[Closed] What a different 10mm makes... Hardtail Geometry Woes

Fork length or fork travel?

Fork length or fork travel?

Both as specced... 540mm a2c and 130mm travel

I've read your post 4 times and each time it gets more confusing. You originally say that the bottom bracket is 10mm LOWER than spec and you go on to say that you LIKE low bottom brackets.

And to top it all off you then go onto say that the BB is actually 10mm HIGHER......

WHAT IS IT DUDE!!!!???? If you are making simple mistakes like that when creating a series of sentences to explain your situation.... then I can only suggest that your calculations are wrong and you have made a mistake when measuring.

Also, unless this is some weird way of micro-managing the whole Corona Virus lockdown anxiety, I would suggest you chill the **** out and have a wee drink... or a wee toke of something.

10mm? TEN MILLIMETRES?.... Jesus, just let some air out of your tyres, will you?

^ bit of an explosive reply.

I'm going to go with a series of minor variations of random small bits you've not even thought of. Now bin the angle finder and have a beer. 🙂

How are you measuring BB drop?

tyre size and profile is my guess - thats what has made the difference. Fit the original tyres and remeasure.

BB drop is how much lower than a straight line between the axles it is. Are you measuring this correctly?

Tyres won't have any affect on this as it's not a measurement from the ground but from the axles.

Ah - apologies.

I’ve read your post 4 times and each time it gets more confusing.

I've had a strong cider, half a bottle of red, and am now on the G&T, and i've read it back, and it still makes perfect sense to me... 🤷🏻‍♂️

You originally say that the bottom bracket is 10mm LOWER than spec

Someone isn't paying attention... 🤦🏻

I said the BB drop is 10mm LESS than specified... Not 10mm lower! CRUCIAL difference...

WHAT IS IT DUDE!!!!???? If you are making simple mistakes like that when creating a series of sentences to explain your situation…. then I can only suggest that your calculations are wrong and you have made a mistake when measuring.

Don't know who or what has upset you today, but seems I was the straw that broke the camels back! Still doesn't excuse your lack of comprehension and the irrational response mind!

Also, unless this is some weird way of micro-managing the whole Corona Virus lockdown anxiety, I would suggest you chill the **** out and have a wee drink… or a wee toke of something.

Part of the success to a happy life is learning what you can, and crucially can't, control in life... If you're busy getting upset because you're not in control of the Coronavirus situation, I don't know what to say to you other than to take a large piece of your own advice!

As for the drink... Re-read my first comment in this post...

10mm? TEN MILLIMETRES?…. Jesus, just let some air out of your tyres, will you?

You're assuming I have some left to let out... I don't expect you to understand where I'm coming from, I'm asking for experiences of those that might and/or what they've done when they have found that the frame they have purchased hasn't met requirements...

I can tell the difference between 170 and 175 cranks in a double blind test. In fact, I'm confident I could tell the difference between 170's and 172.5's more often than not in a double blind test. Just because 10mm doesn't matter to you doesn't mean it's not very important to someone else! In my job, if a product was even 1mm larger or smaller than specified, we would have no end of complaints and would probably have to recall the products until the problem was rectified... Your "WTF difference is 10mm gonna make" response would go down about as well as a lead balloon!

^ bit of an explosive reply.

Tough crowd on a Saturday night eh! 🤔

I’m going to go with a series of minor variations of random small bits you’ve not even thought of. Now bin the angle finder and have a beer.

Angle finder put down the moment I posted this, curry consumed and a few drinks in now... 👌🏻

Still doesn't mean I'm not miffed that the manufacturer has either wilfully lied, or more likely doesn't know how to measure things properly. I've taken into account for all possibilities, it's the accuracy of every single other measurement to my measurements, yet the one single glaring error (by 10mm FFS!) that annoys me... It does at least explain why I feel more "on than in" the bike, which reinforces exactly why I prefer a particularly low BB given the choice.

How are you measuring BB drop?

Any/every way I can... Including...

BB drop is how much lower than a straight line between the axles it is.

👍🏻

Fit the original tyres and remeasure.

No original tyres specced, but I have two sets of wheels and tyres to try for reference... with 29x2.35 on (that I know are 742mm dia) the BB height is 316mm measured (on both sides, on a flat surface). With the other wheels on with 29x2.25 tyres on (that I know are 736mm) I get 313mm... Every which way I measure it, I get a 55mm drop, not the 65mm drop specified.

To all those saying about tyre size, etc - remember that BB drop and BB height are technically different; BB drop is not affected by tyre size, whereas BB height is.

Is the only way you've calculated BB drop by subtracting floor-to-BB-centre from floor-to-axle-centre, or have you set a straight edge between the axles and measured from normal to that to the centre of the BB?

Does the manufacturer only quote BB drop on their geometry table?

Agree with boy, BB drop is an important factor in how a bike feels an 10mm makes a lot of difference.
I ride track frames that have ~10mm less BB drop than road frames and it is very noticeable when riding them, you can actually feel you are higher off the ground with all other things being equal (BB to top of seat)

Saying that, I have never actually measured BB drop and just tend to believe what it says but would feel if it was wrong.

I’ve had a strong cider, half a bottle of red, and am now on the G&T, and i’ve read it back, and it still makes perfect sense to me… 🤷🏻‍♂️

Maybe you need all that before it makes sense 🤔😆

If we are assuming you are bang on with your measurements, is there a possibility your geometry source of reference is the wrong one? Sounds like this is a second hand purchase, so geometry from the web?

Outside chance there is that it isn't quite the model year you think it is when looking up geo?

Other thing - maybe more likely - is that I've had misunderstandings across apparently identical models, especially in one case where the alloy and carbon versions of a frame were sold as the same, but the carbon was actually longer (10mm more reach). This wasn't at all clear and most reviews and sales info simply said they were the same except materials. I personally think the (major) manufacturer was fine with this presentation, as most people used the more fashionable longer figures and probably sold more alloy bikes off the back of it - who measures?

I'm reading this that it's a used frame?.

If so, sell it, and move on, folk are paying silly money now.

when a manufacturer lists its geometry, and the reality is somewhat different to what is listed, where would a consumer stand?

There's usually a little disclaimer somewhere saying that specifications might vary from what is advertised. If you order a bike and the advertised spec isn't available, you can cancel the order.

OK, so I’m not the original buyer here, so I know I don’t have a leg to stand on.

You can either worry about it or not worry about it. It won't make any difference, except that you will be unhappy if you choose to worry about it and less unhappy if you choose to ignore it and just ride the bike.

Crucial missing info:

What frame, what year.

If your measurements are accurate (I'm not supposing they're not) I'd suspect typo on the original spec. Or they changed geometry without telling anyone, or if it was re-sale model like Airborne or Planet X, they haven't measured the bikes properly once they got to the UK. You're not the original purchaser but I'd imagine it would come under distance selling regs as "not sold as advertised"  and you would get your money back, as your not, either suck it up, or move the frame on.

Ooh. Headset? Can't some frames run internal and external lower cups?

“I’m curious as to how often people will actually check”

Hardly anyone? I notice stuff like this, but I know I’m hypersensitive to bike set-up. I’ve never measured BB drop, only BB height, and that’s not easy to do accurately. <5mm out I’d assumed a measurement error but ~10mm and I’d be wondering if the brand or the factory screwed up.

A 10mm BB height difference on a hardtail will change the feel of the bike by a fair bit - it’s not like you can run more or less sag to adjust it, as on a full-sus.

If this is a new frame and it doesn’t meet their specs then consumer rights should be in your favour.

you need to put the frame, only, in a jig and measure, unless you have access to 3d measurement equipment or such like

Do you like the way it rides? The rest is pretty much irrelevant - unless the spec police pull you over and measure your BB drop obviously, in which case you may be in BIG TROUBLE. Lockdown exercise regs specify strict compliance with manufacturers' quoted specs, bike may be seized and crushed. But if you don't like it, that won't matter 🙂

Is this part of the vogue-ish trend where people have started buying 'geometry' rather than an actual bike. I've read ST reviews which seem mostly to have fixated on whether the chainstays are a radical 5mm longer than before. Or the headtube is 2mm shorter (not really). I sort of guess it because I'm super fussy about how my bikes ride and I understand that geometry geekery is a short-cut to getting it right, maybe. But I do sometimes wonder if it all gets in the way of 'riding the bike simply to ride the bike'.

Id get a different ruler 🙂

Geometry listing dynamic geometry and you’re measuring static maybe?

Is this part of the vogue-ish trend where people have started buying ‘geometry’ rather than an actual bike.

"Started"???

“Started”???

Well, maybe it's not a new thing, but there does seem to be more emphasis on it recently. Or maybe I've just become more aware of it as a thing.

You can either worry about it or not worry about it

Or you can actually do something about it and change the frame.

I do wonder about it being from a different year.

I phoned up Orange once to ask about a frame spec. I think I was looking at getting a new frame and wanted to use the old fork. Or trying to work out of I could run a longer fork on my frame old frame

So I phoned then and said that it looked like the geometry hadn't changed between these years can I just slap a longer fork on. His answer was that their target geometry was the same but that the Frames weren't the same. Every year they put in a longer axle crown length and the software and everything gets adjusted to keep the angles etc. the same.

So it does seem conceivable that one year some one said let's drop the bottom bracket 10mm and keep everything else the same. So did the geometry come from correct years archive?

Oh and sympathies it is really annoying

Is the only way you’ve calculated BB drop by subtracting floor-to-BB-centre from floor-to-axle-centre, or have you set a straight edge between the axles and measured from normal to that to the centre of the BB?

Does the manufacturer only quote BB drop on their geometry table?

Both 👍🏻

And yes, they only quoted drop... Not height. Height I’d be forced to take with a pinch of salt, as one person may fit 2” tyres, another 2.4’s on the same frame.

What frame, what year.

If your measurements are accurate (I’m not supposing they’re not) I’d suspect typo on the original spec. Or they changed geometry without telling anyone, or if it was re-sale model like Airborne or Planet X, they haven’t measured the bikes properly once they got to the UK.

I think you have probably hit the nail on the head here nickc...

Geometry listing dynamic geometry and you’re measuring static maybe?

That was my initial assumption, and given a 130mm fork sagged @ 25% would yield approximately 10.5mm extra drop at the BB, it would certainly be a reasonable one... However... Every single other measurement is bang on correct when taken static. I don’t mean somewhere near, I mean absolutely cock on! But the BB drop is exactly 10mm less than quoted...

Do you like the way it rides? The rest is pretty much irrelevant

Only reason I’ve been so pedantic to check everything, is I wondered how and why I felt so perched on top of this bike, when it supposedly has the same BB drop as my last 3 Hardtails. It mostly rides quite well, and arguably if I mostly rode rockier trails the 10mm extra BB height may be seen as a slight blessing, however... It just feels a bit nervous when descending to me, and I was trying to work out why, and I think it’s the feeling of being a bit perched on top rather than more in the bike that’s doing it.

This, like I said, is on 2.35” tyres incidentally. The frame was designed to take up to a 2.6” which would raise the BB a further 5mm again!

I’m reading this that it’s a used frame?.

If so, sell it, and move on, folk are paying silly money now.

Yeah, ultimately you’re bang on. It’s just got me wondering about how often this happens. Or doesn’t. When a manufacturer gets just one single aspect of their geometry quite so wrong.

I’ve spent time looking for a suitable replacement, sadly there isn’t anything available right now! So I will carry on riding this for the time being and get something more suitable ordered ASAP I think and move on...

Worth looking at a Bird Zero 29? 80mm quoted static BB drop (297mm static height with 130mm forks and 2.6 tyres) - nice and low!

Ooh. Headset?

I was racking my brains trying to think of components that could effect bb drop, and couldn’t. But you’re right, lower BB cup height would effect it.

Any/every way I can… Including…

Sorry, what I meant is, how on earth are you measuring it accurately? It’s tricky.

Anyway… it does sound like actual frame and web archive is a different batch, or, as you suggest, that sagged drop was quoted by mistake among the static figures.

Put some 27.5x2.6 tyres on to lower things?

Or run more sag? (Although that will change the angles a bit as well, as would the headset cup swapping thought).

Could it be that the manufacturer measured geometry in one size and the differences you have measured are the result of down or upsizing tube lengths.

I am assuming here as my small BFe has no wishbone where as my medium does. I thought it was to keep geometry constant with different frame sizes, Kelvin or Cy will be able to confirm that.

eccentric BB ?

Welcome to the reality of bike manufacturers!

I imagine if you measured most decent bikes properly, you would find they don’t match their published geometry charts.

“Could it be that the manufacturer measured geometry in one size and the differences you have measured are the result of down or upsizing tube lengths.”

Normally BB height/drop is constant as frame sizes vary. There’s an argument for manufacturers to lower BB with decreasing size because bikes for small people can use shorter cranks but I don’t know of anyone doing that within a given model (although Islabike certainly do it with their kids bikes).

Was just thinking the same… bb drop is normally consistent across all sizes… but should it be?

Unlikely, but it’s not something like the floor you’re measuring from is on a slight slant, just happening to make the angles look correct when they shouldn’t be? Or the floor isn’t quite flat meaning it isn’t a suitable datum?

I do the odd bit of measuring stuff at work and usually find that when something doesn’t seem to make sense ‘you’re looking at it wrong’ is as likely as ‘the designer got it wrong’. Classic example is bolt patterns that seem to be on a weird diameter but turn out to be on a square. Worth revisiting things from a slightly sideways approach sometimes.

If the headset lower cup is external on your build, but the manufacturer measure with a zero stack headset then that could account for a lot of the difference.

There’s an argument for manufacturers to lower BB with decreasing size because bikes for small people can use shorter cranks but I don’t know of anyone doing that within a given model

It's common for road bikes. With terrain varying so much and MTBers on either 165 or 170mm cranks mostly out of choice of riding style rather than leg length I'm not sure it's so important on MTBs.

I've tried measuring a lot of bikes in the garage with a few methods and tools, tbh it's never that accurate. You need a frame jig to do it properly. And alu frame build accuracy on BB drop will be +/- a couple of mm anyway. 10mm / ~15-20% out sounds a bit further off but hard to measure precisely. It may be 3mm off actual and a bit more in measuring method.

A) what frame is it?
B) if you hadn’t measured it, would you have known? (I am judging you on my biking skills here)

So... Took the bike out for a decent ride today. Started to gel with it a little more on todays ride. It's still taller than ideal, but I have a plan for the immediate future at least. I'm going to try some 175mm cranks on it I think, to lower my saddle height by 5mm and effectively drop my riding height for everything except when riding pedals level. I also have a 150mm dropper post I can fit to it, to replace the 125mm that's on it now. Hopefully this should make me feel a little more in than on the bike. I do have some 20mm rise bars I could swap in too in place of the 12mm rise bars on it currently.

A) what frame is it?

I'm not in the business of naming and shaming on a public forum. Especially as I'm not the original owner anyway. I'm happy that it's Caveat Emptor, I was just more curious as to how often this happens, and do other people notice it... I can (and will do sometime) sell the frame on, i'll get my money back so I'm not concerned. Just a little miffed as I had been looking for one for a short while when this one came up 2nd hand (they stopped making it after just one batch), and the geometry isn't quite as listed...

B) if you hadn’t measured it, would you have known? (I am judging you on my biking skills here)

I've already stated that I measured it, precisely because I could feel it... It felt a little tall and ponderous compared to my last few hardtails, and the 10mm less than specced BB drop has confirmed my suspicions.

It’s common for road bikes. With terrain varying so much and MTBers on either 165 or 170mm cranks mostly out of choice of riding style rather than leg length I’m not sure it’s so important on MTBs.

I *should* be on 175's for my leg length, but have run 170's for 15 years or more now because of personal preference and recurring knee issues in the past. I'm gonna try some 175's on it for a couple of rides, see if that improves things at all, and as long as it doesn't aggravate my knees then that may be an OK temporary fix.

I’ve tried measuring a lot of bikes in the garage with a few methods and tools, tbh it’s never that accurate. You need a frame jig to do it properly. And alu frame build accuracy on BB drop will be +/- a couple of mm anyway. 10mm / ~15-20% out sounds a bit further off but hard to measure precisely. It may be 3mm off actual and a bit more in measuring method.

2-3mm out I'd expect and wouldn't really notice if I'm honest. 10mm is quite a way out of tolerance, and is noticeable immediately

If the headset lower cup is external on your build, but the manufacturer measure with a zero stack headset then that could account for a lot of the difference.

It's an internal "Campag style" headset, where the bearings rest directly on a taper inside the head tube, no cups to speak of, and nor can you fit any. So headset is totally out of the equation.

Or the floor isn’t quite flat meaning it isn’t a suitable datum?

The front and rear axles are the Datum... Not the floor!

Welcome to the reality of bike manufacturers!

I imagine if you measured most decent bikes properly, you would find they don’t match their published geometry charts.

I hear what you're saying, there are certainly tiers to quality from some of the manufacturers for sure... I have found many brands really very accurate indeed, some a little less so but only say 2-3mm out here or there compared to their published geometry charts. I have measured MANY hardtails with 13"+ static BB heights though in the past, a combination of people over forking, manufacturer/designer getting confused between sagged and static geometry, and people insisting they need to run 175mm cranks and be able to pedal through rock gardens on their Hardtails without clipping a pedal still! BB heights that tall have no place on a hardtail MTB, unless maybe it's a specifically designed Trials bike perhaps.

eccentric BB ?

It wasn't... I don't think I have put out enough power yet to have ovalised it already! 😉

Worth looking at a Bird Zero 29? 80mm quoted static BB drop (297mm static height with 130mm forks and 2.6 tyres) – nice and low!

VERY keen to get a go on one of these at some point, but I'm not sure I want to commit to an 80mm BB drop and 2.6" tyres... I'm open minded for sure, would need to try before buying in this case though I think.

I was just more curious as to how often this happens

We don’t even know what has happened, so can’t contribute to whether it’s unusual.

Without knowing what frame you are measuring and what geom chart you are comparing it to, I’m not sure how anyone else can add to this thread beyond “might be the wrong version”, or “yeah, the drop cited in the geom chart might be a mistake”, or “measure it again”.

We don’t even know what has happened, so can’t contribute to whether it’s unusual.

It's in my 1st post... BB drop is 10mm less than stated, despite all other aspects of the geometry being absolutely as per spec in the geometry chart!

Did I need to be more explicit? 🤷🏻‍♂️

Stated where?

Did I need to be more explicit?

Yes. What's the frame? Where did you get the geometry info?

VERY keen to get a go on one of these at some point, but I’m not sure I want to commit to an 80mm BB drop and 2.6″ tyres… I’m open minded for sure, would need to try before buying in this case though I think.

Its not as low as you'd think. Its replicating the 11.5" BBs I had on bikes in the 90s. If you run it with a 2.3 (nominally 740mm) tyre its going to be low at 290 but still rideable. There's nothing to make you run 2.6s if you like low BBs, you just need to get the forks set up right so you don't gallop through the travel.

Did you actually measure the axle to crown of the fork or go off specs? I suspect the former but just checking.

Could it be that the fork lengths are different? Not all 130mm forks have the same axle to crown measurement.

Did the headset come with a crown race?

The front and rear axles are the Datum… Not the floor!

Aye, but we don’t (I think?) know how you’re measuring, it could be up to the axles from the floor, up to the bb from the floor and take the difference.

If you’re measuring directly from a line between the axles, which once you’ve got round the bits of frame in between becomes a measurement in 3D space, I wouldn’t be surprised to see a few mm creep in. Add in tolerancing and that could get you towards your 10mm.

The only thing that will effect BB drop is the AC length of the fork and the stack height of the bottom headset cup.

Unless you are using a fork with the exact AC of the original and the exact same headset used to calculate the frame geometry then 10mm is not unexpected.

Headset stack heights can vary by a good few mm between different manufacturers and the tolerance in fork AC for suspension forks can be a few mm too.

If dropping the BB by 10mm is that much of a concern and you really want to keep the frame change the headset for one with a larger lower stack height and that should get you much closer to what you expect.

EDIT: remember as well that it is not uncommon for large quantity frame manufacturers to build to a 5mm tolerance.

The only thing that will effect BB drop is the AC length of the fork and the stack height of the bottom headset cup.

Yes, but I've already established this frame runs an internal style headset, it doesn't run external cups, so that is not an issue. nor is the AC length of the fork, because...

Unless you are using a fork with the exact AC of the original and the exact same headset used to calculate the frame geometry then 10mm is not unexpected.

I'm using a fork of the correct A2C length and the headset that came with the frame from new... 10mm height difference @ the BB would equate to more than 30mm difference in A2C length of the fork, bearing in mind the chainstays are 430mm long and the wheelbase is 1195mm long, and the static HA is 65.5 degrees!

Did you actually measure the axle to crown of the fork or go off specs? I suspect the former but just checking.

Both... Within a mm of each other, and also checked that the fork isn't sucking into its travel at all too.

remember as well that it is not uncommon for large quantity frame manufacturers to build to a 5mm tolerance.

This, if true, is more useful, and what I was angling at... I didn't know what to expect tolerance wise. For reference, having now measured it, my full sus is pretty much bang on with manufacturers claims for given fork length etc. I've gone from 51 to 44mm offset, which has slackened the HA and SA by approx 0.2deg over manufacturers claim, raised the BB about 1mm, and decreased the wheelbase by almost 7mm, but otherwise it's very accurate. I'm confident if I replaced my 44mm offset fork with a 51mm, all of the dimensions would be exactly as stated on the manufacturers website.

I personally wouldn't consider 5mm to be particularly acceptable, especially not if I was the designer, and a far eastern factory sent a shipment that were out by 5mm in one dimension... So 10mm concerns me!

If you’re measuring directly from a line between the axles, which once you’ve got round the bits of frame in between becomes a measurement in 3D space, I wouldn’t be surprised to see a few mm creep in. Add in tolerancing and that could get you towards your 10mm.

I've measured it several different ways, taken averages, found a known flat floor to measure on too, just to be sure... I get the same result every time, a 55mm static BB drop.

It's a good few years now since I did my Engineering Apprenticeship, and they my Engineering Degree, but I'm used to working with tolerances of less than 0.1mm in general, and sometimes quite a bit tighter than that. I am very confident, within +/-1mm, that my measured 55mm drop is accurate.

Its not as low as you’d think. Its replicating the 11.5″ BBs I had on bikes in the 90s. If you run it with a 2.3 (nominally 740mm) tyre its going to be low at 290 but still rideable. There’s nothing to make you run 2.6s if you like low BBs, you just need to get the forks set up right so you don’t gallop through the travel.

benpinnick I am super keen to try it... I was riding back in the 90's too, though as a teenager at the time I was not especially aware of the geometry of the bikes I was riding at the time I'll be totally honest.

Agreed on the forks... Already have a Diaz Runt fitted to the 36's on my full sus for that exact reason. I don't like overly progressive forks that give up too much of their travel too readily, so with you on that one.

Not much help, but I've had this happen twice to me.
1. Kona Explosif 2013- they got the BB drop wrong by 20mm (IIRC) in the chart. I could feel it was odd. You can see a few complained about this on MTBR. I contacted the LBS I bought it from who spoke to Kona. Kona shrugged shoulders and said 'spec may very from description'. MY14 onwards had the corrected geometry.
2. Ordered a custom frame from a framebuilder (will remain nameless)- waited 6 months and then got it built up when it arrived. It was tiny! Did some basic measurements and instead of my spec (medium/long-ish), it was more like an XS size. No apology and stopped replying to my emails. I re-measured everything and sold it for what it actually was.

Opposite end of the spectrum, but proves that everyone can get it (badly) wrong.
Sadly no resolution for either issue.

I personally wouldn’t consider 5mm to be particularly acceptable, especially not if I was the designer, and a far eastern factory sent a shipment that were out by 5mm in one dimension… So 10mm concerns me!

Damn straight.

"I’ve measured it several different ways, taken averages, found a known flat floor to measure on too, just to be sure… I get the same result every time, a 55mm static BB drop."

I trust your measurements. And I'd trust some brands to ensure their production bikes meet their specs (especially smaller rider-led ones like Cotic, Bird, etc) - personally I'd hate to have products out there that I'm not happy with, and if I was in charge and a batch of frames arrived with the BB 10mm higher than designed then they would have to get sold at a discount with the issue clearly stated. But with plenty of other brands I'd expect them to keep quiet and sell them at full price - or be too clueless to even notice!

One possible solution - 27.5 rear wheel with a wide rim and a big 2.6 or small 2.8 tyre. That'll lose you some reach and slacken the angles by about 0.4 deg but you'll gain stack height to offset the reach and if the seat tube is reasonably steep that'll work pretty well. That could feel good with the right casing or an insert to damp it.

I’ve measured it several different ways, taken averages, found a known flat floor to measure on too, just to be sure… I get the same result every time, a 55mm static BB drop.

Fair enough!

Can you not contact the manufacturer just to ask the question?! With no fear of any reprisal they're likely to give you an honest answer! Providing you can get hold of them.

I personally wouldn’t consider 5mm to be particularly acceptable

Neither do I which is why I will make sure the frames I build are to within 1mm max deviation from design and usually get them to within 0.5mm and 0.2 degrees.

I have had frames in the jig from Cube, Trek, Specialised and some others that have been way out of spec. Usually it is the alignment of the rear triangle that is worst but I have a Cube Acid frame in the shed that is 4mm too long and the headtube angle is out by nearly 1.5 degrees. The best production frame I have measured was my 1997 Raleigh M-Trax. That was bang on in every aspect.

Sounds like the spec on you frame was wrong and the BB drop is 55mm as you have measured. I know it has been asked several times already but out of interest, what frame is it?

Also out of interest, which way was that head tube out? Slacker or steeper? And did anyone notice riding it?

remember as well that it is not uncommon for large quantity frame manufacturers to build to a 5mm tolerance.

This, if true, is more useful, and what I was angling at…

Not quite. +/-2mm on BB drop is a common standard tolerance for rigid mass-produced Al frames. +/-0.5 deg on angles. If you were making something higher end you'd expect it to be tighter. Generally they're closer than that ime but eg between a pilot run that sets up the fixtures and coming off the heat treatment racks there can be some variation. As hard as it is to measure accurately you shouldn't get a measurement that far off, you could be measuring within +/- 2mm and both tolerences compounded can't account for the -10mm you get.

Thanks again jameso... 👍🏻

Compounds my suspicion that said measurement on my frame was a cockup, and as it only had one production run, they just went with it expecting people not to notice I guess...

Thought I had some 175 cranks I could sling on it, but can’t seem to find them. Might try and borrow some to give it a try. I’ll fit the 150mm dropper tonight and hopefully try it tomorrow evening, see if it changes the feel.

As for 27.5” wheel with a 2.6-2.8 tyre... A) not interested, and B) don’t think it would actually fit, as where the tyre clearance is on this frame it’s a machined yoke, and reducing the radius of the rim by 19mm and fitting a fatter tyre, would likely cause clearance issues strangely enough. It’s also an XC/trail bike as such. If it was a 160mm travel full sus enduro bike, I’d consider going mullet to drop the BB a little potentially, but not on this bike... I’ll put up for now, then as and when I can get a suitable replacement, I’ll sell it back on and hopefully get most of what it cost me back. 👍🏻