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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. 👍🏻