HT angle question- Mtbr.com

1. ## HT angle question

Looking to build a rigid/rigid SS. If a frame lists a HT angle of 69.5 degrees and shows the fork length as 500 mm, how much will it steepen if a 480mm rigid fork is used? In other words, is there some rule of thumb formula for how the head tube angle changes for say every 10 mm of fork length change?

2. Rule of thumb? You betcha.... math! Specifically, geometry. Ok I kid, I kid. Assuming your both forks are being measured from axle to crown and have the same trail, your headtube angle will be about 70.5 degrees. Keep in mind that it doesn't just change HT angle. It will also lower your bottom bracket and steepen you seat tube angle as well. Roughly speaking, you will change your HT angle about 1/2 degree for each 10mm in A-C change.

3. Thanks Joe. I'm not swapping out forks. The frame I'm looking at has a headtube angle of 69.5 and they assume a virtual for fork length of 500mm. The fork I am looking to pair with it has an actual length of 480 mm.

4. Originally Posted by Cevan
Looking to build a rigid/rigid SS. If a frame lists a HT angle of 69.5 degrees and shows the fork length as 500 mm, how much will it steepen if a 480mm rigid fork is used? In other words, is there some rule of thumb formula for how the head tube angle changes for say every 10 mm of fork length change?
for future reference: HTA is the more common way to write head tube angle. (HT angle to me reads "hard tail angle") also there's no such thing as a rigid/rigid SS, or a full rigid SS for that matter, it's redundant. it's just rigid SS. sorry, pet peeve.

(someone correct me if i butchered the numbers) 500mm is the A-C which is for a 100mm travel fork if we're talking about 29ers. generally rigid forks are designed to account for sag on a sus fork. so a frame designed for a 100mm fork (490mm a-c) assuming 20% sag will most commonly be paired with a 480mm ish rigid fork. likewise, an 80mm sus fork commonly gets replaced by a 465-470mm rigid fork.

there's no hard rules, you can choose a fork that preserves the HTA or steepens it a little which is common when swapping to a rigid fork. you can also fine tune with a different headset depending on how you want it set up.

like Joe said, it's maybe 1/2 a degree different from a sagged sus fork. not enough to make a big difference. 480mm rigid fork is the correct length.

5. Are you talking about a rigid fork or a suspension fork? I know you want to run it rigid, but is the geometry chart using a suspension fork or rigid? You have to be careful if it's suspension, as some list the angle with the fork at max length, and some with the fork at recommended sag.

6. All good info above, high school geometry has been much more useful than I ever thought it would be at the time.

Originally Posted by bikeny
Are you talking about a rigid fork or a suspension fork? I know you want to run it rigid, but is the geometry chart using a suspension fork or rigid? You have to be careful if it's suspension, as some list the angle with the fork at max length, and some with the fork at recommended sag.
Good to check, but I've always seen it as compensated for the suspension fork sag.

7. Originally Posted by *OneSpeed*
. . . also there's no such thing as a rigid/rigid SS, or a full rigid SS for that matter, it's redundant. it's just rigid SS. sorry, pet peeve.
. . . .
heh heh. so what would you call a bike with a rear shock and rigid fork? besides KOOK?

http://forums.mtbr.com/29er-bikes/re...id-682323.html

8. Originally Posted by Natedogz
All good info above, high school geometry has been much more useful than I ever thought it would be at the time.

Good to check, but I've always seen it as compensated for the suspension fork sag.

The HTA is a static measurement, not a range (at least in my case) and it is assuming a 500mm A-C. So if I have a 480mm A-C, I am going to assume the HTA will be ~70.5.

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