With the PRX integrated rubber straps, we decided to fit the straps with double knob quick release spring bars (similar to the spring bars on the OEM bracelet), but with a longer stem compared to the spring bars on the OEM bracelet. While it may seem like double knob QRSBs are all the rage and should be used as a default on all integrated rubber straps and bracelets, there are also some downsides that we've discovered.
A quick look at how the bracelet can be removed and strap inserted with double knob QRSB. Notice the shorter knobs on the bracelet, and how you still have to use a tool to remove the bracelet because of this.
First, the good. Double knob QRSB greatly improves on the ease of inserting and removing the strap. With two knobs that sit taller than the case, you can insert and remove the strap using your fingers. With no sharp metal tools involved, it reduces the chances of scratching your watch.
Can you achieve the same outcome with a single knob QRSB, like those found on leather straps? Yes and no, and the main reason comes down to whether the strap / bracelet is compressible.
YES, YOU CAN DO THE SAME WITH SINGLE KNOB: If the strap is slightly compressible, like a leather strap or a rubber strap without any plastic or metal insert that gets in the way, you can actually insert and remove the strap with just a single knob QRSB. You can insert one side of the spring bar, compress the single knob (and the strap), insert the entire strap within the lugs, and release the knob.
NO, YOU CAN'T DO THE SAME WITH SINGLE KNOB: If the strap / bracelet is not compressible, like a metal bracelet or a rubber / leather strap with a plastic or metal insert, you cannot use a single knob QRSB to insert and remove the strap. This is because the strap occupies the entire space of the lugs, and you cannot compress the strap sufficiently to position the QRSB.
While double knob QRSB may seem like it can do no wrong, our experience in using them for the Tissot PRX rubber strap has shown that there are some downsides, which could possibly explain why it isn't more widely adopted by all brands where possible.
1. It is extremely labour intensive (read: costly) to install a double knob QRSB. Unlike non-quick release spring bars where you simply slide the spring bar into the hole (takes <1 second), or single knob QRSB where you insert it via the small notch cutout, (~5 seconds), there's no magical shortcut to installing double knob QRSB.
One knob is fixed and welded for additional security, while the other knob has to be screwed into the spring bar after the spring bar is slotted through the strap. Given the size of these spring bars, it is very tricky trying to screw the knob into the bar, and with a lot of focus, it can take a skilled person (with a lot of patience) around 15-25 seconds to install the spring bar and screw the knob in. With two straps, that's about a minute just to install the spring bars, compared to at most 5 seconds for non-quick release spring bars. And with production, more time needed equals to higher cost.
2. Knobs can fall off over time. There is a certain level of failure rate in these small parts, and even with regular, single knob QRSBs, the knobs will fall off in some of the spring bars that we see. Our solution to this problem for single knob QRSBs was to develop proprietary spring bars with knobs that are welded instead of a simple wedge or screw. Welding greatly reduces the chances of the knob falling off.
However, if you understand how the installation is done for double knob QRSB (see the video above), you'll know that it is not possible to weld both knobs to the spring bar, as the knobs will get in the way of installation. And since one knob has to be screwed in, the chance of it coming loose over time and falling off is there, even if it may be ~ 1 in 1000 spring bars that we see.
Note: one side of the double knob QRSB that we use is welded in, while the other side is screwed in.
Why We Decided on Double Knob QRSB for PRX Rubber Strap
Ultimately, we made the decision to install double knob QRSB in the PRX Rubber Straps, mainly because of the benefits it provided in terms of ease of use.
Yes, it was possible that the screw-in knob on the QRSB could fall off for a small percentage of users. However, we didn't want to adopt a sub-optimal solution in our product (single knob QRSB or even non-quick release spring bar) and shortchange most users, in order to eliminate the possibility of the knob falling off, since that will only happen to a small percentage of users.
Moreover, even if one knob were to fall off, the strap can still be used as intended. Firstly, you can still screw the knob back in (if you had the patience to) and it will be back to normal. Secondly, even with just one knob, you can still install and remove the strap without any issues.
Installing PRX Rubber Strap with a Single Knob QRSB
In the event that one of the knobs on your PRX strap has come off and you are unable to screw the knob back in, fret not! You can still install and use the strap without any risk of the strap falling off the watch. Here's how:
If you are still unable to install the strap, or uncomfortable with having just one knob, feel free to reach out to us!