The “Bottom Flooring Compartments” problem…

Some of the content of this post is taken with permission from 

I have thrown up a reblog of the whole post (found here), but I thought since it’s such a long post, I’ll just post the actual email in a separate post.

The email is from Robert Bell who is considered an expert in this field. Robert Bell was CEO at Britax Nordiska throughout the 90s. Today he own Sakta (Sakerhetsbutikken) where he sells car seats and other safety equipment. He trains staff in car seat safety, is a member of the SIS-committee, and is involved in the development of car seats, so works closely together with the manufacturers.

Here is he’s email answering our question to the everlasting debate: IS it safe to use a filler in a bottom floor compartment, or if long enough, extend the foot-prop leg of the car seat all the way down to the bottom?

          “I can try to give you a bit more information, that may make you a bit more comfortable with your choice of CRS. That Cybex seat is not my favorite, but that’s because I think it 1) has little leg room for the child and therefore 2) leads to forward facing kids earlier than with other choices. Here in Sweden we like kids to stay RF as long as possible. I’m sure you’ve read more about this on

My background is that for the past decade I have run a specialist store in Stockholm that has sold and installed CRS for almost 40 years. Prior to that I ran a few CRS companies, including Britax here in the Nordic markets and Britax in the US. Prior to that I ran the same store for the first decade. Not an engineer by training, I have nonetheless managed or contributed to the design of many CRS, both here and in the US. I’ve been on the committee that developed ISOFIX from the outset. So I know a bit about manufacturing CRS, the requirements and standards, and the practice of CRS installation. I think I am one of the few private individuals who have owned their own crash rig, so I know a bit about testing as well.

Here are your questions:

  1. Have any car seats been safety tested with the foot extended to the floor and/or on top of any filling?
  2. Who is responsible for putting car seats through safety tests? Are there different bodies in different countries?
  3. If it is the car manufacturer or even car seat manufacturer, why have they not tested it with the foot extended/on top of some kind of makeshift filling or manufacturer recommended filling, surely they want a part of that market?
  4. Are there written studies/videos anywhere that I can view safety studies on this?

To answer all this I have to start with question 2, which is about how CRS are tested and approved.

In Europe we have a very strict approach to the approval of CRS, which involves something called “Type approval.” A manufacturer, let’s say Britax, first designs the seat and produces test samples, and at the same time, a set of drawings and specifications that describe the product in great detail. Britax has a set of crash rigs, very good ones in fact, so the actual performance of the product is known prior to the formal testing. Britax however is not allowed to test the product itself and proclaim it to fulfill the standard (and I’ll come back to this).

Britax then takes the product and goes to a certified test house (there is an ISO standard for how to run a test house) which may be publically owned or privately owned. Let’s say they choose the very well known TNO in Delft, the Netherlands. TNO runs the product through a very extensive battery of tests. These tests are defined in a standards document, in our case the older ECE 44 revision 4, or perhaps the newer ECE 129 (called iSize). These documents are in fact very similar. The tests are designed to answer many questions: how durable is the webbing? Does the head protection absorb energy at the right level? Is the buckle hard enough to open? Does the adjuster device work after a number of years of simulated use? What happens if the seat is exposed to fire? Does the cover contain banned chemicals? Does the harness work for the size of children for which the CRS is intended. And of course the central question, does the seat work well to protect the head and chest of the child in a simulated accident, when installed in a manner that reflects a fairly lax installation on an average European vehicle seat.

Once this testing is complete, TNO then examines the tested samples very carefully to ensure that they are exactly in correspondence with the submitted drawings and documents. The experts also review the crash testing and other testing and decide if the product is fully in compliance with the 150 or so pages of demands in the standard. If so, TNO issues a document which states that the product is in fact in compliance with the standard ECE 44 for use in certain parts of a motor vehicle for certain sizes of children.

That, however, is not the end of the process. In the case of TNO the documents are passed on to a Dutch governmental body called RDW (Rijksdienst voor het Wegverkeer). There the next group of engineers examines the test results and the submission documents and decides if TNO is correct. If so, they issue a formal type approval of the product. If not, the testing has to be redone, or documents added or revised, or whatever the RDW requires. It is also possible for the RDW to reject the approval altogether, though that does not happen often. This can be done in many countries, and in Europe, and approval to the ECE 44 issued by a European government is accepted as an approval by all other countries in the Union.

So that is the answer to how CRS are approved. There is always a government body that issues a formal approval. That document allows the manufacturer to claim that a certain product is approved, for example, in all vehicle seating positions for children in the mass range 0-18 kilos.

In the US, the process is utterly different. In their set of rules, the manufacturer tests on his own and certifies that the product is in compliance with the US standard (called FMVS213). The government may and does often test on its own, and will issue a judgment if the product is in compliance. Often they are not, and this results in one of the innumerable recalls that all US sites on child seats list. The standards are also quite different in some regards, so it is not possible to make a seat that fulfills both at the same time.

Now to answer your questions about floor support, I have to digress a bit into CRS history. The floor support or floor prop is, as far as I know, a Volvo invention. Volvo made its own CRS for many years, some of them quite advanced. Volvo decided that a RF CRS with a floor support would work better in rear seats. These seats were approved in a special way in Europe: the ECE 44 allows for a so called “vehicle specific” approval, which means that the CRS is approved in a vehicle and only in that vehicle. When Volvo tested their seats, they used a full car seat body on their crash rig (and the Swedish equivalent of TNO actually sent its engineers to Volvo to run the tests on Volvos own rig).

Floor supports appealed to other CRS manufacturers, because they seemed a good way to reduce CRS motion in a crash test. This is always a good thing: the less they move, the lower the load passed on to the child.

But here is where the problem arises that you are facing with your seat. The ECE rig is not a car body. It is testing device that has been used since the 70’s, long before Volvo and others began using floor props. So the floor on the rig is not defined in the standard, and the use of floor props is not covered by the wording of the standard. There are committees within the ECE process that discuss these kinds of things, so there is a correct practice for the way in which floor props are installed, but the device itself is not part of the testing.

And here is where the use of floor props becomes difficult for good companies that try to make good child seats. As a design engineer, I don’t actually know what the floor of the vehicle is like. The compartments that you have on that Ford are only one example of many problems that surround the vehicle floor. To start with, there is no reason (apart from floor props) for a motor car to have a strong floor. Most modern vehicles have a floor made in high strength steel with a thickness of 0.6-0.7 mm. That is a remarkably thin piece of metal, and it is not designed to take sudden impacts from the interior of the car (why would it, aside from this use of floor props?). Now, if the floor bulges under stress, that would in itself absorb some energy and be good for the child. Volvo has actually followed up on their designs, and in some case the floor prop punctured the floor (again, not necessarily a bad thing). Furthermore, many floor areas are perforated, with drain holes for chassis rust treatment, break lines, and other things that need holes in the metal.

So from an engineering point of view, the floor prop is a headache. We think they probably help, they certainly help on the test rig, but if we are careful, we make sure that our designs will do well even if the floor is like a Swiss cheese.

If you follow me this far, you can then understand that the manufacturers are not altogether excited about telling parents that that their well designed floor prop may well be doing very little to help the child survive a crash. Nor are they keen to say a lot about the obvious problems of floor pockets and the invisible problem of floor perforations.

But the very good news is that the more conservative CRS manufacturers make sure that their designs work well even if there is no floor at all. That is the correct engineering conclusion of all this: let’s make sure that our seats are very strong structures that can handle all the crash forces even without the floor support at all.

Britax has done this with some (or perhaps all) of the extended RF child seats. Maxi-Cosi has done the same with the Mobi. This I know from being present at the testing of these products. And I expect that many others do exactly the same thing. I must state that I do not know if Cybex works in this way.

Furthermore, the US standard has a certain bearing on this question. The US standard is in some ways more conservative. It may be changing on this score, but if I am correctly informed it is not possible to certify a seat that depends on a floor support for achieving correct performance on the standard crash test demands. As a result, US child seats do not use floor supports. The US Britax seats, for example, the Roundabout, use exactly the same shell as the Hi-Way II in Europe. But the Roundabout is approved without floor support. It may be that Cybex also does a Serona for the US, and in that case, you can also draw the conclusion that the item will perform to US levels without a floor support.

So, to answer your first question and third questions.
My guess is that Britax has done exactly the testing that you’d wish they had done, because Britax has a very large crash rig in Andover which can (and does) support the testing of full car bodies. They will no doubt not share this work, but I would be surprised if there is not considerable knowledge of what happens when a seat is mounted like yours. So the answer to question 4 is that there is probably no public information on this score.

What do we say to parents here? Remember that Volvo invented the floor prop to improve rear seat performance. If a CRS rests on the dash in a front seat installation, a floor prop is not needed, because the dashboard is so very strong. When we encounter floor pockets, we do suggest that users 1. fill them or extend the prop to the actual floor and 2. place the CRS against the seat back of the front seat. The latter will give considerable support in most vehicles.

If you had turned to me as a Swedish parent, I would say to use your seat exactly as In Car Safety recommended. No matter which seat you had chosen the issue would be exactly the same, and I would go so far as to say that the performance of these various ISOFIX seats would be quite similar in a crash. And that performance would be very good.

Here is why I say.
Remember that an ISOFIX seat is very stiffly attached to the vehicle at the rear end. In a crash, the forward motion of the seat will be inhibited, and the seat will then rotate down into the vehicle seat cushion, and at the same time, load the floor prop. On the ECE crash rig, this rotation is very limited, because the rig has a “floor” that is solid steel of a thickness of 5 or 6 inches. But when you test the same seat without a floor prop, that rotation will be absorbed quite well by the vehicle seat, and the forward motion of the seat is quite limited even without the prop. This is a good outcome, and I believe it will be the same regardless of which seat you use. In your case and in an actual crash, I expect the floor prop placed on the actual floor metal of your Ford will retard motion quite well. In a very violent happening, it may well deform the floor pan, and that does not hurt the child.

There is a working group in the ISO process that is charged with devising a way of making a defined part car floors strong, so as to improve the function of floor props. That way we can begin to assume that the vehicle floor will work consistently. When we are agreed on that, your concerns will be answered. But for the moment I think you and other mothers can feel confident that your child restraints will protect your child in a crash regardless of the inconsistencies that surround the floor prop feature.

The best thing about rearward facing is that the laws of physics are on our side. The seat keeps the child from hitting the vehicle interior (which often happens in FF seats) and the energy of the crash is absorbed over the entire back and head as it rests against the EPS liner of a stiff CRS shell. There is none of the violent motion and neck loads associated with forward facing seats. So the way the floor prop functions is really a detail, not unimportant, but at the same time only one component of many that ensure that the child moves very little in the crash. I usually end up saying to parents that it does not matter which child seat you choose. The only thing that matters is that it is a rearward facing car seat. You can read more about the statistics at but that is the really important thing to convey to your readers.
Let me know if you want me to explain something of the above in more detail, or if something seems unclear.”

I want to thank Robert Bell from the bottom of my heart! He’s made is SO clear as to WHY this no longer needs to be debated!

If you have come across someone who is telling you that it is NOT safe to either extend the leg to the bottom of the floor in the compartment, OR that it is NOT safe to buy a filler and fill it, and as a result, put the foot prop on top of the compartment, you now have the answer.

Photo cred:




Therese has completed the ‘Advanced Child Car Seat Training Course’ at TRL (Transport Research Lab) and is a CPD accredited car seat expert. She blogs about in-car safety, car seats, tips, reviews, give-aways and advice. She’s a mum on a mission to change the law and raise awareness. She is also a breast feeding advocate and gentle parenting promoter who loves cloth nappies, baby-wearing, BLW and co-sleeping/bed-sharing.