Trailer tyre pressure calculator Page 1 / 2

dublo, Dec 3, 5:04pm
Once upon a time I found, via Google, a calculator which told me how much pressure was needed in my light trailer's tyres for a given load weight on each wheel. Now (of course!) I can't find it. This came to mind the other day when I took car and trailer into the shop for WoFs. The mechanic drove slowly over a speed bump in the lane next to the workshop and the trailer's wheels briefly became airborne.
I occasionally carry a light but fragile load in the trailer (model railway layout) and wanting to give it as smooth a ride as possible reduce the 175/80R13 tyres to about 18 pounds pressure, from the normal 25 when taking a load of prunings to the rubbish dump. It still bounces about a little on rough roads. I checked the trailer's weight with household scales: empty 92 kg on each wheel and 30 kg on the towbar. giving a total trailer weight of 214 kg. The light load is about 62 kg, giving about 123 kg on each wheel, a lot less than the average of about 350 kg when the wheels were on a car . Now to find out what pressure I need in each tyre - anyone know where to find that elusive calculator?

tweake, Dec 3, 5:29pm
tires going airborne has a lot to do with the suspension.
is it single axle or double?
most seam to be set up to only really work when you have a decent load on. you really need progressive springs and a some decent shock absorbers.

i dislike calcs on tire pressure because tires can vary quite a lot.
for eg our trailer tires run about 80psi.
the problem with light pressures is that it can be to soft and it will heat up and destroy the tire. might be better to gauge by how the tire sags or by how hot it gets after being used.

tygertung, Dec 3, 6:23pm
You will probably want very low pressure, like 8-10 psi. Radial tyres are more tolerant of low pressure.

Try putting 8-10 psi on them and see if they look squishy. If you are not going far and not at high speed they won't get very hot most likely.

Motorcross riders and mountain bikers often run very low pressure in their tyres, they have special rim locks to stop the tyre rotating under hard acceleration and braking.

dublo, Dec 3, 7:34pm
Thank you, both. When I bought the trailer, new, in 1998 the factory in Te Puke asked what springs I preferred and fitted longer ones than standard, saying they would give light loads a better ride. ( They gave a cubic metre of topsoil a VERY smooth ride!) I'll try lowering the pressure a little until the bulge looks about the same as it does on the car's same size tyres.

(Back in 1972 I bought a new 13 foot runabout and its "custom" trailer with 12" rims. I forget what pressure we used but guess it would have been about 25 pounds. The springs had virtually no flexibility and gave a ride as hard as the hobs of hell. Dad suggested removing the second leaf from each spring, which was almost as long as the top one. That improved the ride tremendously, the boat wasn't shaken to death and all was well when I sold the combination over 20 years later.)

bill-robinson, Dec 3, 8:38pm
puy your delicate load on an air matress and put correct pressure in the tyres

pettal, Dec 3, 9:57pm
18 psi -- the thing would just about be running on the rim .

franc123, Dec 3, 11:06pm
Some tyres do not look visually flat until they are below 10-12 psi. Even on cars.

tygertung, Dec 4, 6:48am
You'd be surprised how low tyre pressures can get. Even on my racing bike with the 23mm tyres which usualy run at about 120 psi, you can get down to about 10-15 psi before you notice they are really flat.

kazbanz, Dec 4, 1:29pm
sort of relevant--LT50 tyre pressures 10 psi recommended pressure

lookoutas, Dec 7, 7:07pm
I would suggest a 5 second course on Braille.

3tomany, Dec 10, 9:21am
I go by how it sags and watch the tyres over bumps to make sure they do not distort to much. I run 42 psi if carrying 2ton but will drop them for light loads to around 25psi and find that is good for my tyre type. Just depends on the tyres and some cheap tyres will not be very heat tolerant.

3tomany, Dec 10, 9:23am
I have done 18psi but when you look at them distort in the mirror over bumps it gives me the poo pooe's.

mrfxit, Dec 10, 10:48am
Generally speaking, 30psi on any trailer tyre is the sweet spot.
Under that for light weight trailers & light loads
Over that for heavy trailers & or heavy loads

All my trailers run at 35psi on any tyre from empty deck to 1T load.

tweake, Dec 10, 6:26pm
i would not say ANY trailer tire.
plenty of tires out there that require 50+ psi for a light load.
its highly dependant on what tires your using.

mrfxit, Dec 11, 5:56pm
True but this appears to be about common domestic tyres, not commercial types.

tweake, Dec 12, 1:48pm
domestic, commercial, they all use the same tires. its all about weight limits rather than use.
i've never come across "domestic" trailer tyres.

martin11, Dec 12, 2:00pm
4 or 6 ply tyres verse 8 ply ones !

tweake, Dec 12, 2:28pm
i have not checked but i think your comparing car tires to trailer tires.

edit: there is no reason you cannot use "domestic" trailer tires in commercial use. obviously limit by whatever weight limit they have. i see some 4ply trailer tires listed, no mention that they are "domestic" use only.
they are tires, the weight limit is what counts. the tire doesn't know what type of object is on the deck.

mrfxit, Dec 13, 7:30am
Domestic tyres & common domestic usage & weights.
Eg: Std small/ medium size car tyres on 6x4 single axle trailers commonly carting 100kg upwards to rare 500kg loads of garden rubbish/ kids bikes etc, most days pulled by a small to medium size 4cyl car of low to modest performance.

NOT, commercial usage or single / twin axle heavy duty chassis trailers that commonly carry builders mix/ builders supplys/ small diggers etc & have tyre ratings to match the loads.

Mom & dad trailers = domestic usage with light loads
Business duties = commercial usage heavy loads.
Applies to trailer AND tyre design AND usage intentions

mrfxit, Dec 13, 7:36am
Is this a fairly modern trailer design or the old style design.

Older trailers tend to have long springs with short travel (work well when set right)
Newer trailers tend to have very curved & short springs that only really work right when loaded with reasonable weights (Ie: springs flatten out a bit)

tweake, Dec 13, 10:53am
Mom & dad trailers caring tons of soil/gravel/cement
Business duties caring tons of soil/gravel/cement

xs1100, Dec 13, 12:17pm
i run 50psi on my boat trailers 165-13 after talking to a boat builder and as he said tows a hell of a lot nicer and then on my other trailer with low profile 13s i was told to run 80 psi

mrfxit, Dec 13, 1:31pm
Std common 6x4 "Mum & Dad" trailers bottom the springs out getting close to a ton & very rarely get that much unless they have a garden project on the go, never mind the lack of ability's those matchbox cars they use to pull those weights

Builders/small commercial trailers tend to have heavy duty tyres fitted most times

mrfxit, Dec 13, 1:34pm
In reality, you need better tyres more suited to the weights you are carrying.
LOL, Low profile tyres were never intended for trailers, they simply don't have a sophisticated enough suspension to cope properly with them

franc123, Dec 13, 1:55pm
Thats unsurprising it doesnt tow well. I'd be checking what the max load pressure for that tyre is on the sidewall, before following that "advice".

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