I'm fortunate to have had more than my share of Attitudes and Adroits roll through my garage and so I decided to set up a small database of numbers and try to correlate those numbers to features which can be attributed to specific periods during Klein production phases. My hope was that I would not only figure out the numbering sequence and be able to determine which sequence came after which and so on.
I've compiled a database of approximately 300 (02/13/2015) serial numbers spanning 1990-1993 Attitudes and Adroits. Here are my findings:
First of all I think Klein used the Hexadecimal numbering system. This would easily explain numbers like 1F3 = 499 and OD2 = 210. This make sense as Klein had been in business for a while and they would have to deal with numbers greater than 999 and since they used the rear dropouts there wasn't enough room for a frame identifier (AT for early Attitudes, AD for early Adroits) and numbers in excess of 999. HEX can express very large numbers by using a combination of letters and numbers. I can't help but wonder if Gary's time at MIT didn't help influence this??
I've reviewed this with Gary and he confirms that he did in fact use HEX since the very early days. Which explains why even some of the 85 Mountain Kleins have HEX numbers. I've never taken much interest in road frames, but the little bit that I've looked at indicates that those too used HEX numbering for the S/Ns.
The commonly known features of a 1990 Attitude include:
1. Box crown (mitered) fork
2. Solid black decals
3. MC1 headset seal stamped with KLEIN (K) logo
Klein stamped headset cap
The less known features of a 1990 Attitude include:
1. Early Attitudes had the down tube cable inlets rotated up from the centerline
2. Early Attitudes had a 56cm top tube, while later ones increased the length to 57cm
3. Early Attitudes had bottom bracket bearings with red/rust colored seals, later ones had black ones
I believe that early Attitudes started with the ATxxx serial numbers. Each serial number was preceeded by a small Klein logo (K). I've read on old forums that the first batch of Klein Mission Control headtubes were either undersized or reamed too much causing problems with the bearing fit in the frame (either too tight or spinning). Some Klein employees seemed to remember that once the problem was resolved the serial numbers were changed to BATxxx. I think the change over happened around #302 made as Attitude B(K)AT12E had a B added in front of the (K) symbol implying that the problem was found, fixed and that one frame which originally would have received the bad head tube was fixed and that's why the B was added later in front of the (K) symbol. Following that transitional frame I see a majority of BATxxx Attitudes and only a couple more ATxxx Attitudes, implying a switch over to BAT.
After collecting numbers for another year or so I've found that ATxxx frames continued on (up to AT28d or 653) for a while after AT12E (#302) implying that either the problem wasn't resolved right away, or that the addition of the B to the serial number wasn't correlated with the head tube issue. It's possible that the process changed and the S/N schema was revised later on for another reason. More and more I think that the change had more to do with subtle frame redesign including the cable inlets, top tube length and the head tube fix. That would make more sense to me, but I don't have any confirmation that this is what actually happened.
I believe that ATxxx Attitudes preceded BATxxx Attitudes because all ATxxx Attitudes have the downtube cable inlets located above the centerline, a feature known to be found only on very early bikes. While very few BATxxx Attitudes are found with those same cable inlets.
ATxxx cable inlets - rotated up from centerline
BATxx Cable inlets - on centerline
All of the ATxxx and BATxxx Attitudes I've identified had box forks, solid black decals and the Klein stamped headset seal. So, I think it's safe to say that if your Attitude has an AT or BAT serial number it's a 1990.
Fork serial numbers in 1990 would start with 5A.
In total I believe that Klein made around 2300 Attitudes in 1990, give or take. The highest fork serial number is right in that range so that seems to make sense.
1991 / 1992 Attitude: (REVISION PENDING)
While 1990 was the introductory year full of discoveries and changes, 1991 and later 1992 were both years of mass production and relative consistency of features.
Main features of 1991 Attitudes include:
1. Uniklein fork (altough the catalog still showed the box crown fork)
2. Solid black decals (later switched to black with white outline Attitude logo closer to seat tube)
3. Smooth MC1 headset set
1991 was the year Klein introduced the legendary Uniklein (round crown) fork. Evidently the early box crown forks were somewhat prone to failure and sometime in late 1990 early 1991 Klein switched to the stronger and somewhat less harsh Uniklein fork. I've identified very few 1991 Attitudes with factory installed box crown forks, but they're out there.
I believe that 1991 Attitude frames had a serial number format Axxx and the forks had a format AOxxx. The earliest Axxx Attitude I have recorded is #92 and the highest is #3498, while the highest fork comes in at #3757. The higher number might account for extra forks built to replace broken 5A box crown forks. At some point, either in late 1991 or early 1992 Klein relocated the serial numbers from the rear dropout to underneath the BB shell. This provided more space and allowed them to use a longer string of characters. This led to the new ATxxxxxx format. Out of the 60+ Attitudes I've only spotted two with this format ATT000111 and ATT002433. Although I do not have enough data to prove this it seems that sometime around late 92 Klein switched the fork serial format to Axxxx, with the highest number in my record of #2203.
So, it would seem that Klein built somewhere around 3700 Attitudes in 1991 and perhaps another 2400 in 1992. The fork numbers don't seem to corroborate this, but this is still a fairly limited data set. It's also very likely that the number of rigid forks would be less than the number of frames since 91 and 92 saw wider adoption of suspension forks, hence fewer rigid forks required to outfit frames.
1993 / 1994 Attitudes: (REVISION PENDING)
The 1993 Attitude was the final version of the MC1 Attitude and it was the model year most closely related to the Adroit. Unlike it's predecessors which had a 1 1/4" (27.2mm seatpost) seat tube the 93 Attitude went up to the 1 3/8" seat tube (31.6mm seatpost) used on the Adroit. This reduced the seat tube extension above the seat tube from 6cm to about 2. Also, unlike the earlier Attitudes with a 1 5/8" down tube the 93 Attitude received a larger 1 7/8" downtube, closing in on the massive 2" downtube on the Adroit.
Also Klein stopped using decals in 1993 and started debossing their frames.
1993 also marked the introduction of a new serial number scheme. The fairly non descript ATTxxxx was first replaced with T for Attitude then MMYY for month and year of production followed by xxxx denoting the number of the frame in the overall production line. At some point Klein added the frame size (19 or 20...) in front of the T.
The highest number 1993 Attitude I have recorded was from April of 93 at #1695. The previous number was from February at #1271. So it's safe to say that another 200 or so were made before production switched to MC2 bikes in July 1993.
Although 1993 was the last official production year for the MC1 Attitude the rollout of MC2 bikes was slow and Klein sold a small quantity of MC1 Attitudes in 1994. I have very little data on this but I think it's safe to say that 200 more MC1 Attitudes were built and sold as 94 model year bikes.
So, in total this would bring production of 93/94 MC1 Attitudes to around 2000 give or take.
All in all I estimate between 8,000 - 10,000 MC1 Attitudes were made by Klein between 1990 and 1994.
The data set supporting these conclusion is relatively small compared to the total production numbers, but it is the largest such data set ever compiled. So, although these conclusion are made on limited data they seem plausible.
If you want to help please submit information for your bike or any others that you know of for sure via my blogger contact form.