Framebuilders - More stability

I know we've been there before, but I'm still not sure what makes a bike more stable (less "twitchy").

What contributes to stability more, long chain stays, more fork rake or more trail?

I have a steel cross bike I ride on the roads (72.5 HTA, 45mm rake, 43cm chain stays) that is darn solid to control compared to other bikes I've owned. I'm looking for a custom frame, but don't know what geometry to go with for a bike that is even more stable at speed. 46cm chain stays? 71 degree head tube angle? Both?

Thanks for your help.

43cm CS are long enough. I'd go for a lot of trail: 72 HTA and 40mm fork rake.

Head tube angle and fork rake, both affect trail, and that determines your stearing stability. Chainstay length is more of a feel thing related to the bike having a big ride. Not more stable, but like being in the front seat of a car rather than in the back with your seat over the wheel. Bikes can have lots of stay length but then they start to fall out of the norm. One biking engineer who is race oriented in his riding but does mostly long training ride type stuff, just uses the whole chain stay, whatever it will allow and has no problems riding at speed, climbing etc...

Nessism, isn't chain stay length a way of designing the frame's weight distribution? You can make the ST more upright if you need to move the rider forward, but can't the knee then move past the pedal spindle? Some riders might not want that. You could design the setback based on knee position and crank length, then adjust rear wheel position using chainstay length. At least in one of my bikes, where I've been studying balance, I need to move my butt, the biggest and most massive element of the whole system, forward 3.3 cm to get 43/57 distribution. That 3.3 cm could have been in the chainstay instead.

I would then have a Mondonico with 44.3 cm CS rather than 41 like I have.

Road Fan

Nessism, isn't chain stay length a way of designing the frame's weight distribution? You can make the ST more upright if you need to move the rider forward, but can't the knee then move past the pedal spindle? Some riders might not want that. You could design the setback based on knee position and crank length, then adjust rear wheel position using chainstay length. At least in one of my bikes, where I've been studying balance, I need to move my butt, the biggest and most massive element of the whole system, forward 3.3 cm to get 43/57 distribution. That 3.3 cm could have been in the chainstay instead.

I would then have a Mondonico with 44.3 cm CS rather than 41 like I have.

Road Fan

Yes, long chain stays move the weight distribution forward. They also lengthen the wheelbase.

Seat tube angle should be set to get the proper rider position up and behind the bottom bracket. Chain stay length is independent of this.

44.3cm CS's are pretty long. I'm not the most experienced in terms of geometry extremes so not sure why Mondonico would go this far. Maybe for carrying a load?

What contributes to stability more, long chain stays, more fork rake or more trail?

They're both important. However, there is IMHO two parts to the way a bike handles.

1) Steering geometry.
2) COG in relation to wheelbase.

Both contribute to stability, and it's not really productive to say one is less or more important when both can be incorporated into the design.

Yes, long chain stays move the weight distribution forward. They also lengthen the wheelbase.

Seat tube angle should be set to get the proper rider position up and behind the bottom bracket. Chain stay length is independent of this.

44.3cm CS's are pretty long. I'm not the most experienced in terms of geometry extremes so not sure why Mondonico would go this far. Maybe for carrying a load?

Mondonico didn't go that far. Mine has 41 cm CS's. If I were to be balanced "correctly" on it, it would have 44 cm CS all other things being equal.

Do you design frames on a custom basis? How do you achieve good weight distribution? What do you do if that confilicts with a desired knee-pedal position? Seems to me once you have the contact points defined for knee-pedal and arm reach (cockpit), the remaining degree of freedom is chainstay, once the front end is nailed down steering stability and response.

There can be cockpit differences based on the pattern of use. Spinning vs, different cadence or handling needs. But on my road bikes I have all the same cockpit measurements, and the only varience is for seat post and saddle differences. Beyond that I vary chainstay length and head tube angle, And trail. All three vary wheel base. I don't worry about the weight distribution between the wheels since for loaded touring there seems to be a wide variation possible. I'm not into racing so I don't have that factored into my thinking.

They're both important. However, there is IMHO two parts to the way a bike handles.

1) Steering geometry.
2) COG in relation to wheelbase.

Both contribute to stability, and it's not really productive to say one is less or more important when both can be incorporated into the design.

Do you see fore/aft COG placement as more or less important than vertical, say, based on BB drop?

Do you see fore/aft COG placement as more or less important than vertical, say, based on BB drop?

My pref is for a high CG / high BB (or lower BB Drop if you prefer). YMMV.

Long CS’s put a little more weight on the front wheel compared to short CS’s. Don’t forget though that riders sometimes get out of the saddle and sprint which transfers a lot of weight forward. Race bikes have short CS’s for this purpose I think – puts a little more weight on the back wheel when sprinting.