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The question might sound confusing (my apologies), but here are the specifics.

I have a bike that's fitted with a Racktime rack, which has a maximum load of 25kg.

I also recently got a child seat that's rack-mounted (and frame-mounting isn't an option), the Hamax Caress. Now, Hamax state in several places on their safety booklet that it's designed for racks which meet the standard ISO 11243:2016 with a maximum load of at least 27kg.

What I'm wondering is whether this is merely an extra precaution, or genuinely something to take as seriously as possible. My thinking is that since my child weighs only 9-10kg at the moment, and the seat (I haven't weighed it) is around 5kg, for now at least we should be fine.

The rack adapter from the seat fits the rack fine, and meets the tube diameter and rack width specifications – it's even 11243:2016 compliant, in fact. Ought the -2kg max load to make much of a difference at this point?

[Edit: see Update 1 below] Complicating matters further, when I looked for the US product listings (I live in Europe and bought the seat in Europe), it lists everything, naturally, in imperial measurements. The same product is advertised as safe with any rack that has a max load of 55lbs – which is, essentially, 25kg!

Are the seats likely to be different across the regions? (I doubt it.) Perhaps it's just a different regulatory environment or something, I don't know. But it throws into question whether I really ought to worry about this 2kg difference in the max load of the rack.

I could buy a new rack, and ultimately would if it was unsafe, but I'm wondering if I'm rather just being paranoid instead of following the logic -- even if it states the rack's max load must be at least 27kg, that's only likely to be a risk for a rack with a 25kg max load when the combined weight of child and seat approaches 25kg.

What are your thoughts and experiences with this? Obviously the safety of my child is paramount, but I don't know if the weight limit here is actually a non-issue.

Update 1: I've just seen that there is a 2023 version and older versions of the child seat. The newer version, which I have, uses a different rack adaptor to fit the seat to the rack, and specifies a different compatibility with rack max loads - the old one states 25-30kg, the new one states 27kg. So that explains the inconsistency across stores – I think the older version is still retailing in the US, but not in my European locale (Germany).

Update 2: According to the rack manufacturer website (item 10, under heading: Mounting), they (like Tubus) deliberately under-rate their max load to avoid the automatic child seat compatibility rating they'd get if they claimed it was 27kg or more. Their specific rationale has given me pause, though. Here it is in full:

Please do not attach a child seat to your racktime carrier! Our carriers are very resistant to bending and other loads that occur during riding. However, if add-on parts (such as child seats) are fastened by means of crimp clamps, this can lead to breaks on the carrier, because racktime carriers are made of a thin-walled tube. Damage of this kind is not covered by the guarantee.

Please note the following: A child seat that is attached to one of our luggage carriers by means of crimp connectors and the carrying of a person on a luggage carrier in this way invalidates your guarantee coverage and, more importantly, poses a danger to the life and limb of the child or person using it!

Now, this is the rack mounting adapter I have. It is clearly a clamping fit system. To fit it, you use an alan key to tighten/loosen two long bolts that bring the clamps closer together (as well as a safety strap around the seat post). What do people here think about this fitting system and the racktime claim that their "thin-walled tube" could fail on account of the clamp pressure? Is this a normal design for racks? It strikes me from looking and feeling the rack itself that this would be a very unlikely outcome, but I am not a mechanic or engineer of any sort.

Incidentally, while looking for their safety booklets, I found the Hamax adapter digital booklet has accidentally not been updated (unlike the child seat booklet). So it states the max load rating of a compatible (and standard-compliant) rack must be at least 25kg, not 27kg or more. Barring the possibility of a typo, this indicates to me that internally, Hamax probably rated it such to be in line with previous regulations, not anticipating the +2kg requirement needed for the newest regulation change.

I think all this probably settles the basic concern I had: that the weight is probably itself a non-issue, handling worries aside (where I would be using the bike, every man and his dog uses rear-mounted bike seats, the roads are slow, and the drivers respectful of cyclists--so I'm not too worried about handling issues; it doesn't strike me as hard at the lower child weight end of the spectrum). But it's opened up a new concern as outlined by racktime. What do you think?

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    The rack load numbers for racktime and a few others such as tubis are listed lower because of liability reasons centered around exactly what you are trying to do. They are almost half of the actual load rating. I will try and find the information and submit a proper answer when I do.
    – Nate W
    Commented Jun 13 at 23:27
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    Consider the 3rd answer to this link. forums.electricbikereview.com/threads/… Also consider the last paragraph titled "Bottom Line" from this link realtruck.com/blog/breaking-strength-vs-working-load-limit
    – MindDBike
    Commented Jun 14 at 0:51
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    Thanks - those are both helpful replies. I am wondering now, after looking at the rack manufacturer website, if the issue is less the potential weight than the rack-mounting mechanism. I will update the main post accordingly.
    – randomtask
    Commented Jun 14 at 11:14
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    I once assisted at an accident site where a kid had been riding on the rack directly and their foot got pulled between the spokes and the frame. The parent who was riding the bike didn't seem to get how reckless they were, even after the accident (and not being able to separate the kid from the bike without my help). I think you are the opposite of that parent, you are overthinking it instead of not thinking enough. Good on you for asking the question, but based on how you wrote the question I think you are able to adequately judge the safety yourself. PS: The kid had no lasting damage I think
    – Nobody
    Commented Jun 14 at 21:45
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    The thin-wall tube issue is key, as is load placement on the rack. Clamp connections and a high center of mass of the load are both going to introduce larger local forces on the tubes than would be imposed by normal bike luggage. (Kickstands clamped to bikes not designed for them present a similar situation, where it’s relatively easy to crush one’s chainstays, even though the bike as a whole may be capable of taking larger forces than are applied to the kickstand by the ground.)
    – RLH
    Commented Jun 15 at 6:02

2 Answers 2

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Tubus, a rack manufacturer says on their website:

ISO 11243:2016 requires that all luggage carriers with a carrying capacity of 27 kg or more automatically be approved for the attachment of child seats, too. In our opinion, transporting children on luggage carriers is dangerous, so we voluntarily restrict the carrying capacity of tubus luggage carriers to max. 26 kg, thus excluding the attachment of child seats and/or the transport of children on our carriers.

(for reference, on the specs of their products: max load 26kg, tested with 40kg)

Whether a rack could theoretically support the extra weight is one thing, but safety also includes the handling of the bike, and conventional bikes are not really designed to handle large loads on the rear. The center of gravity of the kid is very high, which raises significantly the center of gravity of the bike. That and the weight distribution impacts the handling of the bike - in most conventional racks, the platform is behind the wheel axle. Of course, the actual impact depends of the weight of the ride, but for lighter riders, that can be a serious issue (I know some (light) people who are reluctant to use the bike for groceries because of that).

For what it's worth, I live in the street of a kintergarten/primary school (4-12y), in a relatively bike-friendly city. The most popular "form factor" I see for parents bringing their kids to school are longtails with 20" rear wheels (the size of the front one depends on the model), probably because this form factor addresses both the center of gravity issue and the weight distribution.

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  • Thank you for this! I've found some similar rationale on the Racktime website just now. Though it leaves me even more hesitant (detail to follow in a main post update). Re the trailers -- where I live, all of the kinds are pretty common: cargo bikes, trailers, child seats (front and back), and often a combination of them. The roads are slow and the cars respect the bikes. I'm not too worried about the bike handling given that my child is pretty light (when he gets a heavier, a trailer or cargo will likely be the way to go!).
    – randomtask
    Commented Jun 14 at 11:17
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    @randomtask other kinds are also common (front loading cargos, normal bikes), but 20" longtails is the dominant kind (probably because a front-loading cargo is very complicated to store if you don't have a garage), while the good 20" long tails don't take more space than a normal bike and can carry 2 kids. Very little trailers though, probably because it's more dangerous if you have to ride on the same roads as cars — bike infrastructure is being developed, but the city speed limit is by default 30kph.
    – Rеnаud
    Commented Jun 14 at 11:31
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At the danger of being dissed here but: What would you do if you had zero documentation for either component? You'd mount it and pull and push and see whether it gives you the impression that it holds with a safety margin, imagining border situations (bike almost falling over, thus all weight pulling laterally; a collision or going down a curb, causing large dynamic loads; that kind of forces). After understanding the hollow tube construction I'd also unmount it once and look at the tubes at the attachment points. If in doubt, perhaps use a sleeve of some kind to prevent point loads on the walls.

This goes both ways: Even if the manufacturer says "no problem" — if it looks bad, don't use it either!

Bottom line: Consider that manuals in the U.S. these days are spammed with blurb rejecting liability of any kind to the point of being unusable, and do the sensible thing. Downside: If something happens it's your own damn fault, you can't sue anybody.

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  • I see big kids jump on the rear rack of any bike, often the bikes of friends. I am sure many of those are over the 'safe' weight. And cycling with a big kid on the back of your bike is a skill, these kids all learn that skill.
    – Willeke
    Commented Jun 14 at 18:54
  • @Willeke I take that as agreement? Commented Jun 14 at 19:18
  • It was meant to be, but do not due me if it goes wrong.
    – Willeke
    Commented Jun 14 at 19:56
  • You make a good point. Though ultimately I am not mechanically minded enough to really trust my gut on these things -- e.g., whether the hollow tube worry is likely to be something that will show first with minor faults or could just fail all at once. Given all this, I'm presently leaning to getting another rack, though even with ones suitably rated for child seats, I still don't know about their construction (just have to hope the manufacturers are responsible when they purposely rate their racks as taking 27kg+ weight, knowing what that means for child seat safety ratings).
    – randomtask
    Commented Jun 15 at 23:21

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