Hello Everyone:

Recently purchased a used Dahon Impulse folder and it came with brand new Primos Comet 20 inch x 1.5 inch tires.

When I got it they were inflated to 100 psi and the ride is harsh.

What PSI would you recommend for this tire/bicycle combination.

Depends on the load. I, at 70kg with no load and on pavement, would inflate those 1.5" wide tires to around 50-55psi in the rear and 35-40psi in the front.

For tires that width on my regular (non-folding) bike, I use 50 psi front, 70 psi rear.

The pressure depends of both the total load (bicycle weight+cyclist weight with clothes+weight of anything carried on the bike) and the type of tires.

Primo Comet 37x406 are fast rolling, high pressure tires, they are specified for 100PSI.

Rims can be damaged by too high a pressure, do you know if those rims will be safe for the long term at that pressure? I do not.

No idea, I doubt that these relatively expensive Primo Comet aimed at high performance road oriented bikes were factory installed on a low end utility folding bike like the Dahon Impulse, its probably the previous owner who mounted them without taking into account that they require a relatively high pressure due to their lightweight structure.

With a too low pressure, rims can also be damaged when riding in a pothole or border.

The safer is probably to swap them for more rigid, lower pressure touring tires.

Like all things engineering, tire pressure is based in compromise.
What works with one person will not necessarily work for you.

So let's work through this methodically:
1. Limitations.
a. Upper: 100PSI
b. Lower: When the rim hits (for a 1.5, around 35-40psi)
2. Considerations
a. Rider, bicycle, cargo weight on average use.
b. comfort/speed compromise
c. weight loading, typically 40% front 60% rear, will change with cargo panniers or riding positon

There are charts and graphs to help you pick a pressure or you can try manually.

I'd suggest 60psi to start.
Too harsh? reduce by 5psi
Bottom out? increase by 5psi
Find a happy compromise.

TouristMSN's suggestion is probably pretty close, but figure it out for yourself since only you know you.

Or replace with 20x2 Big Apples (if they will fit your frame) and enjoy riding on soft air suspension at 30-40 PSI

I agree. That's why your own recommendation of 70psi gives me pause, particularly since you have failed to include rider weight, your own weight in the recommendation. 70psi suggests, at least to me, over-inflation and peril to the rim, or that you are carrying 85-90kg on your person. To be clear, no recommendation is useful and credible unless it is accompanied, at a minimum, by rider weight. You (pl.) know this.

I learned a lot from this thread. Thanks! That's why I (most of the time) lurk and (occasionally) post.

Unfortunately, crucially we still haven't learned people's weights, including that of the OP, or the weights for the sundry pressure recommendations proffered here. With one exception, of course.

For example, I use to run quite recently and for a couple of years a few years back, these 1.65" (nominal, narrower in reality) tires at 40/30 psi on my 71kg body frame. On 1.50", I'd add 10psi and ride 50/40 psi. 70psi seems excessive unless one is packing some serious body weight and needs to be caveated as such. Remember, the higher the pressure, the higher the stress on the rim and the spokes, and obviously, the bouncier, the harsher the ride.
​​​​​​

42-406 nominal


42-406 nominal
​​​​​

The OP said 1.5 inch width tires, which I assume means 37 to 40mm. I noted that I use 50 front and 70 rear for that width on my non-folding bike.

To add more detail, I weigh about 80 kg, or roughly 175 pounds. My rims on the bike with that tire width are either Salsa Gordo (discontinued), Velocity Dyad, or Mavic A719.

Of these three bikes that I use tires of those sizes, the only rim that I actually know the max rating is the A719, photo below is from my A719 rim. This rim is about 20 years old, I am pretty sure I built up the wheel with this rim in 2004.



I am just trusting that the other rims have similar capability. They do not state a pressure on the rim and I have not found a max pressure on Velocity website. The Salsa rim has been discontinued for quite a few years now, if they published a max pressure years ago, I am unaware of it. I think I built up the wheels that have Salsa Gordo rims in 2010. And I built up the Dyad wheels in 2017.

On my recent bike tour where I was carrying four panniers, etc., I ran 60 psi front, 80 psi rear on the 37mm tires, thus an extra 10 psi for the load, this bike had the Dyad rims. I am guessing that my touring load on the bike was an additional 55 to 70 pounds counting the weight of touring racks and empty panniers. That said, I only added air weekly, so the pressures were lower some of the time. This bike without racks is roughly 32 pounds.

Some people have proposed the 15 percent tire drop rule of thumb, I usually run pressure slightly higher than that in rear but that rule of thumb I think has front tire pressure way too low, so I never run a tire down at that pressure.
https://www.renehersecycles.com/wp-c...BQTireDrop.pdf

I suspect this is more detail than most people wanted, but since you asked.

For the same type of tires, a small wheel diameter tire must always have a higher pressure than a bigger wheel tire for the same total weight of the bike+cyclist+baggage.

Schwalbe not only give a maximum pressure for every tire type (model, diameter, width) but also a minimum pressure, you can see that smaller diameter tires have always a higher minimum and maximum pressure than the same tire with a bigger diameter.

On a paved road in good condition, a higher pressure reduce the rolling resistance, so the pressure is also a trade-off between comfort and rolling resistance.

Contact patch area times the pressure equals the weight on the tire. Max pressure is twice the blow off pressure. As a practical matter, minimum pressure is the lowest pressure without pinch flats.