Son_Rising

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Maxwell

In general, a larger wheel will roll a tiny bit faster, but smaller wheels will be stronger and stiffer. For the heaviest loads, 24” or 20” wheels are better. Biggest thing is making sure whatever size wheel you have, that its spokes are tensioned as high as the rim will tolerate. The more spokes and the stiffer rim, the more you can carry.

tim24k

PLUS ONE
As long as your wheels are round, no worries.

Harhir

The only issue what I came across a few times is trailer tongue length. Meaning some trailer tongues are too short for bikes with 700c wheels.
When we moved to the US in 2001 I brought my trekking bike from Germany. 700c wheels, fenders and tail light on the rear fender. And when we started shopping for a kids trailer a few months later I found out that not all trailers had a trailer tongue long enough for that bike with 700c wheels and the fenders.
Same with the Burley Travoy trailer I have for shopping. I cannot mount the hitch on the seat post since the trailer will not clear the rear wheel. I had to fabricate a bracket on the rear rack to move the hitch back.
But I also tow the Travoy with my BikeE which has a 20" wheel in the rear and a 16" wheel in the front. No issues here either. Wheel size does not matter.

CliffordK

It'll depend a bit on the bike/trailer combo.

I use kiddie trailers that typically have a low front footwell.

When I tow with the 20", the foot well hangs mighty low, for greater risk of rubbing on everything.

andychrist

I’ve been towing trailers for almost two decades, first on on uprights and now on recumbents, with both 700c and 26” rear wheels.

The more important factor IMHO is tire width not wheel size. I wouldn’t use anything under 2.0” to haul heavy loads especially in hilly environs. You need a lot of grip to avoid slipping/fishtailing.

700c might roll a little better but as far as I can tell that difference is fairly negligible. OTOH 26” wheels will offer lower gear inches on a comparable drive train — For cargo bikes, a good set up would be either a 40/32/22 crankset (10-speed) and 11T-40T cassette, or 44/32/22 (9-speed) matched with an 11T-36T. Hollow pinned platform pedals work great for this application as well.

So, my preference is for 26” wheels with 36 spokes. Disk brakes help too.

I also recommend the Schwalbe Marathon GT Tour 26x2.0, it’s an all weather tire with just about the highest durability and puncture protection you can find. Can be had for even less that the GRX GG from TradeInn/BikeInn. Great bargain even when including cost of shipping from the Netherlands (which I have to admit is really slow.)

The Ghost Shifter by andyXchrist, on Flickr

RANS XP Ibera Disk Rack MTB Reflector Rear Planet Bike Grateful Red Tail Light by andyXchrist, on Flickr

Note the Radical Designs Trailer Hitch on the rear hub of my Stratus XPC pictured above.
26x2.0 Schwalbe GT Tour on 24mm [?] Velocity Aeroheat rims

Bacchetta Giro 20 w/ panniers & trailer by andyXchrist, on Flickr

andychrist

Bacchetta Giro 20 New Shimano Deore 10-Speed Crankset + FD, KMC 10-Speed Black/Silver Chain by andyXchrist, on Flickr

Bacchetta Giro 20 New SunRace 11T-40T Black Cassette New Shimano Deore 9-Speed RD New Black Derailleur Extender by andyXchrist, on Flickr

RANS Stratus XP New KDLITKER BL70S XHP70.2 by andyXchrist, on Flickr

44-32-22 Crankset 11-36T Cassette Schwalbe Marathon GT Tour 26x2.0

CliffordK

I've towed mostly with narrow (25mm or less) 700c wheels.

One thing about brakes on flat land or ascending hill is that if you have a pedal power only bike (not electric assist), then you're limited to about 200W of go power, and the brakes will stop that.

Descents depend a lot on the environment. My hills are moderate, and mostly rural. So, there is less of a need to slam on the brakes. In those rare cases where I anticipate needing to stop on a descent, I plan ahead, and just don't take it fast.

Weight, of course, is an important factor.

I think my kid's trailers are rated at about 100 lbs, but the well built ones should go up to about 200 lbs. And they're fine for my road or cross bikes.

Beyond that and I move to my heavy hauler which for fun has a FAT tire, but really, I would have been better off with a standard size tire.

Reynolds 

I don't think there's a great difference between 26 or 700 wheels, except maybe in extreme situations. For loads under 50kgs and not terribly steep hills both will do OK.

CliffordK

I suppose I run 700c wheels on a frame meant for 26" wheels...

Reynolds 

Not sure what you meant there - 26" x 2 wheel diameter is similar to 700 x 23 and I have seen a few conversions.

Ballenxj

Having never towed with a bicycle, but understanding wheel and gearing a bit in cars and trucks, this is what I would think, with the smaller diameter of the 26 incher offering a lower gear ratio that would lend itself better for towing.
Nice photo's btw andychrist.

Reynolds 

I took it for granted that one chose different sprockets for 26" or 700 wheels to achieve the desired (in this case low) gearing.

Ballenxj

You most likely would customize your gearing for what you would need, but I was talking strictly wheel size with stock gears.