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I've got very old (10+ years) Author Mystic bike. It has 26" x 1,95" tires with suggested pressure set to 2,9-4,5 bar. Currently I have inflated them to 3,4 bar. I'm around 120 kgs and riding on such tires brings no problems for me.

I'd like to start riding with my older daughter in bicycle child seat. She is less than 20 kgs (near border weight for that child seat).

Answers to this Duru Can Celasun's question assured me, that in general my bike should handle our together weight of 140-150 kg without serious problems. Though, we may already talk about overloaded bike with such weight of user.

Here, I'd like to ask, what would be perfect tire pressure for both my lonely rides (120-125 kg) and for together trips (140-150 kg)?

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    I would check the wheels if they're in a good condition and if they're strong enough, as that's a critical point. And if you're not sure about your tires and you want to be on the safe side, then you could buy some touring/ebike tires, the biggest that would fit your frame. Schwalbe has recommended max load per tire listed on their EU site. As I see, most of their tires above 1.6" can support over 100kg per wheel which is comforting. Maybe you could also check your tire's age, I wouldn't trust too old tires, like 5 years or older, to support this kind of load. – PPatrik Aug 3 '17 at 23:48
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Generally, the tire pressure range marked on the tire is somewhat loose - the marketing department wants a big range since it will sell better, the legal department wants a small range so liability is lower and the engineers have something probably bigger than whats marked on the tire as safe. Your tire pressure should be high enough that you don't get pinch flats but not give a harsh ride. The links in the recommendations show roughly what tire deflection should look like. Note that lower pressures are generally more comfortable.

150 kg on a bike is a lot - most companies will mark this as higher than the max weight for their non-touring/cargo bike models (so you're on your own legally). That being said, I have carried a lot more on bikes marked significantly less, you're on your own with that.

As for tire pressures which are best, you have to find this out on your own - its dependent on terrain you're riding over, your daughter's stomach, etc (and the best tire pressure (which is largely subjective, hence why I'm voting to close the question) will depend on the load on the bike so if you wanted one pressure, you'd have to trade off between the no daughter and daughter rides).

There are some recommendation charts and manufacturer recommendations to start with - tweak your tire pressures to your liking.

  • With moderately large tyres you should be able to go up to the sidewall pressure at the back without the ride being too rough for the passenger. A seatpost-mounted child seat is supposed to have some flex compared to a rack-mounted seat; the question doesn't say which it is. At that much weight, unweighting the saddle to use your legs for suspension of your weight over bumps (and avoiding big bumps such as kerbs) would be a good idea. Realistically you're on your own legally unless it falls to bits on the way out of the shop anyway. – Chris H Sep 10 '14 at 19:52

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