Please comment on this geometry proposal

[David Tollefson]

What's the difference in BB drop?

[cfrisia]

new: 70mm old: 68mm

[PJN]

edit: p.s. I'm a bit curious about the Dogma geometry on first page. the lower sized frames yield something like 73-75mm trail, I can't imagine that handles nicely.

Look at how tiny the front-center dimensions are. It's more complicated than 'trail'.

And your chainstays are still way too fucking long if this is going the be a "race bike."

[cfrisia]

well

That's where it gets interesting. The front-center of my geometry is even smaller right now -
OLD version:

Drawing4.jpg

After reading a lot of reviews of the Dogma 65, I understand why you posted it. One review specifically pointed out how well the smaller sizes handle - and then the angles got me thinking - instead of 0.5° steps for example, they have these funny jumps -

Following your recommendation, I adjusted the HTA and chainstays -

NEW version (changes in pink)

Drawing5.jpg

I wish I knew the HTA of similar sized Gaulzettis.

[e-RICHIE]

well


I wish I knew the HTA of similar sized Gaulzettis.

You don't need to know it. Clearly you have some puter program there that spits out this info.
Paste in the front center you want and change the head angle thingy until the hub axle results
in the number you are looking for.

[blasdelf]

jerk doesn't list the front-center either: Corsa Geometry — Gaulzetti Cicli | The Racing Bicycle Reborn

kind of weird to list the chainstay length and not that atmo

[dsaul]

Is there a reason for that 73 STA? It would seem that pushing the STA closer to 74 would push the front end out and get you a longer front center. I only have experience building for myself, so I don't know what I don't know about building for others. Just my 2 cents...

[Amaro Bikes]

Agreed, as I said I'm super happy with my position, yet I don't have the knowledge to make out the differences of 2-3mm trail or 0.5° HTA variation in my head.

Unless you're ultrasensitive, you won't feel that much difference in handling on 0.5º variation, just much more important the overall consistency of the frame geometry based on the sacred three contact points. Once you got those points clear, just adapt the rest so you have reasonable front center dimension, chainstay length, wheelbase, etc.

edit: p.s. I'm a bit curious about the Dogma geometry on first page. the lower sized frames yield something like 73-75mm trail, I can't imagine that handles nicely.

That's exactly a sample of the case, smaller riders need a certain handlebar/saddle/bb position which if you only focus on head tube angle usual measures (73º for example) will mean both toe overlap issues and a too short wheelbase, so all you need is to play with the head tube angle till you reach that reasonable front center dimension having the handlebar position "fixed".

And sorry if I sounded harsh in the previous post, but it just casued me a bit of surprise how someone could decide to build a frame without getting to know a bit deeper what frame geometries are and how they are suposed to work (obviously with each one's own point of view flavour), otherwise instead of a framebuider you could just become a tube welder.

Speaking about your drawing, obviously is impossible to know much without knowing who's going ot ride it (I have almost same saddle height but 595mm saddle-to-bar distance, so there you go how different each one is), but just considering some "general" questions, definetly might be worth a lower head tube angle so to gain front center distance without changing the handlebar position (this way you also gain wheelbase) and shorten the chainstays. But again, it's your bike and you should be the best one to decide what fits you better, both ergonomically and on frame use (clearances, fencers, races, gravel, etc).

And of cours, just a personal opinion, perfectly arguable

Cheers

[Lionel]

Even easier than the Pina geo chart, look at the traditional Colnago geo chart. It has all sizes in 1cm increment. That would be a very good start for a sound geometry.

[drwelby]

The difference will be the shallower head angle will increase the turning radius of the bike. You may perceive it as a bike which doesn't take corners as sharply as you would like.

Could you explain more? Do you mean that the turning radius at a given handlebar angle is greater?

Another effect is that the slacker head angle / higher fork offset will have more wheel flop at a given trail. This means it might wander a bit more in crosswinds or when you're riding sloppy. Or seen another way, it might want to dive into corners harder which could be good or bad.

The wheel flop effect is also proportional to the weight on the front wheel, and the OP's short front center and longs stays aren't going to help.

[cfrisia]

Unless you're ultrasensitive, you won't feel that much difference in handling on 0.5º variation, just much more important the overall consistency of the frame geometry based on the sacred three contact points. Once you got those points clear, just adapt the rest so you have reasonable front center dimension, chainstay length, wheelbase, etc.

That reminded me of this:


Those three points are already clear.
I'm aware of the effects that certain changes in the geometry have, but not to which extent. "reasonable" front-center, trail etc. is the hard part, not the three contact points.

dsaul: increasing the STA doesn't affect the front, it'll just lengthen the seat stays a bit and make the seatpost look unappealing to me (with zero offset).

Lionel: I took the pinarello chart for the raving reviews about their handling.

drwelby: As I'm saying above, it's very hard to imagine how much wheel flop the resulting geometry will have and how it will therefor change the subjective handling characteristics. Most motorcycles have way more trail, and wheel flop becomes a problem for an inexperienced driver (specifically when stopping a heavy machine with the front wheel turned - the bike definitely tip).
However, I have never actually read of people complaining about wheel flop of mass-production frames.

[David Tollefson]

Is there a reason for that 73 STA? It would seem that pushing the STA closer to 74 would push the front end out and get you a longer front center. I only have experience building for myself, so I don't know what I don't know about building for others. Just my 2 cents...

If longer FC is the goal, then lengthen it and don't mess with the STA, imo.

[drwelby]

As I'm saying above, it's very hard to imagine how much wheel flop the resulting geometry will have and how it will therefor change the subjective handling characteristics.

It's a numerical value, like trail. Trail is the lever arm around the steering axis of horizontal forces at the contact patch. Wheel flop is the same, only from vertical forces at the contact patch. You can calculate it here:

Bicycle Trail Calculator | yojimg.net

Converting a wheel flop value to a subjective handling trait is another problem, and factoring in changes in weight distribution is another. But it might be a useful value to look at.

For example, many early 29" mountain bikes tended to use steeper head angles to get similar trail values to 26" bikes when using forks with similar offsets. So you would often see 72 head angles with 38mm offsets. My first 29er frames were like this, and I noticed that they were harder to turn in sharp corners. There are other things to blame for this: longer wheelbase and heavier and larger wheels primarily, but I was curious if wheel flop was part of the equation. I started playing with the numbers, looking for a combination of rake and head angle that had the same trail as my 26" reference bike, but slightly more wheel flop. Because the bigger wheels have more inertia around the steering access, I figured a little extra wheel flop would help the front end steer. I was hoping to harness the wheel flop as a bit of 'power steering'. What I ended up with was a bike with a 70 head angle and 58 fork offset. This resulted in a really long front center, which I mitigated slightly by using a bit longer stem, and lengthening the chainstays for balance. Compared to the previous 29er it was still 2" longer in wheelbase, but it was paradoxically a far better bike around switchbacks. Other people also came to the same conclusion, with Trek moving to their 'G2' geometry with 45mm offset forks, which now seem to be a common option in 29 inch suspension forks.

[dsaul]

"dsaul: increasing the STA doesn't affect the front, it'll just lengthen the seat stays a bit and make the seatpost look unappealing to me (with zero offset)."

It does if you keep the top tube the same length and forget about that KOPS stuff, but that is another discussion. Just my opinion, yours may vary.

[blasdelf]

dicking around with your seating position to change where the front wheel goes is literally ass backwards

outside of aerobar positions, your setback with a particular saddle is basically inviolate, has nothing to do with kops and everything to do with hip angle

why the fuck would you unbalance yourself just to keep a tube artificially short while lengthening the actually meaningful measurement (reach)

[christian]

Wait, can we go back to the beginning? Why on earth do you need 12cm of drop and 52cm from saddle tip to center of bar. It would seem you would be better off trading drop for reach. (Arms bend at the elbow after all.) What about a stock 54 or 55 with 8 cm of drop and more reach wouldn't work for you as a basis for this geo.

[mjbabcock]

The feedback is cool and all, but with the time spent tweeting a program I think you might be better off buying a $150 tube set from Nova or Hank and brazing something up. Maybe braze two. Avoid lugs to save some money and plan on cutting them up to verify your brazing is good.

Once you've felt how they ride, come here and ask for translation of what you felt.

[cfrisia]

dsaul, christian: As I said, position is dialed in. You balance the frame, not the rider.

mjbabcock - sure this would be the easiest solution, but - I don't want to reinvent the wheel, I am trying to find a recipe.
Now while this might sound a bit naive first, keep in mind that it also means looking at manufacturers with experience, reading reviews, listening to the pros here and generally get a discussion going about experiences, pros and cons (also, the time I am able spend at a workshop is very very limited).

I took the geometry of some frames (Pinarello Dogma 65.1, Merckx EMX-525, Colnago C59) which admittedly, by various sources, handle "good" and I'm checking if I can find any analogous/correlative functions in there... I'm aware of the high probability to find chaos, but wouldn't it be awesome if there was a formula?

[e-RICHIE]

dsaul, christian: As I said, position is dialed in. You balance the frame, not the rider.

mjbabcock - sure this would be the easiest solution, but - I don't want to reinvent the wheel, I am trying to find a recipe.
Now while this might sound a bit naive first, keep in mind that it also means looking at manufacturers with experience, reading reviews, listening to the pros here and generally get a discussion going about experiences, pros and cons (also, the time I am able spend at a workshop is very very limited).

I took the geometry of some frames (Pinarello Dogma 65.1, Merckx EMX-525, Colnago C59) which admittedly, by various sources, handle "good" and I'm checking if I can find any analogous/correlative functions in there... I'm aware of the high probability to find chaos, but wouldn't it be awesome if there was a formula?

There is a formula - it's called riding. Doing it long enough informs position. You don't want a geometry, you want contact points.

[christian]

dsaul, christian: As I said, position is dialed in.

With all due respect, I don't believe you.