Ramblings on tyre width and pressure

I originally wrote this for NZ Road Cyclist‘s Workshop section (which I write every issue) but it contains a lot of relevant info so with their permission I’m going to post it here as well.  With every 23mm-wide wheelset we sell we supply tyre pressure recommendations – this article serves as an overview of this.

Over the decades road tyres have slowly been getting wider – for years 19mm tyres were standard, then 23mm became normal. In the past few months there has been a wave of interest in 25mm tyres.

25mm tyres have been around for years lurking in the shadows and generally seen as the type of wide, cushy tyre that Luddites and grumpy old men use. In the past few months, the perception of 25mm tyres has really changed in the ranks of both professional cyclists and weekend-warriors. Take a look at the bikes being ridden at the Tour Down Under or Paris Nice and you will notice that most of the professional peloton has adopted wider tyres.

Why wider?

Back when bikes were steel and breasts were real tyres measured 19mm wide. The steel bikes of this era were flexy and comfortable.

As bikes have developed they’ve become stiffer…much, much stiffer. Modern bikes have wider bottom brackets, huge tubes and fork steerer tubes which have grown from 1” to 1.5” in diameter. All of these changes have resulted in bikes which are much stiffer in both the lateral and vertical dimensions. An increase in vertical stiffness means a decrease in comfort.

To add some comfort back into modern bikes the rims and tyres are getting wider. Since the dawn of road cycling rims have been 19mm wide, but now most modern rims have grown to 23mm.

With an increase in rim width the tyres can also get wider. The wider the rim and tyre the more air will fit inside, and therefore the lower the air pressure needs to be. It’s this decrease in air pressure which results in a smoother, more comfortable bike with more grip.

I’m going to repeat that last bit because it’s super important: Wider tyres and wider rims won’t make your bike more comfortable. Lower tyre pressure will. Wider tyres and wider rims will allow you to run lower tyre pressure.

Why not just lower tyre pressure? If you lower tyre pressure too much then two things will occur:

1) The tyre will squirm around and feel under-inflated.

2) You’ll suffer from pinch-flats where a sharp-edged impact will pinch the inner tube between the road and rim and result in a flat tyre.

 Comparison of height and width

tire_height_width_thumb

23mm tyre 25mm tyre Change:
19mm rim Width: 23.2mm

Height: 20.7mm

Width: 25.4mm

Height: 22.8mm

Width: +2.2mm

Height: +2.1mm

23mm rim Width: 25.5mm

Height: 20.3mm

Width: 27.4mm

Height: 22.9mm

Width: +1.9mm

Height: +2.6mm

Change: Width: +2.3mm

Height: -0.4mm

Width: +2.0mm

Height: +0.1mm

 Why not wider?

Clearance:

The biggest inhibitor of larger tyres is your frame and forks’ ability to fit them. Many modern road bikes are designed with short chain stays and fat tubes and that means a reduction in clearance. Many ‘aero’ road bikes are especially tight.

Keep in mind that the wheel and tyre will deform under normal riding conditions, and mud, dirt, and rocks need extra space around the tyre so if 25mm tyres are a real squeeze while stationary they might not work while riding.

tire_clearance_thumb

(note the clearance around the tyre under the fork crown and brake caliper)

 Drag:

Two things will cause a tyre to feel slow: rolling resistance and aerodynamic drag.

Contrary to popular belief rolling resistance actually decreases as you lower tyre pressure. The softer tyre can deform more easily to imperfections in the road (we’ve got lots of these in NZ!) and forward momentum is less affected. Below about 30kph rolling resistance makes up for most of the tyres drag.

Over around 30kph aerodynamic drag accounts for the majority of tyre drag. The larger the tyre, and the more aerodynamically awkward the transition from tyre to rim the more drag will be caused.

It’s worth noting that many modern 23mm-width rims are aerodynamically designed around 25mm tyres and will have less aerodynamic drag than the same 25mm tyre on a ‘standard’ 19mm rim.

 How low can you go?

Basically, you can continue to lower your tyre pressure until either the tyres feel underinflated or you suffer from pinch-flats. You might be surprised how low you can go, and how much better your bike will feel as a result.

As mentioned above a wider rim and a wider tyre will each allow lower tyre pressures without any negative effects. We’ll treat a traditional 19mm rim and 23mm tyre as the baseline and then look at what a wider rim or wider tyre will allow.

tyre pressures baseline

23mm tyre 25mm tyre
19mm rim Baseline – see graph Drop 10psi
23mm rim Drop 10psi Drop 20psi

Here at Wheelworks we build and sell a lot of wheels and we get a lot of questions about whether a customer should be running 23 or 25mm tyres. My suggestion is to try them for yourself – tyres aren’t especially expensive but will change how the bike rides and feels. Just remember to try these tyre pressure suggestions to get the best out of whatever width you choose.