter stem) , so effectively you lose stack from the system.

In actual numbers, with a 73 degree head tube you gain 2mm of stack for every 10mm of stem reach. So the best part of ~5mm of stack if we're talking typical transitions between frame sizes (i.e. 500/520/540/560).

[callmeishmael]

As a smaller rider (I've ridden 51-54cm frames for 20+ years, but default to a 52) I've got very used to 90mm stems, simply because many manufacturers produce frames that actually have very similar reach measurements from a 52 to a 54; they just steepen the seat angle to reduce TT length (that's a cheat, IMO, but off topic. Another off topic is ludicrous 75 deg seat angles on road bikes, but I won't go there, either).

It used to bother me, but doesn't really now. I'd also say that on most bikes, most of the time, I genuinely can't feel a difference between 100mm and 90mm stem - in terms of handling, that is. A genuinely fast, snaking descent on a bike with a steep HTA might be one of them, but they are relatively few and far between in my neck of the woods. What I often can feel, though, is mild hand discomfort after 90 minutes to 2 hours, if I go too long at the front end, as well as an increasing, almost subconscious reluctance to use the drops.

So these days, if I feel comfortable on a bike, in a balanced position, and the bike handles ok, I tend not to lose sleep over those kind of differences. The crucial thing is to get that position absolutely dialled in.

[callmeishmael]

Originally Posted by fourflys

eRichie is famous for taking your body measurements and figuring out what the angles and lengths need to be..

It would never happen, but I'd be fascinated to see what he'd come up with...

As an aside, a friend of mine ordered a Casati (just pre Covid). He sent bike fit data and his physical measurements. The chaps at Casati apparently said something along the lines of 'We'll build it to your fitter's numbers if you want, but they seem wrong based on your measurements. Will you trust us to make the bike how we think it should be?'

He did and he says he's never been so comfortable on a bike. Just goes to show the value of experience and good instincts, I guess!

[wackyjack]

Here is the target geometry. I have steepened HTA by a .5 degree to 72.5 and adjusted top tube length to have perfect fit with 100mm stem. As others say, and I generally agree, with the 'smaller' frame there is a better adjustment window as body ages. In fact my personal assessment of my mobility is between 'a brick' and 'Pinocchio', so I'm not a racer-type person by any means.
Screenshot 2023-03-24 at 22.03.52.jpg
Crucial point for me here were saddle-bar drop and BB to saddle tip distance, while BB to saddle length (along the ST) was almost perfect even before the fit. In classic reddit terms, feel free to roast (joking).
For the reference, bike is built around 35mm tyres on 25mm internal width rims, so I expect they're blow up to 38mm or so, which is, in turn, max tyre size that I specced for this frame. For gravel shenanigans I have a dedicated gravel bike with chunky 43mm tyres (frame can take 47mm in fact).
One more thing to note is that I'll leave a 10mm spacer on top of the stem to have some adjustment possibilities later on. However, that doesn't affect the geometry.

[robt57]

979mm WB, make sure you want that is my main takeaway.
I see no practical purpose unless you are crit racing IMHO.

Especially if you wind up with long cockpit and forward riding.

That will be a small fore/aft CG window IMO.

Me, RR geom up. CX correct of smaller OK if not too small... larger fore/aft CG window to stay in with CX/Gravel CS/WB/FC etc