Hardtail reach increase with fork sag

I’m trying to work out the increase in reach from static values to sagged values for a hardtail with 150mm fork sagged at 25%.

Initially I thought this was straight forward but then realised the bb also moves too…

I mainly want to do to get an idea of how a hardtail frame compares to a full sus frame, but working it out is also annoying me!

Please note this isn’t a discussion as to whether reach does or doesn’t increase with fork sag on a hardtail, as it does. Reach is the horizontal distance between the centre of the bb and the top of the head tube. With a hardtail the bike pivots around the rear axle when the fork says.

https://bikegeo.muha.cc/

Won’t this do it?  Just swap the a2c length between sagged and extended to see the difference.

Much discussion here, though does debate the does or doesn’t a fair bit. Some comments on difference between full sus and hard tail though.

https://singletrackworld.com/forum/topic/suspension-and-effect-on-reach/

Far simpler though to just try some bikes. Use the figures to compare like with like and find one similar, try it.

Is reach not the distance from the bars to the seat?  this will not change.  the bars will alter in height yes and if you are taking an imaginary horizontal distance it will alter a tiny amount but next to nothing.  Or have I misunderstood?

Is reach not the distance from the bars to the seat?

No, it isn’t.

Anyway, if the manufacturer doesn’t give you the sagged geom for a hardtail … move along.

Edit: googled a diagram for you…

Trigonometry innit?

I bet it’s roughly 2/3 of ***all though.

Really? Noticed how much the reach changes when you fit longer/shorter forks?

Last time I looked, a 130ish fork increases “reach” by 15mm when you’re on the bike.

Which makes me wonder why lots of hardtails are being designed with similarreach number as FS models, which means that when you’re on the bike they’re even longer.

But then lots of hardtails seem to have similar head angles to FS bikes “static”, and then steepen by 1.5degs when you sit on them.

Dunno.

Ta Kelvin.  Every day is a school day.  Funny terminology but if thats what is used………………

No, it’s really good terminology! Actually, I think Brant may well have been the first person I noticed singing the praises of stack/reach as a far more useful indication of bike handling than TT length etc. Of course, those old school measurements are still useful for seated riding fit… but all the fun stuff is done unseated.

And, manufacturers really should be stating sagged measurements for hardtails, otherwise comparison with other bikes is nonsensical, as Brant has shown.

Shirley you “simply” work out the rotation of the bike brought about by the sag and recalculate the reach?

Which makes me wonder why lots of hardtails are being designed with similar reach number as FS models, which means that when you’re on the bike they’re even longer.

It’s odd, isn’t it? New HT stuff’s too long I reckon, not my cuppa at all, and there’s nowhere to go from there is there? Going shorter in a year or so will take some bullshitting marketing 😆

Can’t help you with how much your reach will change with various fork/frame/sag combinations.

I can however help with over 30 years of riding lots of different types of frames and geometry, to give an approximation of how much difference it will make.

F8ck all in real terms. Just ride and have fun.

(I appreciate it’s not a very good answer to your question, but really? is it THAT important to work this out?)

I think just as important is what happens to the stack height too.

I’ve realised lately that I should of been running a higher front end than I’ve always thought/been told to do. I have a Stanton Slackline which has a standard 140mm fork on it ,some other changes led me to try putting my 160mm fork off my other bike onto it just for an experiment-making an almost Frankenstein like bike.Everything improved – climbing,seated position,jumping position,cornering and  staying much more planted on my flat pedals(and indeed staying on them-this was the recent problem that led me to put the taller forks on after recently transferring  onto flat pedals).

I thought this was a great little bike before doing this now I know it can be much better for me .

I think just as important is what happens to the stack height too.

I’ve realised lately that I should of been running a higher front end than I’ve always thought/been told to do. I have a Stanton Slackline which has a standard 140mm fork on it ,some other changes led me to try putting my 160mm fork off my other bike onto it just for an experiment-making an almost Frankenstein like bike.Everything improved – climbing,seated position,jumping position,cornering and  staying much more planted on my flat pedals(and indeed staying on them-this was the recent problem that led me to put the taller forks on after recently transferring  onto flat pedals).

I thought this was a great little bike before doing this now I know it can be much better for me .

Your post reminded me of this-

https://singletrackworld.com/forum/topic/davey-push-bikes-full-moto/

My Switchback’s built up similar (tallish front end) and it works well 🙂

is it THAT important to work this out?

Who’s saying it’s really important? I think the OP is just interested.  Don’t criticise him for wanting to understand stuff, because it’s that attitude that has given you most of the technology you take for granted both old and new.

Which makes me wonder why lots of hardtails are being designed with similarreach number as FS models, which means that when you’re on the bike they’re even longer.

But then lots of hardtails seem to have similar head angles to FS bikes “static”, and then steepen by 1.5degs when you sit on them.

Make some sort of logical sense.

1) Hardtails are slower and need more weight on the front but without the longer wheelbase.

2) It’s easier to market than trigonometry

This DIAGRAM from Bird shows how various measurements change with sag.

It’s based on 130mm forks and 30% sag but gives a good indication of percentage changes.

@molgrips
<div>Yep, you’re right. Wasn’t wanting to criticise per ‘se. I reckon i’ve just been looking at the forum for too long.</div>
<div></div>
<div>MUST LEAVE NOW!!!!</div>

Wow just looked at that post about the Daveybike and everything rang true with me.I was just about to say I wondered if the guy that was driven to make that for himself has what I have-relatively long lower body with maybe a relatively short upper body/reach and then clicked through to his website where the first thing he says is that is indeed the case.Very interesting.

A 150mm fork with 25% sag is the same length A2C as a 112.5mm fork static. Rotating frame forwards and down at the headtube around the rear wheel has the following effects:

BB height drops by about 30% of the decrease in A2C length.

Head angle steepens by ~0.4 deg per 10mm decrease in A2C length.

Stack decreases by ~60% of the decrease in A2C length.

Reach increases by ~40% of the decrease in A2C length.

The changes are significant – like the difference between a 120mm trail bike and 160mm enduro beast in the head angle and almost the difference between a medium and a large in reach.

https://bikegeo.muha.cc/

Won’t this do it?  Just swap the a2c length between sagged and extended to see the difference

Thanks have done that and it says the reach will increase by ~20mm. Doesnt seem much but the difference in reach to the next size up is 15mm!