Why are bike rear rack weight limits so low?

sdtr443w

New Member
Consider the two racks in this picture.

The one on the right came with my Bolton Foxbat. Weight limit: 25kg/55lbs.
The one on the left is an Axiom Fatliner. Weight limit: 110lbs. Also, it doesn't actually fit on my bike and the hardware for the rear axle shims sheared while taking them off. So it's going back.

Just looking at these, I would think the one on the right is the chonky thing that'll hold anything. I just realized that I've picked the rear end of the bike up from the rack just fine and probably instantly violated the weight limit in the reverse direction. No problems. Is there a major variation in the metals used? Is the Axiom made from forged fairy moonbeam ingots while these default ones are made from compressed Chinese melamine?

Going into other bikes: I tried an Evelo Aurora Limited. Its rear rack is integrated into the frame. You couldn't replace it if you wanted to. Weight limit? 45 pounds. While looking at other bikes, I was looking at the Dost ones with the integrated rack. Those have a 100 pound limit, which seems more reasonable to me.

Some other things I considered:
1. I figured the actual weak points are the bolts. The fittings look identical. I don't think the Axiom is using, say, grade 9 fittings--or really any other kind of fitting different than the one of the right. I wouldn't be surprised if they came off the same assembly line.
2. The shims. The Axiom shim is something like half as thick as the ones on the right. It doesn't look that way in the picture.

So what's up with this? I have some gigantic Ortlieb panniers coming in soon and I'm hoping to just mount them on the ones on the right and load them suckers up at the supermarket. It won't be 110 pounds but with a few 12 packs of cans and some milk, I've exceeded 75 pounds of stuff before.
whats_up_racks.jpg
 

Timpo

Well-Known Member
I'm not an engineer who designed or manufactured those racks, so I can only speculate..

1) The Bolton (or ebike) racks may actually be inferior, the material used, or even the weak bolt as you suggested.

2) The Bolton (or ebike) racks are actually stronger than normal bicycle racks, as it is beefier. The manufactures are just "playing it safe" so that people don't put heavy loads.
 

jim6b

Active Member
Do you think it matters that Bolton, the bike manufacturer, is saying it is okay for the rack AND the bike?
Whereas a rack maker is only concerned with the rack and perhaps knows that to sell aftermarket racks he needs to have a high weight guarantee.
 

Sierratim

Well-Known Member
Specialized provides Racmtime tacks on kost of their Vado series ebikes. The racks have a higher rating than Specialized publishes. They explain this by referencing balance, braking power, frame strength, etc. So in this instance a higher rated rack won't help, the ebike manufacturer limits the cargo load.

There are of course racks that can handle high loads. They need to be paired with a bike that has equal capabilitieis.
 

Nova Haibike

Well-Known Member
I know that the rack manufacturer Tubus intentionally limits the weight capacity 26 kg. to prevent their racks from being automatically approved for use as a mount for a child seat: "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." But they test some of their racks up to a load of 40 kg.
 

sed6

Member
I know that the rack manufacturer Tubus intentionally limits the weight capacity 26 kg. to prevent their racks from being automatically approved for use as a mount for a child seat: "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." But they test some of their racks up to a load of 40 kg.
This must be it, or some version there of. It regulation and lawyers. Can't go over 28 mph and cant load more 55 lbs on the rack, because you're stupid, and we're the government and we know what's best for you!
 

Ready

New Member
Other things to consider:

  1. Rack as an extension to the bike. It's not how much it can carry but how it can affect the dynamic of the bike. It can affect the balance of the bike greatly-to a point where it might do a wheelie if you hit bump. Can the rear wheel handle the weight? Suspension? Brakes?
  2. The low weight limit is to protect against extreme dynamic load. 25kg at rest is very different then if you hit a bump or a jump (look up dynamic load vs. static load).
  3. The parts the rack connect to may be not be designed to handle the load. The weight be be straight down when things are going straight, but when you turn, torsional shear stress is introduced.
There are just so many variables to consider.