[spincrazy]

Ok, so I'm calling on the vast stores of knowledge here on this board, at least I hope so.

What up with frame geometery? I understand some of the basics, but can someone please explain the details to me?

Headtube angle
Seatpost angle
Top tube length

XC race
DH
DS
Road race
Trials
etc.

Gracias

 

[spincrazy]

nothing huh? oh well.

 

[oldfart]

I don't type well enough to give you a proper answer and I would draw you some diagrams too to save a couple thousand words. But Bicycle head angles seem to go from 68 in a dh bike to 74 on some road bikes. XC bikes are around 70-71 degrees. Most road bikes vary from 71 - 74. The steeper the angle the quicker the steering. The shallower the slower but more stable it is. But you also have to factor in trail. That is how far back the contact patch is from where the steering axis intercepts the ground. I need to draw here but you'll have to do that yourself. The more trail you have the more the bike wants to go straight and the heavier the steering will be. Seat angle is less important in my view. Simply because saddles are adjusted fore and aft making the angle somewhat irrelavent. As I see it XC and road bikes are set up for pedalling efficiency whereas DH bikes are set up for handling efficiency. For pedalling your trying to get a comfortable yet powerful position. Bolt upright don't work neither does folded in half. So on a road bike where you also want a flatter more aerodynamic position, that seat angle might be steeper so the rider can get low but mainatain a reasonable angle between the legs and torso. It will be less steep on a XC bike because handling on downhills would be compromised with a road type position. for DH its much less important. Put the seat low and out of the way for the most part. Pedaling is usually done standing. Top tube length is a function of sizing really. But given the same head and seat angles top tube length will affect wheelbase to a certain extent. Often head angle is shallower than seat angles so as head tubes and seat tubes lengthen, top tubes shorten. So for bigger bikes the top tubes are longer and wheelbase too to preserve a similar riding position for the bigger rider. I could on and on but I'm not even close to being an expert on geometry. But surf about you can find lots of stuff on fit and geometry. www.rivendellbicycles.com has some road touring type info that will get you thinking.

 

[spincrazy]

Wow. thanks a lot oldfart. Helltaco's right when he says you are a sage. When I see your name at the end of a thread I know that there is going to be some INFO there, good info. Thanks again.

 

[oldfart]

Thanks. That's one the advantages of old age...experience. The other advantages are...........well OK there aren't any. That's why I won't grow up.

 

[Gutty]

Sorry i didn't reply sooner.......Does this help at all?