Does anyone know how much of a change in head angle an extra 20mm of fork travel would give?

I have an Inbred which, according to the On-One website, has a HA of 70 deg with a 100mm fork – I have a 120mm forks, what do you reckon my current HA would be.

Ta.

I did think I’d read that it was 1 degree per cm of change but you’re best measuring the axle to crown of your forks – the difference might not be 20mm.

if you have an iPhone you can get an app to measure…

It’s 1 degree per 17mm for the average bike, which is about 25mm before sag.

1″ give approx 1deg static change.

Measure it?

Do you have a protractor?

Nice, thanks for that folks..

No, I don’t have a protractor. Not since GCSE maths anyway.

You don’t need a protractor.

The Cotic website has a lovely simulator. Click and spend hours here

Who said trigonometry is useless..
get a protractor, make yourself a plumb-bob from a piece of string and something suitably weighty, use these new tools and a tape measure to get your numbers, find out the the axle to crown length of the new fork and do the math. this would give you the static head angle, for your actual on the bike riding H.A. subtract your sag from your axle to crown length.. 😀

This may come in handy

Bike/Fork geometry calculator

As well as changing your head angle, adding a longer fork will add more stress to the frame, which may cause the frame to fail and invalidate the warranty. I dount it’s too much of a problem going from 100-120, but it’s something to be aware of.

And it’s lift the bottom bracket up a touch too, which might be a bonus of you’re catching rocks a lot.

gearfreak – Member
As well as changing your head angle, adding a longer fork will add more stress to the frame, which may cause the frame to fail and invalidate the warranty.

I’m not sure that’s the case. IMO this issue arises because people with longer forks will hoon bike more, and that’s why mfrs have limits. IIRC Brant agreed.

I’ve got 120mm forks on my Inbred. When I bought the forks I also got some spacers to reduce the travel to 100mm. I thought I’d just try them at 120mm first and it rides great, so I never bothered changing them.

HTH

nice link duntstick

adding a longer fork will add more stress to the frame

I’ve heard that loads of times over the years, but don’t understand the theory behind it.

Why does it stress the frame more?

The fork acts like a lever, the longer it is the more force it can apply to the head tube.

but a typical fork is about already 500mm long…

adding 20mm to this makes … 4% difference to any leverage induced stress.

i wouldn’t lose any sleep over it.

ahwiles – you’re right in this case but fitting 888s to a frame designed to take a RS SID is a different story.

I see the lever example. But wouldn’t that hold true mostly when riding on the flat?

The greatest forces would occur, I’d guess, while you’re going downhill (faster travel). At that thime, the bike is angled down the hill anyway and most forces would be traveling up the fork leg by the time it reached the headset, any lateral force from impact with the terrain (at the headset) would actually be towards the rear of the bike. No?

Forces are still going back through the bike even if you’re on a slope.

Anyway

poisonspider – Member
The fork acts like a lever, the longer it is the more force it can apply to the head tube.

The fork will be compressed when there are significant forces acting on it.