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Praxis Works Conversion Bottom Bracket – PART 1

 Praxis Works Bottom Bracket
Praxis Works Conversion Bottom Bracket
Praxis Works Bottom Bracket

 

— PART 1 —

 

 Just like with Consumer Reports, a lot can be said about actually buying aPraxis Works Warranty product to test. In this case, the tester is not beholding to the manufacturer. I bought this bottom bracket through my own funds and will share an honest review of my thoughts and experiences.
On a second note, this seemingly simple review turned more complicated than originally thought due to ‘bearing issues’ which you will read about in Part 1. This article was originally going to be a short 2,000 word maximum review on the Praxis’ Bottom Bracket solution to the creaking and squeaking seen in most press-fit bottom brackets. Part 2 will review the Praxis technology.

 

INITIAL OBSERVATIONS

 

Since there are several versions of this bottom bracket available (pictured above), you will need to make sure that you pick the right one for your application. Praxis currently supports SHIMANO, SRAM and CAMPAGNOLO in BB30 and PF30 in 68mm ROAD and 73mm MTB as well as Specialized OSBB. To make this selection even more complicated, frame manufacturers have been known to change their BB standards half-way through a model year run which might cause any Bottom Bracket, including Praxis to be a ‘non-fit.’ For example, frame manufacturers might make an engineering change such as using a customized aluminum shell with a machined lip instead of the spec BB30 C-clips, or even switching over to a BB360EVO from BB30 or PF30.

6805 bearing 2437 bearing

For the SHIMANO Hollowtech II solution, you will need to first decide on which bearings you want as there are two options, (1) an $85 set of steel bearings or (2) a $195 set of hybrid-ceramic bearings. Currently, all Hollowtech II solutions come with 2437 bearings sourced from different suppliers. No industry standard 6805 bearings are available from Praxis and if you choose to swap out the 2437 bearings for the 6805 bearings, you WILL void the warranty.

As an engineer and industry consultant, I am very familiar with bearing technology including materials selection,  machining quality and fit and finish, so to me, voiding the warranty on the whole bottom bracket seems a bit harsh, especially since it is only the bearings that are being swapped out. In my opinion, as well as most other manufacturers of bottom brackets, the bearings are separate entities as are the cups. In fact, in many bottom brackets, the cups can be left in and the bearings popped out and a new set easily pressed back in after a season. So, it seems a little peculiar that you will void the warranty on the entire bottom bracket just by changing bearings, especially since the 2437 and 6805 bearings both have a 37mm OD and are both 7mm thick. The only difference is that the 6805 solution (which is the bearing Shimano has chosen as their standard) requires a plastic dust cap shim that reduces the bearing ID from 25mm to 24mm for a perfect fit for the 24mm OD Hollowtech II crank spindle.
Since almost every BB manufacturer lets you change out bearings, I’m not really sure whatPraxis Works Conversion Bottom Bracket Praxis is thinking?
But, to be fair, I called Mr. Adam Haverstock, Director of Marketing and Sales at Praxis and we had a lengthy discussion. He clarified several of my question, but, left others unresolved.

 

BACKSTORY

 

About a year and a half ago, I was asked to test a manufacturers first generation internally routed Di2-ready frame (spec’d for 24mm crank spindle). Everything came in the box except the bottom bracket. My choice was to either use a TPS press-fit bottom bracket, or try one of the new Praxis BB’s. I opted for the Praxis since their engineering and technology looks rock solid. I say rock solid since they solved the squeaking experienced by many of the traditional press-fit models.
Initially, I thought this was going to be a fairly easy article to write – what worked, what didn’t. Easy, since a bottom bracket is made up of 2 separate sets of items, bearings and cups – one set per side. But, this review turned out to be a lot more involved than I had initially thought. I ordered the standard steel bearing Praxis BB Praxis Works Conversion Bottom Bracketand immediately noticed that there was no plastic reducer sleeve that is usually seen in every other 24mm BB solution. This sleeve is usually required to reduce the 25mm ID of the 6805 bearing to 24mm so that any 24mm crank (i.e., Shimano) will fit. Instead, the Praxis BB was supplied with a set of Enduro 2437 bearings. So, before installing the BB, I decided to do a little research. This is where things got a little complicated.

 

DISCUSSIONS WITH MANY

 

The main question I wanted to answer was – “For a bicycle bottom bracket application, which is best, a metal crank spindle running (a) on a metal bearing surface (called Interference-Fit), or (b) on a plastic reducer?”

“It turns out that the first 6 words in the question above
are the critical key to ultimately arriving at the answer.”

In starting my research, I spoke with 20 Southern California bicycle shop mechanics and they ALL said that they have experienced bottom bracket creaking and squeaking with direct metal-to-metal contact / interference-fit bottom brackets.
Next, I spoke with Mr. Haverstock where I questioned him on this point. He stated that pro Praxis Works Conversion Bottom Bracketteams use these exact BB’s with no issue and if these BB’s are good enough for a pro team then they are good enough for the average rider. He also stated that he remembers only 2 BB’s ever returned for warranty issues.
I then recontacted all of the mechanics and discussed what I was told by Praxis. Again, all 20 mechanics told me that they would never warranty any BB for creaking and squeaking. What they would do is just slap some extra heavy-duty grease onto the crank spindle and bearings, some Loctite on the cups then reassemble the BB.  They all told me that this simple fix usually lasts customers for a full year, or at least until the next time that the bike needs servicing. They also mentioned that this is an easy fix that makes the customer happy, happier than if they had to return the BB back to Praxis, forcing the customer to wait 1-3 weeks for a replacement BB – which will end up probably making the same noises as before. So in response to Mr. Haverstocks’ comments, since BB’s are not returned for warranty, Praxis has skewed data as to the actual number of BB’s that have made noises and have had maintenance work performed on them.
A second point that the mechanics made was that since Pro-Tour teams have their own dedicated full-time mechanics who work on team bikes daily and changing out parts before they fail, you really can’t compare this to a LBS mechanic and their customers. LBS customers only bring in their bikes when something is broken. Essentially preventative maintenance vs fixing a broken part.
“At this point, I want to again mention that this is not an issue with the Praxis BB cups, but an issue with the bearing & interface technology chosen for this application.”
So, based on these mechanics experiences, it looks like there IS an issue of creaking & squeaking.

 

INTERFERENCE FIT or PLASTIC SHIM?

 

Next, I interviewed six Mechanical Engineers (ME’s). Three were professional engineers in their given industries and three were full professors with PhD’s. Turns out, one of these PhD’s also Praxis Works Conversion Bottom Bracketconsults for Specialized!
All ME’s told me that an interference-fit
(metal-to-metal contact) is best, BUT,
that it also depends upon the application.
Metal-to-metal contact: For example, a bearing used in a high speed motor application is heated then slid over a shaft that has been cooled. The bearing slides on easily and once back to room temperature, these 2 parts are ‘locked’ together. NO squeaking, NO creaking, NO movement between the bearing and the motor shaft. So what happens if a bearing goes bad? To change a bearing, the technician will probably heat the bearing with a torch and use a bearing puller to slide it back off the shaft.
For a bicycle bottom bracket application, the solution is not so easy. Referring to Praxis Works Conversion Bottom Bracketthe picture to the right, to assemble a bottom bracket built to the same net-zero tolerance spec as industrial motors, you would need to heat up the drive-side bearing while cooling the crank spindle. Slide the bearing over the spindle, then do the same for the non-drive side. For this season, there would be no issue. For next season, there would be a problem. To disassemble, you would start by removing the left (non-drive side) crank arm, but now what would you do? How would you get the right (drive-side) crank arm off? Remember, the non-drive side bearing is locked onto the crank spindle. A bearing puller won’t work because the bearing is set into the BB shell so there is nothing for the puller to grip onto. Heating up the bearing isn’t an option since you would destroy the carbon fiber.

 

SO WHAT’S THE BEST SOLUTION?

(for an in-depth analysis [From Slocum A.H., Precision Machine Design, Prentice Hall, EnglewoodDr Slocum MIT professor Cliffs, New Jersey 1992, pp 387-399], see excerpt from Professor Slocum’s book and class notes at the end of this review)
The pro’s and con’s for a bicycle application are;
  1. PLASTIC REDUCER SHIM/DUST COVER (Shimano solution)
    1. PROs
      • Can build to slightly looser tolerance since the plastic shim will ‘take up the slack’
      • Inexpensive piece that protects the crank spindle and bearings
      • No mechanical difference when compared to metal-to-metal contact
      • Eliminates virtually all squeaking between crank spindle and bearing
    2. CONs
      • Some aftermarket replacement plastic shims are made from a cheap brittle plastic and will crack when being reinserted into a bearing
  2. METAL-TO-METAL CONTACT
    1. PROs
      • A very reliable and rigid connection when done correctly, but also with the correct application
    2. CONs
      • The key phrase is “when done correctly.” Even though you will get a very reliable and rigid connection – after heating up the bearing and cooling the crank spindle, you would never be able to remove the crank in a bicycle bottom bracket application
      • If BB bearing seizes, friction with metal-on-metal contact, while continuing to pedal, will overheat the crank spindle destroying its heat treating, and possibly ruin the crank – I’ve seen this on a customers bike
      • Increased manufacturing time (i.e., increased cost) is needed to guarantee a precision interface, but still needs to be a looser fit between the bearing and crank spindle interface
      • To prevent too high of a stress, the typical interference needs to be fairly small, nullifying most of the advantages of this solution. Problems include;
        • Radial or circumferential stresses
        • Buckling while pressing crank spindle into the bearing
        • Clearance of the bearing can be exceeded forcing the balls to jam into the races
      • If interface between crank spindle and bearing fit is too loose, crank might feel like it is wobbling or slipping

 

PART 1 SUMMARY

 

In general, direct metal-to-metal interference fit is the best solution for a shaft/bearing application, but, in a bicycle bottom bracket application, for all things considered, it turns out that the plastic shim is the best solution. So is it worth completely voiding the warranty in order to switch out the bearings? I would say yes since next season you would need new bearings anyway.
For those interested in a copy of Dr. Slocum’s Interface-Fit design and calculation spreadsheet, please click here, or contact me at biketestreviews@gmail.com.
I want to again give a big thank you to Dr. Slocum for providing the overall engineering analysis for this review.
Doctor Alexander Slocum earned his PhD in 1985 in Mechanical Engineering and is currently a professor of Mechanical Engineering at Massachusetts Institute of Technology. Dr. Slocum holds over 5 dozen patents, has published numerous papers and books and his interests include design, precision machine design and physics via sports.
 
Dr. Slocum was also gracious to include his excel spreadsheet for those who wish to perform the design analysis themselves.His personal information is located here – http://meche.mit.edu/people/?id=80
Doctor Alexander Slocum Shafts: Interference Fits
Click above excerpt for full size
8/2015
Next time – Part 2 where we review the actual Praxis Bottom Bracket cup technology

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