Vacuum cup based racks like those available from SeaSucker and others have been mentioned at times in the forum. I am starting this new topic as the home for discussion about those and any other vacuum / suction cup related product.
I happen to have a 25+ year history working at Wood’s Powr-Grip which makes vacuum powered materials handling products. Long story, but Wood’s was actually the originator and first to patent this type of product back in the 1960’s.
I design, test and build vacuum cups as my daily job, so I have more than a bit of experience with vacuum powered products. I’ve also had a hand in elements of a couple of the rack makers below, since they started by using our actual products for their racks. Let me know if you have any questions.
I had decent success with these but, what I disliked, was having to pop-out the little vacuum mechanism to lube (i’m super lazy) and, I noticed w/ my Subaru Impreza hatchback, that I always had a problem getting the 4th cup to adhere to the rear window, which I think was due to the curve in the rear glass. Probably user error but just a heads-up.
Out of paranoia I always poured water on the suction cups (as per instructions) before mounting it unless it was for a quick <30 minute jaunt and I definitely noticed a difference in how they would stick (how much of the suction indicator was showing) but I’m sure it still wouldn’t “undo” even if you didn’t do that.
Wetting the cups or surface is not ideal, IMO. One aspect of the way these vacuum cup racks work relates to the coefficient of friction. When you introduce water between the vacuum cup and car surface (glass or paint), you reduce the functional friction. We may not be talking about enough to lead to a slip failure, but a bad idea is still a bad idea in this case.
Clean the surface to remove any dirt or debris, with a small amount of moisture remaining after that cleaning may be fine. But deliberately adding water for better “stick” is counter to how the vacuum pump system is designed to work.
Did not realize this either. They say this on their FAQ: Sometimes we say “wetter is better” around here, but don’t take that in a weird way. SeaSuckers will typically hold better when wet, so the rain is no problem.
Legitimate question, why do the windshield suction cups work better when theyre wet? Is it because it’s not a perfectly clean surface?
I’d need to review their info for full clarity. But based on our testing and general experience, moisture reduces coefficient of friction. As a result, it may allow the vacuum cup to deform a bit “easier” when applying the vacuum pump. This may give the impression of a “better” attachment from an external view. However I am confident that most typical rubbers lose CoeF with water between the rubber and attached surface. Simple pull testing on our fixture has repeatedly shown that lower forces are possible with moisture.
This comes down to the method of air removal under the vacuum cup. Simple flip, slap and screw type cups (common on many simple device mounts) are often better with water for attachment.
Like above, the moisture allows the rubber to slide more freely on the glass/metal surface. As such, it can get “flatter” which gives a better vacuum holding force.
As they say, “There is no free lunch”. In this instance, water on the cup will allow easier deflection, but it also serves to limit the CoeF between the cup and surface. Give and take. As long as you aren’t relying largely on the friction aspect of the connection, you are likely fine. Most of these simple cup options are loaded with more moment forces than sheer/friction forces. As such, it can make the wetness less of an actual “problem”. But is still not idea IMO.
For the more advanced vacuum cups with a manual pump (like Seasucker and our stuff), the pump does the main work and pulls the pad to the surface as it removes air between the cup and surface.
This usually results in a higher vacuum force (negative pressure) than the simple flip and slap cups above. As such, even if the friction between the cup and surface resists the attachment a bit, the brute force of the vacuum will force the cup to the necessary shape to match the surface.
Seasucker may be getting away with the water recommendation due to friction not being the main support component in some loading cases. That and the likely excess via design factor in the loads the vacuum cups can support give some wiggle room there prior to failure.
That’s well more than we probably need to cover in the Deals thread, and I may well just pull this into a separate vacuum rack topic if anyone wants to discuss this boring stuff more
Interesting, I too always add a little water to my seasucker cups, and find that with water they can literally hold for days without a repump, whereas dry they generally start showing the orange band after the course of a full day’s trip.
If we are talking purely about “sealing” and retention of vacuum (negative pressure under the suction cup), the water can help in some ways. The core function of the suction cup relates to the “sealing edge” which is usually a ‘V-shaped’, rounded or flat area at the outer edge of that pad. This creates the primary seal to hold the vacuum from the atmosphere.
That edge can be fouled if there is dirt or debris on it that allows air to freely or more easily pass over that area than a clean sealing edge. In that sense, getting water on there which is likely “cleaning” the sealing edge before or during the vacuum application may help.
If the pad is in poor condition with either dirty or damaged sealing edge, water is a stopgap that will potentially lead to problems if/when it evaporates.
But one thing the water may help with is how the sealing edge moves and finally settles during application. Some sealing edges may move in irregular ways as the pad compresses and conforms to the surface. On occasion, with some sealing edge shapes and sizes, this can lead to a less than ideal seal. If the water allows the rubber to move better, and achieve the best shape when compressed, this may aid in the attachment. LOTS of variables here from the actual surface condition, pad design and attachment process can all influence the “holding” results.
Generally speaking, if the vacuum pump is showing a leak in shorter period of time, that is a sign that the vacuum pad sealing edge, mated surface or both are somehow flawed. This assumes a properly functioning and clean vacuum pump that is not leaking excessively on it’s own.
No doubt! I’m exclusively referring to the visual indication of suction provided by the orange indicator on the Seasucker pump. Anecdotally, with a little water, it’s easy to get the initial suction when mounting the rack, and it seems like it extends the time before that orange indicator is visible again (though even without water mine don’t lose pressure prematurely.)
As someone else mentioned earlier, Seasucker seems to recommend moisture in their own instructions, and that’s why I always default to using it. I’ll avoid it on my next trip and simply clean things down more thoroughly, and see if I notice a difference in performance.
Yeah, as usual, following MFGR recommendations is likely best. I just find it at odds with the many tests I’ve done and what we recommend for our products. There is a notable difference in risk & liability between one device that holds a bike, and some of ours that may be used on material over 1 ton in weight
I was looking at these earlier this year and one of the manufacturers (I forget which) had a recommended elevation range - i.e., not recommended >5000 ft elevation (~1520 m). I live in the foothills in Reno, right around a mile high, and I’d be looking to take my bike up to the mountains. So, the question: if I stick with any of the big brands, do you see any problems sticking a bike to my roof and going 75 mph over an 8000 ft mountain pass?
Great question, and I will have to dig for more info. But in short, higher elevation IS a concern with any vacuum lifting or holding device. The higher you go, the less atmospheric pressure you have. Considering that these use vacuum (negative pressure) as the primary source for holding force, it can and does reduce potential capacity.
The key here is the relative rating of the device, with respect to it’s ultimate holding force at any given elevation. If you give enough Design Factor to something, you can rate it and not worry about anything you can drive too. But, the DF is not universal and the relative rating is actually dependent on a range of factors to include the CoeF between the pad and material, as well as things like bending moment and dynamic loading.
We have specific ratings for all of our lifting products with this at the core, so it’s very valid to consider. I have not dealt with the specific considerations of a rack since we don’t sell those, so I’d be guessing as to their relative ratings. As above, following MFGR specs is likely best. If you have questions beyond that, I’d ask them directly.
ETA:
For reference, here is one example of a “regular” product spec for one of our hand cups:
We also offer a “high elevation” version of this with a different vacuum pump installed, and a reduced 100lbs [45 kg] capacity up to 12,000 ft [3,658m]. So it’s possible to handle higher elevation with some pump changes and likely a reduced capacity.
Chad, probably an impossible question to answer but given your expertise and while I “have you on the line,” what would cause a suction cup to lose function? One of my seasucker cups no longer works (luckily they supply you with an extra to switch over to.) I took it apart, cleaned and relubed it, and switched it over to a new pump. Nothing seems to work, it simply won’t hold air, and there’s nothing aesthetically wrong with it—no visible damage to the cup, no creases or kinks in the rubber, etc. If you had to take a wild guess, what’s the most likely cause?
Here are the two places to start, which seems you’ve at least considered:
The sealing edge on the vacuum pad. Inspect the pad face at the location of the sealing ‘V’, round or flat surface. Look for any tears, cuts or missing material that would potentially lead to a leak when under vacuum.
Remove, clean and relube the vacuum pump itself. The main area of concern is the “U-Cup” that is the functioning part of the pump. Inspect it for damage or debris. Clean and lube. Also inspect and/or clean the inside of the pump assembly. If there is excessive dirt, or a lack of lube, that can lead to leaks.
Sounds like you took a good swing at #2, so I would make sure to check #1.
For reference, here is one of our Troubleshooting guides, for more detail:
