if youre modelling this do you also take into account that the wheelbase lengthens if you compress the rear suspension?

Quote from tristancliffe :

Ben's new term is a good one, and is something not usually quoted on bike specs (not that you'd expect the nuances of suspension geometry to ever be).

Given that it's entirely dependant on the rake angle, I don't see why it would need to be explicitly stated. I'd still like a name for it though.

Quote from tristancliffe :

Trail varies with suspension compression only if you take dive angle into account, which you probably would.

Aha, I hadn't thought of that. Not sure if I need to manually take that into account. Need to have a think about it.

Edit: Don't think so. The rake angle relative to the world is changing, but that is irrelevant. The rake angle relative to the bike is not, so I don't need to worry about it. Trail is a world-space length rather than a bike-space length, and should be calculated using the rake angle in world space, so will change dynamically.

Quote from Shotglass :

if youre modelling this do you also take into account that the wheelbase lengthens if you compress the rear suspension?

Well, the bike is a hardtail, so no.

Quote from danowat :

The hypotenuse of the trail triangle reduces when the suspension dives, therefore the adjencent side (trail) reduces proportianally?

Nope. It would change a bit from tyre compression though. Trail is calculated based on head angle, offset and wheel diameter. Only the latter will actively change due to compression. If the rest change then things become complicated and you're likely not using a telescopic fork (or if you are then you shouldn't be because it's about to break).

his post was worded a bit badly
the head angle does change with dive which is equal to the hypotenuse of the trail/wheel-radius shortening

Quote from Shotglass :

his post was worded a bit badly
the head angle does change with dive which is equal to the hypotenuse of the trail/wheel-radius shortening

I still do not see why the head angle would change with dive.

If the suspension compresses at the front, the wheelbase will shorten, causing the bike will pitch (specifically, dive), and thus the angle of the fork, from straight up, will now be different (less positive, in fact).

because the whole bike nicks down
seen from the side compared to a standing bike during dive the whole bike will rotate torwards the front wheel which also changes the head angle (in road coordiantes not bike coordinates of course but since the contact patch position is what matters for trail those are the appropriate ones)

Ah, true that - I stand corrected. I was considering the permanent angle as calculated between fork and the frame itself.

curiously enough the changes seems to be beneficial in most situations
if im not fundamentally mistaken trail is the parameter that influences how much the fork wil auto correct any lean
when youre braking hard (think moto gp) the trail will decrease making the bike less stable which helps with a fast turn in... on the contrary during acceleration the trail increases and helps to keep the bike stable and straight for optimum accel

Quote from Shotglass :

curiously enough the changes seems to be beneficial in most situations
if im not fundamentally mistaken trail is the parameter that influences how much the fork wil auto correct any lean
when youre braking hard (think moto gp) the trail will decrease making the bike less stable which helps with a fast turn in... on the contrary during acceleration the trail increases and helps to keep the bike stable and straight for optimum accel

Indeed so - I imagine that the amount is pretty much controllable though through slow bump damping.