Please comment on this geometry proposal

[cfrisia]

Hi,

Quite some time has passed for planning my next frame - First of all, a big thank you to this forum for being a source of inspiration and learning.

My first and current frame is a testbed that I'd like to build on for my next project.
The impressions of about 2000 mls riding under various conditions, changes in components (saddle and pedals, specifically) and some modern frames - Gaulzetti has been a big inspiration for this - have given me some ideas where I can improve.

Here's the geometry I would like to do:

Drawing1.jpg

Nothing spectacular? Here's the point: I'm not sure.

Would you share your thoughts on it?

- Do you think the headtube angle is too steep? Current frame has 73°, a 110mm stem (albeit -10°), 55mm trail and was a bit wiggly during a crit
- Current frame has a wheelbase of about 955mm - Is this more important for stability than headtube angle?
- Do you think the chainstays are too long?
- What do you think could improve the handling of this frame?

I'm mostly concerned about stability. I want this thing to be planted, but not sluggish. But it should be rather leaning towards sluggish then towards wiggly.

I know I'm asking a lot to wrap your heads around this. But I'm happy with any contribution.

Kind regards

Chris

[PJN]

Hard to go wrong starting at the right point. Just copy the one that fits from the above table and forget about it.

[hampco]

chainstays are too long, stem is too short, and the 43 rake fork is too little for this size frame - try a 50 rake and reduce the HA

all imho, of course

or, build it as is and use this as a learning experience

[zetroc]

Build one that way and build one the way hampco suggests.

[cfrisia]

thanks for the replies...

mr. hampsten: I'm having a hard time figuring that on out - my current frame has a TT length of only 515mm with that -10° 110mm stem. By using a 90mm stem, I'm trying to get some more wheelbase. Should I trade ~10mm wheelbase for 10mm more stem?

PJN - that is indeed a good starting point - I heard those frames handle very good as well

zetroc - not enough money/time

[e-RICHIE]

That's an extremely unhelpful schematic because in the OP because it ignores the
saddle height, its horizontal placement, and the distance from it to the handlebars.

[theflashunc]

With a 90mm stem, I'm guessing you haven't read a lot of what the Gaulzetti guru Mr Jerk has to say about short stems...

[cfrisia]

sorry - I didn't think to include them because they're dialed in / fixed

Drawing1.jpg

the thing is with stems - I need the 522mm seat length and 128mm drop. I can't do more. More stem would mean less top tube would mean less wheelbase would mean less stability -
so this is what worries me.
theflashunc, do you agree with hampco that I should trade some wheelbase for a longer stem?

[e-RICHIE]

You have a position very close to what I use for cx.
70cm saddle height, 51.5cm reach, and a 4cm setback.
That geometry design needs a huge rethink to yield these contact points as well as a bicycle that works well beneath a rider.

sorry - I didn't think to include them because they're dialed in / fixed

Drawing1.jpg

[cfrisia]

Richard - not sure if I understand, can you think of something specific?

[e-RICHIE]

Richard - not sure if I understand, can you think of something specific?

Forget I wrote that. I looked at the 70mm and didn't see the 755mm number. But I can't process why
someone with that saddle height sits so, so, sooo far forward as to need a sub 13mm setback position.

[cfrisia]

What do you mean by sub 13mm setback position? The setback is there purely for aesthetic reasons btw

[shand]

I think Richard's missing the 70mm setback and reading the 128mm drop as the setback dimension? (the dimensions are a bit unclear)

[e-RICHIE]

I think Richard's missing the 70mm setback and reading the 128mm drop as the setback dimension? (the dimensions are a bit unclear)

Ya got me.
The schematic implies that the saddle nose is 12.8cm behind the central movement.

[theflashunc]

theflashunc, do you agree with hampco that I should trade some wheelbase for a longer stem?

I'm nowhere near the expert that a lot of these guys are in design. All I know in my hamfisted life of riding a bike over two-plus decades now is that every short-stemmed bike I've ridden felt like I was back behind the wheel of my dad's Ford Jubilee tractor. Steering inputs were vague, didn't have enough weight on the front wheel, etc etc...

Obviously there's a ton that goes into it beyond just stem length, but methinks a custom frame with a 90mm stem is either compensating for some extreme physical limitations, or is missing the mark in basic dimensions.

[cfrisia]

cleaned it up a little -

disputable stuff in orange, fixed dimensions in green



edit: as you may notice, I got the wrong dimension on the saddle height before. now its fixed.

[NBC]

cfrisia 01.jpg

i dont know if this is any help at all ? similar to what hampco was suggesting, albeit with a 100mm stem instead

sorry that there's a couple of measurements missing (wheel diameter 668mm, fork length 367mm) 69mm bb drop if you want to work it out that way

HT length is for a chris king headset, 10mm of spacer, 42mm clamp height stem in 84 degree rise/drop

wheelbase 953mm

NBC

[cfrisia]

wow thanks a lot!
I'll take that into account, especially the 71.5° HT angle - seems like I have to find a fork with a bigger rake.

edit: can you see the picture I posted before? It's now gone, at least for me(?)
attaching it again..
Drawing2.jpg

[NBC]

cfrisia 01.jpg

sorry, revised image, the other one was drawn with the incorrect saddle height !

once again, bb drop 69mm, wheel diameter 668mm, fork length 367mm

i believe that 3t supply their forks in 49mm rake, and enve do a 50mm rake in theirs, wouldn't make a huge difference

NBC

[Scheisserad61]

That design with that saddle height (now it is 766?) looks questionable at best. This is the problem with trying to analyze a frame design WITHOUT a rider. How about pushing the CAD aside for a bit and simply post the following photos of you in riding gear with your current bike held in a trainer and levelled :

1. Side shot : hands in the hooks pedal at 3:00
2. same as above, but hands on hoods and pedal at 6:00

Make sure the photos are well-lit, clear (decent camera no fuzzy phone pics), and the lens should be around the same height as the saddle, maybe a bit higher. Try and have the front wheel in there so we can get a feel where the front axle is. Try and get a neutral background so you and your current bike are clear.

Hopefully this might help sort out some of the numbers on that last post.