Can an install be TOO tight?

BlueEyedFloozy · Sep 20, 2007

It turns out that part of my problem with my new Marathon is that I'm just not strong enough to deinstall it when my husband is the one who has installed & tightened it before me. He gets it so tight that the red buttons on the LATCH connectors just do nothing unless the latch straps are loosened, and I can't loosen the latch straps either; I press until my thumbs are about to fall off. If I'm the one who has put it in, deinstall is tough, but I can do it.

So is DH putting it in too tight? And if there's no such thing as too tight, then which is better--the tightest possible install, but my total inability to remove it if necessary? Or a slightly less tight install, but the capability of both adults to take it in and out as needed?

Avery'sMama · Sep 20, 2007

Try puting your weight into the seat while pressing loosening them. Like you would if you were installing it. It's the only way I can get mine out!

Cindy&Connor · Sep 21, 2007

I've heard both. I personally like it as tight as possible. To loosen my LATCH straps, I get in the seat on my knees and lean to one side while I'm pushing the gray button and wiggling it loose. Just make sure not to lean on the foam and break it. When I install mine I can't just push the red buttons either.

Defrost · Sep 21, 2007

Yes, you can get it too tight (I used to think there was no such thing)! I posted some more info in your other post.

As long as there is less than an inch of movement at the belt path, it's tight enough. It doesn't have to be so tight that you can't remove it easily!

mtnharmony · Sep 22, 2007

can you post a link about too tight? I wondered that yesterday when I pressed the button on the seatbelt and the seat flew up and the buckle flew out so hard it nearly broke my nail.

Defrost · Sep 22, 2007

I don't know of any links, sorry. But the seat needs to have less than one inch of movement at the belt path, before attaching the tether anchor. Somewhere between that tight, and so tight it's difficult or impossible to un-install is what you're aiming for.

Also remember to put pressure/weight on the seat when un-installing it, the same as you did when installing it, which will often prevent the belts and buckles from ricocheting around.

broxbourneDELETE · Sep 24, 2007

Defrost said:
Yes, you can get it too tight (I used to think there was no such thing)! I posted some more info in your other post.

As long as there is less than an inch of movement at the belt path, it's tight enough. It doesn't have to be so tight that you can't remove it easily!

I agree with the above. I try to apply the minimum tension on the carseat LATCH webbing to get less than 1" movement all around.

If webbing is anything like seatbelts, it is supposed to stretch a little during a wreck. Seatbelts have special weaves and rip-stop stitching to achieve this. If the web/belt is pre-strained, all it means is that it will not strain as much during the crash. I think the little bitta stretch increases the precious 'ride-down' time (that the real techs know more about than I do). If I were installing with a seatbelt, I would try not to super-strain the belt to install a carseat.

So ... the main benefits a seatbelt provides are offering the stretch to increase ride-down and then physically restraining you from leaving the general area defined by the belt path. If you overtighten, it would seem that you are throwing out, or at least diminishing, the first part.

southpawboston · Sep 24, 2007

broxbourne said:
I agree with the above. I try to apply the minimum tension on the carseat LATCH webbing to get less than 1" movement all around.

If webbing is anything like seatbelts, it is supposed to stretch a little during a wreck. Seatbelts have special weaves and rip-stop stitching to achieve this. If the web/belt is pre-strained, all it means is that it will not strain as much during the crash. I think the little bitta stretch increases the precious 'ride-down' time (that the real techs know more about than I do). If I were installing with a seatbelt, I would try not to super-strain the belt to install a carseat.

So ... the main benefits a seatbelt provides are offering the stretch to increase ride-down and then physically restraining you from leaving the general area defined by the belt path. If you overtighten, it would seem that you are throwing out, or at least diminishing, the first part.

i disagree that the force applied to a LATCH belt in a really tight installation is sufficient to consume a significant amount of the stretch potential of the belt. IMHO, probably >99% of the "tightness" of an installation is caused by compression of the vehicle seat, not stretching of the LATCH belt. the force applied to the LATCH belt tightening strap by human force is insignificant compared with the force applied to the strap in a high-speed crash.

i also doubt that having a less than tight install assists in the deceleration ("ride-down") of the CRS. any looseness of the install is taken up very early in the energy transfer of the crash and probably does not contribute much to the ride-down. the majority of ride-down is going to be provided by the stretching belt, not the slack in the installation.

broxbourneDELETE · Sep 24, 2007

southpawboston said:
IMHO, probably >99% of the "tightness" of an installation is caused by compression of the vehicle seat, not stretching of the LATCH belt.

WADR ..The vehicle seat foam is pushing back onto the carseat, which is straining the belt webbing. The belt jumped upon release and broke the installer's fingernail. Everything is under stress and anything that can strain (is compliant) will strain. When you cinch your leather belt too tight around your waist, there are definitely signs on the leather that it's under both stress (if you can feel radial pressure from the belt, what do you think the belt feels?) and strain (you can see that the leather near the buckle pinhole looks funny and has thinned a little). Your spleen is not the only thing that suffers.

southpawboston said:
the majority of ride-down is going to be provided by the stretching belt, not the slack in the installation.

I don't disagree with this. But not everything less than a nail-breaking installation is to be considered slack. I do not see the advantage of a nail-breaking installation, or an installation that restricts movement further than 1" in any direction.

Are you saying that built-in strain afforded by the specially woven/stitched belt during a crash is large enough that it will make you wish you'd installed the carseat tighter? That could be, and something I had not considered.

southpawboston · Sep 24, 2007

strain and stretching are two distinct terms. a webbed belt can withstand more strain before beginning to stretch than we are able to put on it with our own force. we can put about 100-200 lbs of force on the belt, which is not sufficient to make it stretch. a crash may place 1000's of lbs of force on the webbing, at which point it will certainly stretch.

and yes, i am suggesting that since there is *so much* stretching of a belt already (i forgot the number... i'm sure someone can chime in on that fact... something like up to 12"?) that can take place with enough crash force, which allows movement of the CRS, why wouldn't you want to minimize the movement of the seat prior to the stretching?

as i understand it, the 1" of slack that you are preventing with a really tight install is 1" less movement of the CRS during a crash, but with no less ride-down effect of the stretching.

i'm not even 100% sold on the ride-down benefit of stretching... rigid LATCH does not afford ANY stretching, but does allow a slight forward rotation of the CRS which provides some ride-down. flexible LATCH allows the same forward rotation, but then adds in undesired movement of the CRS in any direction. it would be interesting to hear if any techs have seen the load numbers from tests with seats secured with rigid LATCH versus regular LATCH (e.g., the baby trend LATCH-loc versus the regular LATCH version). it is my understanding that the seats are otherwise the same, save for rigid LATCH versus LATCH belt. that would either prove or disprove the benefit of the so-called ride-down effect of flexible LATCH versus having a really tight install such as is provided by rigid LATCH.

Defrost · Sep 24, 2007

I just want to clarify my point - I'm not against tight installs; I prefer them. I like my carseats to rock the car! In a crash, the vehicle seat is going to move, the belt, the carseat... I like to see as little movement beforehand as possible. But I also like being able to get them out without having to get a partner and fighting with them for twenty minutes.

There's a happy medium in there somewhere, I'm sure.

broxbourneDELETE · Sep 24, 2007

southpawboston said:
strain and stretching are two distinct terms. a webbed belt can withstand more strain before beginning to stretch than we are able to put on it with our own force. we can put about 100-200 lbs of force on the belt, which is not sufficient to make it stretch. a crash may place 1000's of lbs of force on the webbing, at which point it will certainly stretch.

and yes, i am suggesting that since there is *so much* stretching of a belt already (i forgot the number... i'm sure someone can chime in on that fact... something like up to 12"?) that can take place with enough crash force, which allows movement of the CRS, why wouldn't you want to minimize the movement of the seat prior to the stretching?

as i understand it, the 1" of slack that you are preventing with a really tight install is 1" less movement of the CRS during a crash, but with no less ride-down effect of the stretching.

i'm not even 100% sold on the ride-down benefit of stretching... rigid LATCH does not afford ANY stretching, but does allow a slight forward rotation of the CRS which provides some ride-down. flexible LATCH allows the same forward rotation, but then adds in undesired movement of the CRS in any direction. it would be interesting to hear if any techs have seen the load numbers from tests with seats secured with rigid LATCH versus regular LATCH (e.g., the baby trend LATCH-loc versus the regular LATCH version). it is my understanding that the seats are otherwise the same, save for rigid LATCH versus LATCH belt. that would either prove or disprove the benefit of the so-called ride-down effect of flexible LATCH versus having a really tight install such as is provided by rigid LATCH.

I never doubted I was a novice on ride-down theory. Yeah I agree the flex vs rigid LATCH would be the acid test.

Did you mean to say that strain and stress are different terms? Strain is [Change in length]/[Original length]. Stress is Force/Area. (I don't know a definition for 'stretch'). Any solid material strains a little if you stress it a little. There is no force threshold before strain happens. A material property, called 'E', defines how stretchy or stiff a material is (before it permanently deforms under tension). The higher the 'E', the stiffer it is. The lower the 'E', the more compliant (stretchy) it is. 'E' is also called the elastic modulus, or Young's modulus.
E - diamonds ~ 1100 GPa
E - nylon ~ 4 GPa

I don't know exactly how seatbelts are supposed to give during a crash. It is probably sort of complicated and involves multiaxial loading assumptions for the belt. One thing I'm sure of, if any belt is straining 12" (!), it has exceeded the elastic limit of the material unless it was made of rubberbands. Energy has been absorbed and damage has been done to the belt. If I were ever in a bad crash and couldn't afford to replace the whole car, I would def want to look into replacing any belts that were in use. Another thing I had not considered up to now.

southpawboston · Sep 24, 2007

wow, now you're making me look like the novice

i'm sorry, i was using the term 'strain' colloquially, as in the same as stress, and not in the engineering context. i thought the two were synonymous (the oxford english dictionary says they are

), but i guess in the engineering sense they are different. no wonder i switched from engineering to biology as an undergrad.

. so let me rephrase my original comment: a belt can withstand a lot of stress before it begins to strain.

and for sure, strained belts need to be replaced!

i think that the load limiter function of modern seatbelts is based on the ability to stretch, at least according to the safety videos on honda.com

. so yeah, once the strain has occured, it's not reversible.

BlueEyedFloozy · Sep 24, 2007

Your debate is wholly over my head. But I can assure there is nowhere near one inch of wiggle room at the seat base or anywhere else, even when I've installed it as opposed to DH. It rocks the car, even when jiggled at the seat top (it's FF). Yet there seems to be enough of a difference between our two installs that in one instance I can't get it out, and in another I can.

Given that the install seems rock-solid regardless, I elect to have the install that give me the ability to deinstall if necessary. I feel that is ultimately the safest for us, since it gives us flexibility in case of an emergency.

Can an install be TOO tight?

BlueEyedFloozy

New member

Avery'sMama

CPST Instructor

Cindy&Connor

New member

Defrost

Moderator - CPSTI Emeritus

mtnharmony

New member

Defrost

Moderator - CPSTI Emeritus

broxbourneDELETE

Guest

southpawboston

New member

broxbourneDELETE

Guest

southpawboston

New member

Defrost

Moderator - CPSTI Emeritus

broxbourneDELETE

Guest

southpawboston

New member

BlueEyedFloozy

New member

Car-Seat.Org Facebook Group

Sponsors

Forum statistics