Suspension and effect on reach

So, I’m trying to compare a hardtail’s geometry to a full sus, but getting confused: Does sag alter geometry figures such as reach? Surely a full sus would be less affected than a hardtail as the back end would compensate for any changes up front? Too damn confusing.

Yeah, as you sag a hartail, reach lengthens slightly as the front drops away form you a little. And yes, the rear sag on a FS minimised that effect.

It depends on the bikes in question and the sag you’re running but adding 15mm to the reach of a hardtail gives you a good idea of how the true reach compares to a full-sus when both are sagged. The more the travel, the greater the effect.

Interesting stuff. Sounds like a large bird zero is the equivalent of most xl bouncy bikes when it comes to length.

Brexit step one should be standardised bike sizing.

It’s OK, your arms have these specially designed bends in them called “elbows” which will allow you to compensate for the minimal change in reach.

I think you’re over thinking this a bit, it’s just a bike afterall.

If you’re happy riding a small or medium or medium or large or large or XL etc, then it matters little. But if you’re one of the many who appear to agonise over buying the right size then it does matter. Many bikes have only 15mm difference in reach between sizes. Buy a hardtail whose reach on the static geometry chart matches your full-sus and it could feel a size too big.

reach is a measurement from BB to bars as far as i’m aware and wont change 1 bit

Reach is the horizontal distance, stack is the vertical distance. When a hardtail sags reach gets longer and stack gets shorter. When a full-sus sags with the usual more sag on shock than fork then reach gets a bit shorter and stack a bit taller.

Reach squared plus stack squared remains constant – Pythagoras and all that…

Hi there, I think youll find that reach is a horizontal measurement from centre of BB to centre of top of headtube and stack is the vertical measurement, these sizes are fixed and as such unaffected by any form of suspension or wheel size, they essentially give you an idea of the size of a frame when riding out of the saddle regardless of seat tube angle.

Hi there, I think youll find that reach is a horizontal measurement from centre of BB to centre of top of headtube and stack is the vertical measurement, these sizes are fixed and as such unaffected by any form of suspension or wheel size, they essentially give you an idea of the size of a frame when riding out of the saddle regardless of seat tube angle.

Except they’re not fixed. The tube lengths and straight line distances between points are fixed, but the vertical and horizontal distances change as the bike moves through its suspension travel, as do the head/seat/other angles.

Exactly. The whole frame tips forwards when you get on a hardtail as the headtube squishes down the fork, whilst on a full-sus the front triangle tends to slightly tip backwards as the bike sags more at the back than the front. That rotation is why a well designed hardtail should have slacker static angles than an equivalent full-sus. Unfortunately marketing reasons are why that rarely is the case with head angles but it is pretty consistent with seat angles.

I see where your coming from but the sizes quoted by manufacturer are static sizes intended to compare frame sizes like for like, so another frame with the same measurements as yours will feel the same stood on the pedals, if you start taking suspension into account you might as well start taking leg and arm movement into it as well.

I can’t be bothered to repeat myself so I’ll quote Last on their FastForward hardtail page:

“If you want to compare the values with your own bike, please notice some hard tail specific effects: In riding position the reach will increase and the stack will decrease due to the sag of the fork. With full suspension bikes the opposite is the case due to more sag on the rear suspension. Effectively the front will feel longer compared to a full suspension bike with the same reach.”

@konanige – I disagree. If you ignore the effect of suspension then you can only use the figures given to compare bikes with the same suspension design and travel. The OP wants to compare hardtails and full sus bikes, and the effect of sag on the measurements will differ significantly and as such so will fit and ride feel. It’s why, in my opinion, sagged measurements would be potentially more useful than static geometry figures, but then that’s opening a can of worms as different riding styles and suspension designs suit differing amounts of sag. A bit of basic trigonometry comes in handy for exactly this sort of comparison.

I have a “large” hard tail, and a “large” full suss. I hope that helps.

Gave up trying to make sense of it. Hard tail –> full suspension, while increasing wheel size to 29er, but decreasing suspension travel . . . Went a bit ocd calculating potential figures then realised it was getting unhealthy. Seems at 185cm I’m at the top end of large, but long body/short legs means i favour a longer reach. Going to try and buy from an actual shop to get sized up properly with scope for swapping if it’s way off.

Problem may stem from ‘old style’ bikes (5-10 years ago) where smaller = fun. Did it balls. In hindsight it meant me at 185cm hunched over an 18 inch frame, seat wedged back, long stem and knee/bar risks.

Anyway, thanks all for your help and melting my brain.

doesn’t change the reach if the suspension is sagged on a hardtail, everything including you and the bike are just tilted forward. think about it

If you’re happy riding a small or medium or medium or large or large or XL etc, then it matters little. But if you’re one of the many who appear to agonise over buying the right size then it does matter.

I found this great way to sort that, I get on the bike and ride it around. If a manufacturer and distributor can’t get bikes to sit on then I’m not interested in their bikes. The bike shop I got my last 2 bikes from have a box of stems etc. and will let people swap and try with cheap stems etc. do decide what nice one to buy.

Surely the only way to change reach is to change the length / height of the stem and the height / position of the saddle?
On any NORMAL bike, the relationship between the top of the seat tube and the top of the headtube is fixed.
Or have i misunderstood?

I’ll have a go. As the fork (on a hard tail) compresses the bike pivots around the rear axle. As this happens the BB moves down and backwards and the head tube moves down and forward at different amounts relative to the rear axle. As a result the reach and stack dimensions change.

An extrem example of this change can be seen when taking all the air out of the fork and sitting on the bike, or fitting a short rigid fork to a bike designed for 160mm fork.

This is the reason why many HT geo tables state that the dimensions are measured at ride height.

Or have i misunderstood?

you have misunderstood.

Its a horizontal measurement of BB to top of headtube, not stem clamp or saddle position or anything like that. If you rotate the frame then this distance changes as its measured on the horizontal, as in horizon, as in reference of measurement does not rotate with the frame like some above seem to think.

Imagine going from a locked out fork to one bottomed out with no air in. The front will be lower and as you are pivoting around the rear axle you will also (if you hold the same body position) be rotated forward. Measure this and you will see the horizontal distance of bb to top of headtube is longer.

If you were to ride like this then to put yourself back in the right place yes you will need to change stem and move your saddle, but this is riding position not the reach measurement we are talking about.

MBR used to just measure the down tube.

Reach stays exactly the same as the seatpost and bars are fixed to the frame.
What changes is the angle you are sat at according to the suspension set up, so your weight might be more on your arms,or more ober the crank/baxk wheel surely?

konanige – Member

I see where your coming from but the sizes quoted by manufacturer are static sizes intended to compare frame sizes like for like, so another frame with the same measurements as yours will feel the same stood on the pedals, if you start taking suspension into account you might as well start taking leg and arm movement into it as well.

The problem being not all frames are measured the same, some are measured with sag, some without. It can make quite a difference, especially on HT frames with 120mm+ travel. This is compounded (as the OP points out) by the fact full-sus bikes sag in a different way, so comparing those static to static is not really appropriate.

You bring up a good point about the standing position though, the good thing about reach (and stack, you MUST also consider stack in any consideration of fit) is it lets you know where the bars will be with the same cockpit regardless of any daft bent seat-tubes or anything like that. Seatposts and saddle adjustment means saddle can be moved about to get them in the same place so TT measurement can vary greatly for the same reach and same saddle position.

MBR used to just measure the down tube.

Was that centre of BB to centre of bottom of HT?

^ I assume so yes. No measurements are much use a single dimension though.

I cannot tell the difference between my hardtail and my full suspension despite the hardtail being 0.5″ longer static.

Also how can so many people on here have absolutely no grasp of a very simple measurement?

despite the hardtail being 0.5″ longer static.

you can’t tell the difference 12mm makes – back of the class for you!! Again so many factors involved getting on the thin seems like the answer. I assume we are taking into account the tolerance of the sweep of the bars and the thickness of the asymmetric grips?

mikewsmith – you can’t tell the difference 12mm makes – back of the class for you!!

Same bars, slightly different stem, different headset, different stack heights, slightly different a to c.

My point being, get something about right and don’t worry about the last 5%

The second part of my post was the smiley for that one…. some people love to over analyse and seen to ride bikes on paper more than the wild. I reckon if you made the measurements up by about an inch most people would not spot it

The distance from the seat to the handle bars will not change as the forks sag. The height of the bars will lower, thats all.

mjsmke – Member

The distance from the seat to the handle bars will not change as the forks sag. The height of the bars will lower, thats all.

and both your arse and your hands will be further forward of the BB.

As above, we are not discussing stretch from saddle to BB, but the value of ‘reach’ in frame measurement, which has a defined way to measure, not just what you think reach is.

probably the best one to sum things up…

It’s useful for sizing because it eliminates the variation in seat tube angles and isn’t affected by wheel size. It’s not perfect though, because the length of the head tube and head angle can skew the measurement. Most manufacturers list reach, so you can compare one bike to another. On a size large, 435mm upwards is a decent size.
Read more at http://www.mbr.co.uk/news/mountain-bike-geometry-326498#8z8L1uoi1Gzsv4qH.99

This is a hard one to measure, because you need a plumbline to do it well. Reach is the horizontal distance between the top of the head tube centre and an imaginary vertical line that runs though the BB centre.

Use it to compare between 2 bike sizes not 2 different bikes.

As we can see here, sag does not affect reach at all.

I should really have measured reach on that… hold on

Here is an example for some of you, which would you choose?

Frame 1. Brand A. XL with 452.5mm reach.
Frame 2. Brand A. L with 440mm reach.
Frame 3. Brand B. L with 440mm reach.

All measured static.

Some would no doubt say Frame 2 and 3 are the same, but they are not. Brand A are measured with rigid forks, of equivalent length to a sagged 100mm fork.

Frame 3 is measured with an un-sagged 120mm fork, so actually has a reach of ~460mm with 30% sag.

Not so simple to just buy a size L cos that’s ‘your size’.

There we go.

Here we can see the static reach value of 46.07 is exactly the same as the sagged reach value of 63.39

F(Flips)Sake this is getting tedious!

– Stack and Reach are very simple measurements
– They DO change with sag (different for HT and FS)
– They are independant of bar position or seat position
– How much they change is easy to work out with a little maths
– How much you care is a lot harder to work out without riding

hopefully this image will clear it up, can’t beleive we’ve got this far with no pictures!

EDIT – (see below re: no pics showing), and well done Podge for taking the time to do a sagged comparison one

I deliberately picked the simplest pic with only stack and reach on there, as angles, effective top tube measurements, stems, bars, seatposts etc. all seem to be confusing people, but go go look at Google Images for more useful and complex examples

Pic on the last page too 😉 the bit I took away was it’s good for deciding if you want a medium or large X but not much good for deciding if bike Y or Z works for you.

Pic on the last page too

how odd, no pics displaying for me 😕 (including my own!) so I take my pic comment back