20x4.0 Fat Tires

The Blackrock X1 runs Kenda 20 x 4.0 tires. That is four inches of tire width on the ground, roughly double a normal bicycle tire, and it is the main reason the bike stays planted on gravel, sand and packed snow where a narrow tire washes out.

Why the width grips

A wide tire spreads the weight of the bike and rider over a larger contact patch. On hard pavement that does not change much. On anything loose it changes a lot, because a narrow tire cuts down into the surface and starts steering itself, while a wide one floats across the top of it and goes where you point it.

The tread does the second half of the job. These are deep blocks with gaps between them, so loose material packs into the gaps and gets thrown clear instead of filling the tread smooth. That is what keeps the bite on gravel roads, on a sandy beach approach, and on the packed snow of a farm lane in February.

There is a comfort effect as well. A four inch tire at moderate pressure carries a lot of air, and that air soaks up small ruts and washboard before it reaches you. On the X1 this works alongside the suspension fork, which has a mechanical lockout so you can firm it up on smooth pavement.

Pressure basics

Pressure is the free adjustment on a fat tire bike, and most riders never touch it. The usable range is printed on the tire sidewall. Stay inside it, and move within it depending on where you are going.

• Toward the higher end for pavement. The tire rolls easier and the bike feels quicker.
• Toward the lower end for sand, snow and soft ground. The tire spreads out, floats better and grips more.
• Check it with a gauge, not a thumb. A fat tire can be well down on pressure and still look fully inflated.

Cold weather drops pressure on its own, so check more often once the temperature falls. There is more on that in the winter riding guide.

The honest trade-off

Fat tires are heavier, and they take more effort to push along smooth pavement than narrow ones. If every trip you make is on flat asphalt, a narrower tire is the faster tool. What you buy with the extra weight is grip and stability on the surfaces where a narrow tire simply is not usable, plus a softer ride on rough ground.

For the full comparison, including who should not buy a fat tire bike, read fat tire versus regular e-bike. The tire spec and the rest of the components are on the specifications table, and the step-thru frame is what lets you get a foot down on the loose ground these tires take you to.