Geometry question regarding axle-crown height and HT angle

[thomas]

Can anyone give me a quick answer to this:

Frame has 73 SA and HA. Wheelbase is about 102cm. Switching to a different fork (same rake) with 368 rather than 373mm a-c distance, how will that 5mm drop to the front end change the head tube angle? I know it will increase very incremently, but wonder if it's like 0.1 degrees or more like 0.5 degrees.

Thanks for any help!

[e-RICHIE]

One less bar in the front tire will change it more than the 5mm you are asking about atmo.

[blasdelf]

One less bar in the front tire will change it more than the 5mm you are asking about atmo.

that's bullshit

you're not going to get that much difference out of a tire that would fit in that fork and have it still be rideable

[blasdelf]

Frame has 73 SA and HA. Wheelbase is about 102cm. Switching to a different fork (same rake) with 368 rather than 373mm a-c distance, how will that 5mm drop to the front end change the head tube angle? I know it will increase very incremently, but wonder if it's like 0.1 degrees or more like 0.5 degrees.

from experience it's in the middle, around 0.3°

you can calculate all the resulting differences exactly using http://bikegeo.muha.cc/

[e-RICHIE]

that's bullshit

you're not going to get that much difference out of a tire that would fit in that fork and have it still be rideable

Hey wordsmith - thanks for the wake call atmo.
I meant the .5 of a degree rather than the 5mm I typed in my reply earlier.
Sue me.

[Mark Kelly]

Easy way to think of this: For small angles, the sine of the angle equals the angle in radians. There are a bit over 57 degrees in a radian, and a bike's wheelbase is about 20 x 57 mm long.

Sooo... each 1mm change in height is about 1/20th of a degree change in angle. 5mm gives about 1/4 of a degree.

Your forks are about 6 x 57mm long, so ceteris paribus one degree change in head tube angle will change rake by about 6mm.. 1/4 of a degree is thus 1.5mm change in rake.

[thomas]

Easy way to think of this....

Maybe easy for you to think about, Mark. Not so much for me! Thanks for the info. And for the other comments posted, too. Always informative (and entertaining).

0.25 degrees seems to be the general consensus - from 73.0 to 73.25 or so.

T

[Andrew R Stewart]

that's bullshit

you're not going to get that much difference out of a tire that would fit in that fork and have it still be rideable

Running a rear tire 5mm larger in profile gets you pretty much the 5mm of height difference that the OP stated. Add any low tire pressure and you're there. This is no BS. Andy.

[blasdelf]

Running a rear tire 5mm larger in profile gets you pretty much the 5mm of height difference that the OP stated. Add any low tire pressure and you're there. This is no BS. Andy.

where are you gonna get a 5mm differential from given the short reach brakes?

to do that with low tire pressure you'd need a hell of a pressure drop

are you really gonna suggest using a 28 and a 23?

[thomas]

Found info here -- Fork Lengths, by Damon Rinard. Carbon forks and steering geometry changes.

Sheldon Brown already figured this stuff out, bless his soul.

[Andrew R Stewart]

where are you gonna get a 5mm differential from given the short reach brakes?

to do that with low tire pressure you'd need a hell of a pressure drop

are you really gonna suggest using a 28 and a 23?

Hey, I'm only helping to answeOP'se Op's question of geometry changes. One way is to work the front eoriginallyonally mentioned. the other is to work the back end. The amount of change is the same. I had hoped by opening up the brain storming to looking at the problem from the other end one might better understand that these amounts of change are not too much in the ovschemescheem of things. It seems to me that the OP stated a way to get this 5mm difference without a tire change. i was saying that one could get the head angle change from a tire change alone. Same result, different cause.

At work I see bikes with vastly different tire profiles often. When people have little $ and need a tire they do what they can. 35mm at one end and 23 at the other, or, a 26x2.1 and a 26x1.5. These people don't complain about how their bike steers. They just deal with what small (and as others haReallyd REally Small) changes resulted.

Do i promote vastly different tire sizes on the same bike? No. Do i see and have I run up to 5mm differences? Yes. Did any of these make any real difference on how the bike handled ( and now the real answer to the OP's question)? No. Having a few extra pounds of weight on your back are far greater a influence as to how the bike handles then a .5 degree.

It's easy to get too caught up in what is repeated over and over, dimensionally, and think that this is the only WAY. Long before the interweb was around people rode their bikes with what ever they could get. They didn't worry about the 2mm more or less trail or the .5 degree more or less head angle. They rode their bikes. More of us should do the same. Andy.

[Michael Gordon]

Consider all of the brave soles of the early 90's who "threw off their geometry" when they swapped out their rigid forks for suspension forks, and took to bump off road trails no less!

[steve garro]

Consider all of the brave soles of the early 90's who "threw off their geometry" when they swapped out their rigid forks for suspension forks, and took to bump off road trails no less!

And they often rode just terrible..........
Real floppy & then bad steering dive.
But, as for the OP I am more to the side of ER then Bladsomething.
Experience over regurgitation.
And to Mark Kelly - that's how I think of it - rotating around the rear axle when you lift the front.
- Garro.

[Andrew R Stewart]

On another interweb framebuilding community this was said.

"Just to emphasise Mark's comment on how little effect these small variations on lower stack height make...I reckon that, with a 1m to the rear axle, you'd need to raise the headtube by roughly 17.5mm to get a 1 degree change in angle,so 1mm will only make around 0.175 degrees."

Actually the change is likely a bit less as most bikes have more then 1m of wheelbase. Andy.

[Mark Kelly]

"Just to emphasise Mark's comment on how little effect these small variations on lower stack height make...I reckon that, with a 1m to the rear axle, you'd need to raise the headtube by roughly 17.5mm to get a 1 degree change in angle,so 1mm will only make around 0.175 degrees."

That's some pretty bad arithmetic. It should say that 1mm will make 0.057 degrees.

Logic check: with a wheelbase of 1m, it follows that 1mm rise will subtend an angle of 1/1000th of a radian = 0.057 degrees.

My 1 in 20 rule uses a longer wheelbase to give easier numbers.

[Andrew R Stewart]

I am at fault of only repeating another. Glad I didn't include their name... Andy.