PVD-RSD SRAM 8-Bolt Spider

Mountain bikes and all-road bikes are some of the harder bikes to design and build. Squeezing frame members around tires and drivetrain parts compound into many other problems to solve.

One of the primary ways of alleviating this is to provide as much clearance on the outside of the bike as can be made. This used to mean that very fancy spiders were needed.  I implemented that back in 2016 (7 years ago), for the SRAM 3-bolt spline cranks. I had to do this for all of my bikes but the rise of commonly available direct mount chainrings has made that a lot easier for mountain bikes. For all-road bikes, the story is different than with mountain bikes. There exist few 3mm offset (5mm actually) direct mount chainrings for tooth counts in excess of 36. So if a need exists, like it would in all-road, for a 38t or 42t ring, there is no support available.

I don’t like that I can’t get what I sensibly need. I find it strange that I seem to be the only person to notice this…for the last 7+ years.

So, it was time, again, to make a part that nobody seems to want. A 3mm offset spider for the newer SRAM 8-bolt spline cranks. It’s time to revise for the new 8-bolt cranks and improve design based on the learning done in that time. 

I wanted to collaborate with Ronen Sarig to produce these. Aside from Ronen being a good friend of mine and in possession of a CNC mill of his own to use, he is working on bicycle designs that are highly influenced by my work. So we share a need for this kind of part. More, Ronen is a senior mechanical engineer at Joby Aviation, a leading silicon valley aerospace company making electric helicopter taxis. Basically, Ronen knows his stuff and that’s a good collaborator.

I did much of the design work with several rounds of review with Ronen. There were things that he wanted to see, ideas of mine that were challenged, and details I had to fight for. This is great stuff. We had it out at times but learning how to keep projects moving is a skill that we both have.

The spline interface and crank arm clearance are reverse engineered as no specifications are available. It’s a simple enough pattern once you figure it out. 

Most spiders are quite loose as they fit to the crank and rings. We wanted a tight fit so we had to be dead nuts on the spec and tolerances. Concentricity is a wonderful thing.

Ronen did the machining of the spiders and they turned out excellent. The finished product looks so dialed against the SRAM Force carbon cranks.

While the 3mm offset spider produces a nearly perfect 49mm chain line with the road cranks for a 142mm hub spacing, a feature of the design was to allow for chainrings to be shimmed inboard when space did exist. This would make the bike more efficient in climbing gears which is what I need while running a 38t chainring on mountain bike trails. Of course, I wanted this for my new 2023 PVD M2-F2 All-Road bike.

2.25mm shims bring the chain line down to 46.75mm with just the smallest amount of space between the ring and yoke.