How much difference does adding 25mm to the fork A-C hight make to the head angle, one degree, or more?

Cheers in advance.

For an "average bike" I calculated 17mm additional fork length = 1 degree head angle.

25mm of fork travel, sagged ~ 17mm, so "yes".

depends
short bike- more difference in angle per cm increase in fork height,
longer bike, vice versa
trigonometry, innit?

Ok, I think trigonometry may be a bit beyond me, so here's some more info 😉

I bought a Viper 29er frame & stuck on a set of 440mm RC31's, it's virtually unridable, so, so skittish that you can't take your hands off the bars, I think it must have been designed around 120mm forks as I really don't think that going from 440mm to 465 will make enough difference, does any one think it could?

Cheers.

No One?

Have you got a link to the stated Geometry?

440mm rigids on a 29er is gonna make it a bit twitchy I'd think, especially as 29ers have got steeper head angles anyway to counter for the larger wheels slowing the steering down a bit.

Going by Cynic-al's findings, fitting a 25mm longer rigid fork is gonna slacken the head angle by almost 1.5 degrees (as a rigid fork doesn't sag funnily enough!), so I'd definitely say it's worth a go.

Not seen the Viper 29er frame, but most 29er hardtails are designed around an 80-100mm travel fork, with a 465mm rigid being a viable option to keep the geometry "normal" without a sus fork fitted.

Model Size (C-T) Head Tube Length Head Tube Angle Seat Tube Angle Seat Post Size Top Tube (C-C) Chain Stay Wheelbase Intended Fork Travel

The 29 17" 90mm 71° 73° 27.2mm 617mm 405mm
The 29 19" 90mm 71° 73° 27.2mm 627mm 405mm
The 29 21" 105mm 71° 73° 27.2mm 645mm 405mm

Doesn't mention fork travel on the 29er, but the head angle is *Very* steep with 440mm, I just don't see how 25mm will make the difference 🙁
I 'chocked' the front wheel by 25mm with a bit of wood, & it doesn't look like it makes much odds, but obviously I can't ride it like that 😉

Cheers.

I'd do the maths but I've had too much red.

Anyone wanna challenge my findings good luck to 'em.

try this

I just don't see how 25mm will make the difference

You'd be surprised, I totally ruined the handling of a frame designed for an 80mm fork by sticking a 100mm fork on it once. I didn't think 20mm would make that much difference, but it did!

71 degree head angle is fairly conventional, if not almost slack, for a 29er. Try a 465 (or even a 470mm fork, there are a few about including the Nuke Proof on CRC).

FWIW Reba 29ers are 490mm at 80mm of travel.

Thanks for the info thus far, it's ineresting that the reba's are 490mm at 80mm, that means they would be 410mm fully compressed and about 470 saged.
I just can't see how 25mm will make the difference, I reckon the frame must have been designed around a 120mm fork, not that I can find out that info.
Cheers.