I am trying to decide between Whispbars and Thule Aeroblades. Gas mileage, wind noise, and safety are all important to me. It seems that the Whispbars may be better than the Aeroblades for MPG and noise, though the Aeroblades are supposedly much better than their older aero bars.
However, when I looked at the Whispbars, the foot doesn't look very strong, and without a backing plate, the locking mechanism is exposed to the elements. In addition, I found the following post where a kayaker describes a failure where the bars meet the feet:
http://forums.mtbr.com/car-biker/whispbar-897753.html (post 12)
The author doesn't specify whether bow/stern tie downs were used, but he does say one kayak was dragged. Do people think tie downs may have prevented this failure, assuming they weren't used? I am thinking that one function of tie downs would be to protect the rack from the forces of a lifting and/or twisting kayak. I may be willing to go with the Whispbars if it seems that the risks can be mitigated sufficiently. Otherwise, the Thule Aeroblades and feet seem beefier (I would still use tie downs).
I doubt it
The poster's description of what happened really doesn't explain the situation well enough for me to figure out what actually happened, but I don't think that matters in regard to your question. Front and rear tie-downs, even if reasonably tight and properly oriented, will only do a little to reduce stress applied to the rack. There's still plenty of "give" in the lines, even with a good amount of tension, and that "give" translates to stress being applied to the rack rather than the tie-downs.
The normal "safety" reason for bow and stern tie-downs (as long as they are properly arranged) is to keep your boat on the roof if the rack attachments or main tie-downs fail. They also help to control side-to-side motion, but no one makes them tight enough to contribute a lot in preventing wind load from being transmitted to the rack attachments.
Interesting input
Thanks. I wonder how much give there is in a tie down. How far will the kayak deflect when attached? I have a plastic sit-on-top and probably tighten more than others.
I thought I’d read that the bow/stern tie downs played a role in preventing rocking of the kayak in addition to acting against lift. In both cases, the force from the tie downs would theoretically reduce repeated dynamic loads on the rack over time. From what you are saying, though, there would be no effect on material fatigue with the use of tie downs…bummer.
I have no idea how much the kayak will rock, but I have 4 feet of overhang on either side of the bars with my 12 foot kayak.
Try this test
I'm not saying "no effect". I am saying it's not enough to matter, unless your bow and stern tie-downs are extremely tight and well arranged.
Try this:
Loosen the main straps on your boat a little, then grab one end of the boat and wiggle it side to side, and up and down, and front to rear. If it won't budge in any of those three alignments, yes, your bow and stern tie-downs are helping a lot to reduce wind load transmitted to the rack. If it moves more than with the main straps tight, then it's mostly the main straps that limit motion of the boat to the degree that you see when everything is snugged down the way you like it. In that case, the forces which oppose those that tend to push the boat around are transmitted to the rack itself, and thus to the rack attachments.
I'm pretty fussy and analytical in how I tie boats down, and am one of the only people I know who typically arranges the front and rear tie-downs in opposition so that both rearward AND forward motion are controlled (and so that neither set of tie-downs can get looser due to the boat shifting position a little (and for most people, it can shift position a lot in the forward direction)). Still, with four tie-down lines all in opposition, the boat is anything BUT rock solid due to those lines alone. The main rack straps (along with either gunwale blocks or special side ties that I often use) still do most of the work to keep the boat in place (the same is true of a kayak in cradles or J-hooks, or even just friction-fit pads).
I’m maybe the second person you know–
I too tie my boats on so that there is a line preventing forward movement and a line preventing rearward movement. Plus the normal lines over the belly of the boat holding it firmly to the rack and tied in a way that minimizes side to side movement.
Thanks again for the input.
It does seem to make sense from this thought experiment that the majority of the load is taken by the main straps even with respect to inhibiting rocking and lift. The contribution from the tie downs may not “significantly” reduce stresses to the rack.
To hijack my own thread…my kayak is 12’, and my bars will be separated by about 4’. However, the bars are located more towards the rear of the car. Would you recommend:
-4’ to rear bar-4’ to front bar’-4’-\
or
/-2’ to rear bar-4’ to front bar’-6’-<br />
?
For the top, the kayak is balanced on the bars without too much overhang in front. However, the rear tie down will not prevent forward movement and will go slack if the kayak shifts forward.
For the bottom, the kayak is unbalanced, and there will be greater lifting forces (and torque) on the front of the kayak that will be transmitted to the front bar. However, the tie downs are arranged so that the rear will not go slack if the kayak shifts forward.
Front Tie Down
I normally only use a front tie down. It is usually reasonably tight and does help reduce the upward pressure of a long boat overhang.
However it is also very useful as a monitoring tool. if it gets looser or shifts during any trip, it as an indicator that some other strap has shifted so you can pull over and check the load before a mishap. I think I have a load shift about three or four times a year. It usually happens when I have multiple boats set up in stacker fasion.
Your results might vary but I typically use at least a front tie down on every trip more than 100 miles.