[pharmaboy]

Just do the Math, let's say you have a bike with 1140mm wheelbase, would you expect it to be different if it was 1144mm? With tyres on there is no way you could even measure it accurately enough - at a guess 1psi in the tyres might make that sort of difference to the effective length of the wheelbase.

Now 35mm versus 51 - you might be able to tell....

 

[pharmaboy]

So doesn't the question then become, if the fork offset is 5mm more, what effect does that have on head angle in degrees - gotta be a competent maths person here somewhere?

IMO, .5 of a degree isn't obvious, 1 degree makes a difference

 

[pharmaboy]

Ok, the internets been what it is, I put a 510mm fork into a 70 degree HA model to calculate the distance between the axle and the perpendicular line going up through the headtube, then added 6mm to that length.

The change in effective head angle was .6 of a degree for a 6mm increase in slackness via offset.

So using head angle , I'd say moving from a 48, 51 down to a 38 would make a quite appreciable difference - 1/2 a degree I'm unsure of, I've had a few bikes this close to each other, and really can't remember noticing big front end differences.

Fork AC lengths are going to make a difference as well I would imagine, and often differe by >10mm for the same travel fork.

This was the calculator I used - didn't even have to remember my trig lol
Right Triangle Angle And Side Calculator

 

[pharmaboy]

Given the same fork length, changing the fork offset does not affect or change the HTA.

It does effect the "effective head angle" though. You will notice that some manufacturers geo pics show actual head tube angle, while others show effective taking into account fork offset.

 

[pharmaboy]

Shiggy, do you know what the word "effective" means?

It changes the meaning of the the words following, and every single geo chart I have looked at assumes a fork on the bike - we don't ride frames.

 

[pharmaboy]

I'm trying to use normal language, it becomes easy to visualise what you are changing because of the movement of the axle out wards . The term "trail", a little searching discovers is used often here on MTBR to describe the effect ( although it's an opposing angle change) .

Forgive me, on my more common biking forums based elsewhere in the world, "trail" is not used often, and more often the effective head tube is used in discussion about this very problem.

Either way, 6mm of fork offset will slacken the front of the bike by somewhere around 1/2 a degree. Is that OK?