Our Engineering Director, Graham Shrive, discusses the new Bottom Bracket for the Factor OSTRO and explores T47, PF30, BBright, BBcorrect, and now Factor
“New bottom bracket standard” has become something of a dirty phrase in the cycling industry. Over the years, special cranks, rear axles, front derailleur mounts, and even rear wheels have been required to facilitate the various solutions that have been invented by the industry.
When it came to developing a solution for our brand new OSTRO VAM, we listened to current feedback in the market and decided that the best way to serve our riders was to adopt the T47 bottom bracket standard. After years of living with the pros and cons of several of the systems, having ourselves built the first-generation O2 with a PF386 system, we were sure there was a better solution than what the industry was currently providing, so we went looking for it.
Taking a different approach is the ‘Factor way’, and we have proudly developed our own take on the new industry standard T47 bottom bracket. It is based on the existing T47 standard, but adapted to allow for use in asymmetrical frames and is actually co-molded into the frame. Co-molding guarantees that both threaded surfaces will be perfectly in line with one another. The benefit is exceptional bearing life and the lowest possible friction for the crank bearings. We’re also able to save some weight on the frame and maintain current stiffness. To top it off, we have also removed the material between the drive and non-drive side which allows for access to brake hoses and Di2 cables. Mechanics rejoice.
This new industry-leading T47 bottom bracket solution is available on our OSTRO VAM.
Read on for a deep dive into how we got there:
When we contemplated bottom bracket standards, one that stood out to us was the BBright open standard developed by Richard Matthews and Don Guichard as part of Project California/R5ca in 2010. In the case of this standard, the choice was to run the absolute maximum width of the Shimano axle, which is 90.5mm. On the drive side, the width is the same as a standard 68mm BB30 axle width or 34mm from centre to face, and the non-drive side is the maximum that can still fit a Shimano axle, which is 45mm, plus 0.25mm for a plastic flange on the BB cup.
Typically in a press fit cup, the drive side bearing hangs outside of the frame, giving lots of clearance for crank spiders. The total width of this system is 79mm +/-0.25mm. This tolerance is quite tight for an outside surface that will later be painted. Because of its high visibility to the rider, and the large loads imparted by pedalling, it typically isn’t practical to face into tolerance. Looking at this standard from the rider’s perspective, while it can physically fit the Shimano axle length, in practice it can cause issues for riders when using different cranks. Or if tolerances are not ‘stacking up’ in a fashion that benefits the rider, leading to potential warranty claims on the frame, or premature bearing wear on the BB. Unfortunately, these issues are not easily rectifiable by the rider through the addition of small spacers because the frame and other components are already at the maximum condition that can fit between the crank arms.
Fig.1 – The BBright standard drawings from BBright.net, as drawn by my former colleague Richard Matthews some 10 years ago. Note the GD&T symbol for the NDS width in parentheses “()” showing that it’s a derived dimension from the total width, which is looking for a tighter tolerance than is typically possible in a production carbon environment
The cumulative effect of the various components in a system’s respective tolerances being combined to be either much larger as a whole, or much smaller, as a whole, than intended. For example, if a frame & BB are on the wider side of their respective tolerances, and a Crank axle on the shorter side, you may get bearing rub.
When Factor looked at what BB standard we should use as a brand, we already knew the benefits of sticking with an asymmetrical design. The primary benefit of this is giving the designer extra width on the non-drive side, so you can effectively gain “free” stiffness from the frame. Having built frames with a PF386 style chimney, we knew this ended up with a net weight gain, and caused quite a few issues with various power meters and small diameter bolt circles.
We decided to proceed with the BBright standard, but made a slight tweak to the approach used with BBright – the result of which we have referred to cheekily as ‘BBcorrect’. In this case, we stepped back from the absolutist approach of running the bottom bracket width all the way to the maximum allowed by Shimano. Instead, choosing to reduce the non-drive side centre to frame face dimension to 43.1 mm unfinished from the BBright 45mm, with an expected paint build-up of no more than 0.4mm. We left the drive side dimension as previous, matching the typical symmetrical 68mm BB30 standard, or 34mm. This total finished width dimension is 77.5mm, +/- 0.5mm. The non-drive side measured from the centerline is 43.5mm, which follows the Shimano guidance for BB width. This gives room to accommodate tolerance stack if it occurs by the use of mini spacers, or preload rings depending on the crank. This also allows riders to use a one-piece, thread together style bottom bracket, such as those by Wheels Manufacturing or Hope. The reduced width allows you to use a normal tool on the non-drive side flange to tighten up the bottom bracket on the spline.
Fig. 2 – Factor’s ‘BBcorrect’ configuration, found on the ONE, O2-Gen 2, O2 VAM, LS, ViSTA and SLiCK
This also quite conveniently complies with the guidance provided by Shimano directly for the recommended width of a frame from the non-drive side, with half of 86.5 being 43.25, which is right in the tolerance band of our press fit BB solution.
This bottom bracket standard is in use on all of our current model bikes mating with our excellent press fit bottom bracket design, which should really give most riders a lifetime of trouble-free use. However, times change and riders have started to really associate press fit systems with increased maintenance headaches and creaking. We are confident our current press fit system addresses most of these concerns by way of tighter manufacturing tolerances. But given the current market conditions, we felt that the best way to serve our riders was to adopt some variant of the T47 standard.
Fig. 3-Shimano Frame design guidance for the BB area
The move to T47 was quite straight forward for Factor, given our choices with the ‘BBcorrect’ standard we’ve already started using. By sticking with the 34mm centre to face dimension on the drive side, we’re able to simply use the standard T47-BB30 adapter (T47e). On the non-drive side, we had to do extra homework with our partners at CeramicSpeed, and fortunately, the industry has moved to adopt a universal standard called T47 asymmetrical, which effectively uses the T47i standard. These cups should be readily available from a variety of manufacturers and will fit all makes and models of BBs, such as Campagnolo Ultra-Torque, SRAM’s Dub system, Shimano 24mm, and native 30mm cranks.
By working with T47 asymmetrical, we also have the benefit that our current frames match the width of the practical maximum for current road cranks, which is also what the T47 asymmetrical standard is set at. By objectively evaluating the needs of the rider and balancing these with the practical constraints on BB width imposed by the various crank standards, we were able to adapt to the current shift to T47 frames with our current ‘BBcorrect’ configuration and offer both T47A and PF4630 frames from the same mold. As seen by our WorldTour team, Israel Start-Up Nation who used press fit versions of our new OSTRO VAM at the Tour de France.
The Factor Difference
When you choose Factor, you know that you are choosing a brand that takes a different approach. This of course applies to our approach with the T47 BB standard. Most manufacturers using T47 are adopting a typical overlapping bonded bottom bracket using a flat interface with the frame and then bonding the cups together in the middle of the section. When used most effectively, this design relies on a positive interface between the frame itself and the bb shell. Typically, this is composed of some type of non-structural material built up on the inside surface of the frame, which is partially machined away and then also bonded to the overlapped sleeves. This approach adds significant amounts of non-structural material in the overlapping cups, as well as the built up area of the frame, which of course is extra weight.
In addition, when we examined this typical way of constructing a T47 BB interface, one of the primary impediments was the actual width of the frame flange interfaces, which added as much as 3mm of unneeded aluminium to each side of the frame, reducing the amount of real estate available to us to improve frame stiffness. Also, the large concentration of stress at the face transition of the flanged surface and the frame can lead to significant concentrations of peel stress. When this surface is finished or painted, this stress can manifest itself in paint cracks or outright debonding of the BB sleeve.
After much discussion and analysis around the issue, our engineering team was able to come up with an exceptional solution. We decided to actually co-mold and post machine the BB shell, ensuring the concentricity of the cups and exceptional strength inside the frame. The primary reason for this is that the co-molding process occurs during the actual curing process, as opposed to a post-mold bonding operation that inheritably must occur at a lower temperature than the Tg (glass transition temperature of the epoxy resin used in pre-impregnated carbon sheets) temperature of the frame itself. This effectively makes the BB and the frame a single piece, similar to the heavy-duty inserts used on full suspension mountain bikes, much more effective than post-mold bonding.
Fig. 7 – Co-molded BB Sleeve pre-machining
Complementing this excellent strength, we realized that by starting with a single piece BB shell and co-molding it into the frame, we can guarantee that both threaded surfaces will be perfectly concentric, or in line with one another. This will give exceptional bearing life and the lowest possible friction for the crank bearings. Of course, being Factor, we were still unsatisfied with the material between the cups not directly contributing to the strength of the system, so another machining operation was added to the process, which removed the material between the drive side and non-drive side cups, decreasing the weight to the bare minimum, and allowing the rider access to brake hoses and Di2 cables while assembling the bike. Something mechanics everywhere will surely appreciate!
In summary, the new industry standard T47 Asymmetrical standard allows us to retain all of the benefits of our previous “BBcorrect” system, while giving the rider more options for service and bearing replacement. By co-molding the threaded sleeve as one piece, the best possible concentricity and parallelism is maintained between the drive side and non-drive side bearings, ensuring long life and smooth operation. This co-molded threaded section reduces the various stresses that have led to threaded BB failures in carbon in the past, and ultimately, the removal of the non-structural material in the centre of the BB section improves the assembly process and reduces weight to the absolute bare minimum.
The new T47 BB standard from Factor can be found on the OSTRO VAM.
Fig. 8 – Finished T47 BB interface