PSI for 2" Kojak Slicks - London to Brighton

ydrol

Hi I am 238lbs in the buff, and have a Carrera Subway 2 with 2" kojak slicks - which I ride clothed and tooled up. So around 280lb load.

The tyre charts I've seen dont seem to cater for road pressures for 2" slicks. I'm doing London to Brighton this Sat and want best combo of comfort & rolling resistance.

I'm thinking about 75psi rear 55psi front ? Or could I go lower, and keep efficiency ?

Barteos

Using a 15% tyre drop method, 50-60PSI max. rear and 25-35PSI front should be a good starting point assuming 65%/35% weight distribution.
75PSI is unlikely to offer any benefits.

t4tomo

25-35 psi front sounds more like gripping in extreme mud / snow on a mTB pressure.

i'd say pretty close to maximum recommended - least contact area = least resistance (ish)

Barteos

t4tomo wrote:

25-35 psi front sounds more like gripping in extreme mud / snow on a mTB pressure.

i'd say pretty close to maximum recommended - least contact area = least resistance (ish)

Optimal pressure depends on the load on the wheel (and tyre volume) and on a hybrid bike front wheel will take only 25-35% of the total weight (280lb).
In this case 25-30PSI is enough for the tyre of that volume and will result in a 15% drop (just like suspension sag).
More than that offers NO BENEFITS whatsoever.

For tyres to roll fast they need to sufficiently absorb the imperfections of the road surface. Tyre deflection (hysteresis) and "contact patch" on their own are just half of the picture...

IanLD

Don't know where you get those figures from, but can only go on my own experience of how a slow puncture due to a leaking valve affected me.

Over the course of a 25 mile ride, the bike started to feel stranger and developed a very aggressive turn in that didn't give me confidence at all.

It was only when I got home that I discovered that I had gone from 70psi down to under 50. Far less of a drop than you are suggesting and it did have a very big effect on how the bike handled.

Kojaks are a large volume tyre and run better when inflated closer to their upper range. I would also run both at the same pressure.

markp80

IanLD wrote:

Don't know where you get those figures from, but can only go on my own experience of how a slow puncture due to a leaking valve affected me.

Over the course of a 25 mile ride, the bike started to feel stranger and developed a very aggressive turn in that didn't give me confidence at all.

It was only when I got home that I discovered that I had gone from 70psi down to under 50. Far less of a drop than you are suggesting and it did have a very big effect on how the bike handled.

Kojaks are a large volume tyre and run better when inflated closer to their upper range. I would also run both at the same pressure.

Barteos isn't referring to a drop in tyre pressure over a journey. He's talking about the amount a tyre compresses when you sit on the bike. There have been tests done which show that the optimum amount of compression is 15%, ie the height of the tyre wall falls by 15% when you sit on the bike. The tests show that increasing pressure to give less than 15% drop do not give any advantage in rolling resistence, but make for a much harsher ride.

For wider tyres, a much lower pressure is required to achieve this drop than for a narrower. My chart only goes up to 37mm width, but for that width, the OP would want 42 front / 80 rear. The pressures suggested by Barteos look about right for a 50mm tyre doing a bit of interpolating.

For a 25mm tyre you'd be looking at more like 150psi for the rear with that load!

Cheers,
MarkP

IanLD

I was making the reference to tyre pressure as it had a big negative effect on my bikes ride and handling. A drop test does not reflect how the tyre deflects during cornering etc and this is what made my bike more and more unpleasant to ride as the slow puncture allowed pressure to drop.

Running a front tyre at the pressures suggested would make it very imprecise and would allow a real risk of it rolling off the rim whilst cornering. It would also lead to an increase in the possibility of pinch punctures.

Different tyres respond in different ways to the pressure in them, but I've found that large volume tyres such as the Kojak, work better closer to their stated maximum. I run mine at 90psi.

On the other hand, when out on my road bike yesterday, I was running my Vittoria Rubino Pros at 110psi which is only 10psi above Vittorias minimum and well below the 145psi maximum.

Trial and error within reason is the best method and I hope the OP found a suitable pressure.

markp80

Sorry IanLD, I misunderstood your post.
I've never ridden with a tyre like the 2" Kojak, so can't speak from direct experience, but I agree with your statement that trialling with different pressures is the best way to go.

Cheers,
MarkP