Presumably for patent reasons. And it's probably not straight forward avoiding having grease/oil being vented as well. Didn't one of the big bike park chairlifts press on a particular fork bleeder when they were loaded?
Edit. I've heard stories of forks boxed up arriving for servicing with oil leaked through the box too.

 

I definitely did not think about that part. I suppose, if a bike is loaded vertically on a lift and the pressure hasn't relieved itself prior to gaining elevation, pooled oil in the legs could escape. Seems like in general, an automatic relief valve would be at least less likely to be accidentally vented than a button? But it could for sure cause a leak I guess.

There already being an extant patent preventing anyone from using such as idea seems plausable.

Physically it seems like such a system could at least be as compact as Fox's pressure relief buttons though.

I definitely recieved a new damper with some oil leaked out of the bleed port. Apparently not enough to require having more added, but it was disturbing to see nonetheless.

 

The system you describe wouldn't work because the pressure relief valve is intended to relieve excess pressure only when the fork is uncompressed. As the fork is compressed the pressure increases, creating the air spring effect. If a pressure relief valve opened at a predetermined pressure, the fork would just bottom out. To verify, equalize the pressure, compress the fork and open the pressure relief valve. A bunch of air will come out. If you then release the valve, the fork will remain in a compressed state.

 

Had to read that twice; either I'm slow or it's late. You make a good point. It could be overcome by having an occluding slider block the port when compressed. That would however defeat the purpose of a simplified automation.

 

Curious to learn if this works. Wouldn’t it create a vacuum?

 

I was coming here to mention MRP. I don't have the "automatic" one shown above. I guess I have the older version you have to remember to press. Honestly, when I've checked the valves on my fork, I have not noticed any air coming out.

 

If someone could develop a valve that relieves positive pressure without (edit) generating a vacuum, that'd be a nice upgrade to sell for all the relief valves.

 

There are a number of things that go into pressure relief valves. Patent protection is a big part of it which is proven by the recent FOX/SRAM litigation. Another part is the execution.

Our patented design features a "pull to release" design which is not only easier to use, but also eliminates the leaking found in a "push to release" system when sitting on a truck tailgate pad, certain resort lifts, and most importantly when just laying flat.

For optimum performance, you need the chassis pressure when the fork is fully extended to be at the outside ambient pressure. As the fork travels through the stroke you use the "trapped" air as part of the overall spring rate characteristic. The challenge with an automated system is how do you design it to be sensitive enough to bleed out 1-4psi to improve performance at full extension, while closing and containing 5-15psi in performance benefiting pressure as the fork compresses.

You're far more likely to achieve an optimum balance with a manual control.

 

The way to solve this issue is an air filter that is always open to ambient pressure. That would be the ideal, although I'm not clear on how you'd accomplish that with current fork designs. Preventing the release of oil mist, while maintaining enough air flow during compression and extension, is difficult. It wouldn't be cheap to implement.

The early Lefty forks(=struts 🤓) did accomplish all of that, by utilizing rolling elements that can use grease, thereby avoiding the oil mist issue entirely. They used a separate air filter to prevent dirt ingress during extension.