# What happens to the dynamics of a bike as a trailer load approaches a significant percentage of the weight of the bike and rider?

An average bike trailer might add 5-10 kilos to the weight of a bike, while not zero its also a small amount of the overall system mass.

How do the dynamics of the combination change, as the weight of the trailer + load approaches the weight of the rider + bike ?

Imagine loaded trailer is half the weight of the rider and bike, and equal mass, and double the mass.

This comes from a comment on Designing Bike Trailers – Especially the hitch and load and aims for a more generic answer.

• @dlu As suggested a while back.
– Criggie
Aug 5, 2017 at 8:40
• A two child trailer could reach 60kg. While that's unlikely to be towed by a light adult on a lightweight road bike it's not impossible. There are no specific warnings from the manufacturers of such trailers (despite many other warnings about stopping distance). I doubt anything is going to happen suddenly. That said I'm familiar with one downside that I'll post as an answer. Aug 5, 2017 at 9:23
• As my rather anecdotal answer illustrates, the weight distribution is also important. Aug 5, 2017 at 9:34
• UK law restricts unbraked trailers to half the weight of the towing vehicle. That might not apply to bikes, or to where you are, but it would probably be unwise to exceed it, and very unwise if you are using a seatpost mounted hitch where braking will tend to lift the rear wheel off the ground if you might ever have to brake at all sharply.
– armb
Aug 8, 2017 at 14:02
• @armb I had that exact thing happen with the 20" trailer onto an axle mount of a 26" wheel. The angle was enough to lift the rear wheel under braking, so its not just seat mounts that have to be careful.
– Criggie
Aug 8, 2017 at 22:28

My heaviest weight ever was two computer UPS.

At home I weighed each wheel and the hitch before unloading and found 42 and 46 kg on the wheels, and 10 kg on the nose, giving a total trailer+load weight of 98 kilos.

I was using my folding bike, which weighs 15 kilos and I'm roughly 95 kilos, for a total of 110 kilos. So Bike+Rider was 11% more than trailer+load.

Yes it was strapped down a lot better for the ride.

My experiences:

• Getting moving is hard work - simply crossing a gutter and up the camber of the road to the centerline was a challenge.

• Moving smoothly was difficult too. One tends to have a power stroke then a lighter moment as the pedals go over the top. The trailer wants to keep moving so it alternaltely resists the power stroke and then pushes your bike while the trailer moves faster than the bike.

• Extreme case, my journey had to cross a railway line with an elevation change of ~1.5 m in about 15 metres so a 5% gradient. I had to drop to bottom gear (15ish gear inches on that bike!) to keep moving.

• Downhills are strange - the trailer's weight pushes your whole bike and it feels like a much steeper grade. You get the feeling that braking is not going to work so don't even try it. I imagine this is how you would feel with no/failed brakes on a downhill grade.

• Bottoming out - You've just ridden through a low spot, perhaps across a driveway gutter or a dip in the road. The trailer is still on the downhill and is pushing your bike up the other side. This reminds me of a simple electrified bike I rode, which was on or off. The sensation might be like a tandem where the stoker has just started pushing real hard unexpectedly.

• Braking - Everything takes about 4 times as long when you weigh 2 times as much. Stopping distances are 4 times longer for the same speed. Although there's only twice as much kinetic energy to dissipate in the brakes, it's not possible to brake as efficiently because of...

• Braking kick - I had a smaller load on my trailer - perhaps 50 kilos of tools going to a bike fixup. I braked to a stop at a red traffic light, using my habitual both brake stop, not fast and not harsh. But the front brake took weight off the rear, and the trailer was able to push the rear wheel up off the ground and then pushed it sideways into the traffic lane somewhat. UPSHOT: when towing a heavily loaded trailer, use predomenantly rear wheel braking, and remember to shift your body weight backwards.

• Cornering dymanics. Your bike can lean into a corner, but a two wheeled trailer can't. I managed to roll my trailer with a 68 kilo load of two landrover wheels. The whole trailer rotated on the coupling (its designed to allow this) and skidded on the road on its left side (outside of the turn) They were too big to lie flat so looked like this: This managed to tweak my rear axle so it needed reseating to be straight.

• Flats Be warned that you're more likely to flat a trailer wheel if you hit something sharp while heavily loaded. If its heavy enough, impacts can damage your wheel rim too. Changing a flat tyre is no fun if you need to unload it all to get it light enough to remove the wheel.

The maximum load of your trailer is going to be the minimum of the material strength of the load-carrying frame, (note mine has an 18mm slab of customwood as a bed, many are just cloth), the load carrying maximum of your trailer chassis, and however much your tyres/wheels and axles/axlemounts can carry.

• "Braking - Everything takes 4 times as long when you weigh 2 times as much. Stopping distances are 4 times longer for the same speed." I think you're confusing mass and speed. Kinetic energy is proportional to mass and to speed-squared. If you double your mass, you have twice the KE so it takes twice as long to stop, not four times (brakes bleed off KE at a roughly constant rate); it's doubling your speed that quadruples your braking distance. Aug 5, 2017 at 12:01
• Did you try any speed bumps? I notice something similar to what you describe under a few of your points even for gentle speed bumps, and the short sharp plastic ones in car parks are interesting (with your loads I'd be very wary of the latter). Also have you compared a bike with full size wheels? Aug 5, 2017 at 14:19
• @DavidRicherby obviously your physics is correct. But because (front, i.e. useful) braking is limited, 2x stopping distance for 2x mass is probably not out applicable. Another factor of 2 as margin for poor handling seems not unreasonable for such huge loads. Aug 5, 2017 at 14:22
• @ChrisH Fair point. I've edited a little to explain that. (Criggie, of course revert or refine if you don't like it.) Aug 5, 2017 at 14:30
• @ChrisH no I don't recall taking it over speed bumps loaded. Imagine they'd have to be really slow, plastic or concrete. The cobbled "traffic calming" lumps in my area have all self-flattened with quakes.
– Criggie
Aug 5, 2017 at 21:34

When towing a trailer of no more than 40% of the weight of me+bike up a gravel road I've encountered one issue that's probably quite common. This is wheelspin. The back wheel needs enough grip to propel the bike and trailer with all the weight that involves up the hill against the rolling resistance of 4 wheels, two of them small and with bearings that aren't as good as bike wheels. When one of the 3 non-drive wheels hits resistance - even just a larger than normal stone, the back wheel can lose grip. The trailer should have a little nose weight but this isn't enough to make the difference.

In this specific case the same slope was easy without the trailer, even deliberately hitting bumps with the front wheel. This was in a hybrid with marathon plus tyres - fairly narrow and slick. MTB tyres would be a good idea if riding with a trailer on gravel habitually, even if your normal tyres would be fine without the trailer. Incidentally a rear mounted seat adds grip by adding load to the back wheel, and doesn't add (much) rolling resistance. I've tackled much worse with the seat on the back.

• Wheelspin is no fun. I've taken that folder up a gravel road climb and had wheelspin, so imagine it would be similar feeling.
– Criggie
Aug 5, 2017 at 21:37

I use mine for fishing tackle as I don't drive. I've towed the heavy trailer to a pond 5.8 miles(9.3 km) away from home, stay the night, and return home the next day. I would say that I have between 65 and 70kg on the back of that set-up and she does the job without issue. It's just VERY HARD WORK.