Calculating reach vs head angle

Does anyone know how to calculate the change in Reach if you slacken the head angle? I’m planning on slackening the head angle on my hardtail by either 1.5 or 2 degrees and wondered how much the reach would grow as the front end drops.

Is there a rough rule of thumb or better?

Ta

Tom KP

Possibly less than how much the headset will shorten the reach by moving the steerer tube back.

Reach is not a driving number for bike fit, I agree with PVD on that much.

Good thinking

I’m not changing frames, just interested in little things like that

I always thought reach would shorten if you slacken the headset?

Not mine but this works well for me…

https://bikegeo.muha.cc/

What you gain from the lower head tube you lose from the offset upper cup, so it cancels out to within a few mm.

The actual effect on reach is going to be minimal, a few mm or so. However it may feel more significant because of the Effect that slackening the steerer will have with regards to moving the clamping ‘zone’ back towards the saddle.

Reach is not a driving number for bike fit, I agree with PVD on that much.

I’ve absolutely no idea who or what PVD is, but for an MTB, I disagree with both of you. On a road bike then yes, but not on an MTB. I spend quite a lot of time standing up, and I can’t see many other attributes that change the fit (as opposed to handling) as significantly as reach in that situation (other than stack)

Edit: oh, he’s someone who makes things like this:

(Which if he doesn’t think reach matters, has one hell of an oil tanker-y cockpit)
Is he just someone who’s trying to sell ‘his’ take on geometry?

This is pretty comprehensive if you have the numbers available to you: http://bb2stem.blogspot.com/

Is he just someone who’s trying to sell ‘his’ take on geometry?

Actually no. He makes bikes only for himself (well and his OH), but he’s actually willing to try stuff out which I respect.
He’s written some interesting blog posts, but basically he likes long front centre, with STA and stem length set to get the saddle to handle bar dimension he likes, in the belief that seated length is more important that reach.

PVD says think of the bike as a system.

A good bike designer will design the bike as a whole, the seat angle the head angle the chain stays the reach, the front centre etc and will think through tbe compromises each gives.

Yeah you can change one part of a system, but what does that do to the whole?

E.g. You slacken the head angle, but what happens to the trail figure? Do you fit a low offset fork? The front centre will increase, does that unbalance the bike because you haven’t also increased the rear centre?

By all means try a slackeriser headset, but it’s not a magic bullet to fix the bike, you will be making other compromises in the geo.

“By all means try a slackeriser headset, but it’s not a magic bullet to fix the bike, you will be making other compromises in the geo.”

All bikes are a balance of compromises. I’ve done a lot of experimenting with my hardtail and my old full-sus and what I learnt was that the way I ride a bike, a full -2 deg slackset works very well on them!

Reach is not a driving number for bike fit, I agree with PVD on that much.

Of course reach matters. It’s probably the most important measurement we discovered, and started to use in the late 2010s.

PVD says think of the bike as a system.

So they must consider reach.

Reach, stack, STA are the best paper numbers to give you an idea of fit.

Anyway, OP – like Chief said earlier, it’s gonna make bugger all difference. What will make more difference is if the offset cups add to the head tube length, and hence the stack. That can affect the reach more than just the head angle and offset bearings.

This is what I found with my old full suspension bike, a -1.5 slackerizer and offset shock hardware dropped the BB a tad and lengthened the wheelbase and improved my Alpine 160 (which was already a pretty damn awesome bike.

The current hardtail rides lovely as it is but I like to tinker. The Reach is pretty similar to the rear centre at the moment as I’m not a fan of super short rear ends. But once never ridden a bike that wasn’t improved by slackening they head angle.

Tom KP