Cache Blog - Custom Titanium CNC Machined Parts
Updated: Feb 19, 2020
When we talk about custom CNC machined parts, the Cache has a whopping eleven of them: 2 in aluminum and 9 in titanium. They range from the most mainstream part (the BB shell – CNC turned from 6-4 titanium to our exact specification), to the highly functional hooded dropouts, to the precise and yet ornate headtube with CNC Lathe live tooling engraved KNOLLY logo.
Custom Titanium CNC parts:
BB shell CNC turned to our exact specification.
Forged headtube CNC turned to our exact specification with live tooling engraved KNOLLY logo.
Cable gusset plate on the underside of the down tube where it meets the BB shell
Seat tube front derailleur bracket mount.
Drive side chain stay yoke.
Flatmount disc mount A.
Flatmount disc mount B (yes, these two rear brake mount parts are different!).
Drive side dropout.
Non-drive side dropout.
Custom Aluminum CNC parts:
Bolt on down tube cable management plate.
Removable front derailleur braze-on style mount.
That is a LOT of parts for a product that is essentially a stretched and slacked out endurance road bike with mega tire clearance. But then again, it’s not a road bike and it needs functionality that doesn’t exist in road or a CX product.
What’s key in this discussion, is the drive side chain stay yoke. The Cache is capable of running narrow Q factor 2x road cranksets yet it still needs tire clearance far beyond what is used in the road bike world. No amount of tube forming would be able to solve the challenge of narrow space requirements of larger tires and narrow cranksets.
Add in the requirement for large single chainring 1x drivetrains and space constraints start getting very, very tight. This requires a machined part which is narrower than any tube can be.
We took the opportunity to add some functionality to the drive side chain stay yoke in terms of the cable management tie wrap location. This is absolutely key to ensuring that the rear derailleur cable (mechanical shift housing or Di2 wire) is located precisely underneath the yoke and away from both the rear tire and crankset chainrings.
Some parts are more subtle in their design. For the flatmount disc brake interface on the chainstay, we need two different parts because the chainstay is angled inward from the dropouts whereas a disc rotor (and hence brake caliper) is parallel to the centreline of the frame.
If we used the same part, one mount would be either too far “into” the chainstay or conversely, the other mount wouldn’t have enough weld area to be strong. The typical height of these mounts is 30mm for the limited off the shelf options and we built our initial prototypes using some of these off the shelf parts just to ensure that we had proof of concept in the design.
However, as we moved into pre-production Cache frame design, I felt that there was an opportunity to significantly improve the design of these parts.
The first issue is that flat mount brake calipers from both Shimano and SRAM ship with fasteners designed for a flat mount brake boss height of 25mm but the vast majority of off the shelf mounts require 30mm hardware. How annoying would it be to order all of the parts to build up your dream gravel bike and then realize that you had to source some generic M5 bolts because the hardware supplied with your brake-set doesn’t work on your frame.
So, we tweaked the flat mount boss design to use the standard 25mm hardware provided by Shimano and SRAM and to also hide the bolt head. Both functionality and aesthetics were improved, while also dropping a few grams of weight. Win, win & win!
Next a key design decision had to be made with the internal cable routing and how to successfully have cables exit the down tube near the BB without having to have a bunch of holes in the frame that we’d have to fish cables through.
A clear solution presented itself from our mountain bike cable routing knowledge and this was to exit all of the cables on the underside of the down tube just above the BB through a custom port. However, this would mean a gusset style plate with M4 hardware, a cable port, and all of this would have to be welded between the down tube and BB shell while still being reliable as hell.
The original design for this part was to be a simple plate with welded M4 pemnuts in place but very quickly it became obvious the superior solution was to just CNC everything into one part and eliminate any variation due to welding.
Probably the most obvious CNC machined parts on the frame are the dropouts. They’re in a high visibility area of the bike and just draw attention to themselves! Personally, I am a huge fan of the “hooded dropout” design for titanium and steel road bikes over the “flat plate” style of dropouts. Hooded dropouts are invariably more expensive since there is a lot more material to remove in machining, but they provide a more classic look and are also a much better structural element at the back of the bike.
I wanted of course for the dropouts to be as symmetrical as possible, but recognized that their designs needed to take into account our standard rear derailleur hanger on the drive side of the frame and the space limitations due to the flat mount disc brake interface on the non-drive side of the frame. I was able to use the same symmetrical base model for both parts but then tweak the design as required for each side’s specific needs.
Given that this dropout is not a universal design and would only be used on Knolly frame models, there are subtle details to the hood design that are designed to match our exact seat stay and chainstay tube shapes. The forward / bottom of the hood is chamfered to match the trapezoid shape of the chainstay exactly, while the top of the hood is radiused so that the round seat stay tube is concentric with this radius.
The final step in the dropout design was to mount the seat stays off centre (behind) the rear axle axis. This helps to build a little bit more vertical compliance into the frame while having no penalty to pedalling efficiency.
The final CNC machined part that I want to discuss is the head tube and it’s CNC engraving. Knolly’s GM Paul Nash is pretty talented in graphic design, but we were struggling to find a suitable head tube badge for the Cache since our standard round MTB headtube badges did not look very good on the Cache’s long headtubes. After a lot of brainstorming, we decided that we would engrave the head tubes! While it’s pretty obvious that each headtube is machined for each specific frame size, what may not be obvious (unless you’re a machinist) is that we also machine the breath holes during the same setup and also the head badge engraving. The engraving is performed with a tiny 0.6mm diameter ball nose engraving tool and takes a LONG time to do each head tube. We think it’s worth it!
For the introduction of the Cache we decided to have a bit of fun and go with a whisky theme for the frame. Can you find what whisky inspired the head tube design?