In a previous edition of Jimmy's tech ramblings (read it here if you missed it) we outlined the ins and outs of the humble rivet nut. While most rivet nut issues can be avoided by following some basic best practices, we've found that the rivet nut is simply not ideally suited for the task of cargo-carrying on carbon forks, so we set out to solve this problem.
Time For Change
Over the years, we've specified several interpretations of rivet nut cargo mounts on our forks. We've used stainless rivet nuts and aluminium rivet nuts, both bonded and not bonded. While some variants offer better results than others, it was clear that improvements could be made, especially as the requirements of bikepackers worldwide became more reliant on efficient load-carrying year after year. Through our own riding experience, we have known this to be a vital improvement area as our products have evolved. Effective fork cargo mounts are essential to effectively distributing your cargo weight in a balanced way.
Essentially, we wanted to do away with the rivet nut altogether and start from scratch to devise a solution that could easily stand up to the rigours of the all-terrain riding we practice and encourage here at Curve.
A little while back we did a complete redesign of the Seek 430 (GMX+) fork, which meant that a new mould was needed, so we decided to make the most of our investment and also give the leg mounts an overhaul at the same time.
Very Good Mounts
So what is so different about these mounts compared to a rivet nut?
First of all, this is a system completely engineered to our specifications. It's not an off-the-shelf product that's been repurposed to "kind of" work. The brief was to come up with something that would be class-leading. After many design iterations and working closely with our CNC parts suppliers and our fork factory to develop the specification, manufacturing and bonding process, we landed on the VGM system. We first introduced it on the Seek 430 FM fork, and we're now rolling it out across the rest of our range of carbon forks too.
Here are some highlights of the benefits of this new system:
Surface Area
The flat part of a regular rivnut, the part that's in contact with the cargo cage of your choice, is approximately 10mm in diameter. On the VGM mount, we've increased the diameter to 16mm. That's a lot more surface area to spread the load to the underlying fork leg and the larger contact patch also helps stabilise the cargo cage.
Stability
A regular rivet nut is only attached to one 'wall' of the hollow fork leg, which means that as the thin carbon wall flexes under load, your cargo will wobble back and forth and potentially, over time, this can lead to rivet nuts coming loose.
The VGM mount, however, is a thru-mount system, meaning that it's supported by both walls of the fork leg. This provides a much more stable platform, and together with the increased diameter the added rigidity of the system is significant.
Thread Length
The average rivet nut has a thread length of approximately 8mm. The VGM mounts have a thread length of 16mm. While you don't strictly have to use the full thread length to attach a cargo cage, more thread engagement means the thread in the mount is less likely to be damaged.
Weather Sealing
The VGM system consists of two parts permanently bonded to each other and to the fork itself. This means that there is no way for debris or water to enter the inside of the fork leg, not even if you ride or wash the bike without bolts to plug the holes. With a regular rivet nut, this is a point of entry for debris and water.
Durability
Because we had the freedom to decide what material to use when developing the VGM system, we chose 6061-T6 aluminium. The "T6" part in the designation means that the aluminium has been hardened via a specific heat treating process, making the material a lot more durable. Additionally, we also opted for an extremely durable "hard anodised" finish. This is not an option for regular rivet nuts. The anodising, together with the fact that the VGM mount has also been bonded to the fork leg with an epoxy, means that the risk of galvanic corrosion between the fork leg and the aluminium is reduced.
Carrying Capacity
We're so confident in this new system that we have more than doubled the allowed load per fork leg. On our previous forks with rivet nuts, the rating was 3kg spread across all three mounts - with VGM, we've increased this to 8kg.
We honestly can't see why anyone would want to carry 16kg of gear on the front of a bike that's not an Omnium or another type of dedicated cargo bike, but as long as you stay within the total system weight rating of 120kg, you could.
Yup. 8kg / per leg!
Single Point Use
We've always discouraged our customers from attaching things to fewer than three points at the same time on our previous fork models. This is because the load-carrying capacity is reduced by a great margin when not using all three points simultaneously. As such, attaching racks or lights to a single rivet nut is not something we'd ever recommend.
With the VGM system, this is no longer a problem. While we would not suggest hanging the full load capacity of 8kg / fork leg off of a single VGM mount - attaching a rack strut to one mounting point on each fork leg is perfectly ok. And if you want to run regular water bottles (2 mounting points) on the fork, that's perfectly ok too.
Testing
The VGM mounts have been tested extensively in both the lab and in real life. The first few tests to prove the concept, before we invested in a new mould and tooling, were done on regular forks that we modified to accept the VGM hardware. We did this in-house here at Curve HQ and we came up with some simple torture tests that clearly showed that the new system was superior to rivet nuts. (Do not do this at home. Your existing fork can not be converted to VGM).
These initial tests gave us the confidence to invest in the new mould and proceed with the production of samples. One of the first samples was immediately taken to South Africa and was used on the inaugural Rhino Run.
Before any of our forks are released to our customers in the wild, they first go through a standard suite of the appropriate ISO 4210 lab tests - these tests are done both at the factory and at a third-party facility. Once these standard tests are completed, reaching the minimum International Standards required (and where most brands wrap up their testing phase), we ramp up the torture beyond the standard ISO requirements. We typically incrementally test to failure to see how the part fares, showing us both when and how failure occurs.
See those big lumps bolted to the legs? Those are heavy steel blocks hanging off the legs during an entire 100,000 repetition fatigue cycle.
The Future
All our gravel and adventure forks categorised under the 'Ride' and 'Seek' series will be using this benchmark system from now on. We've discontinued the Ride 400 fork which was the last fork model to use rivet nuts as cargo mounts and we now have Ride 415 VGM (individual sales available soon, but for now due to stock constraints this fork is only available with GXR and Big Kev purchases), Seek 430 FM, and our new Seek 500 MTB fork all on the VGM program.