Toyota Tech: Siping Tires
Http:// Short Cuts
Author: Joe Micciche. February, 2001

The uncut, unsiped General Grabber MT's.

Tire siping is the process of cutting small slits, or grooves, into the tread blocks of tires in order to provide better traction in adverse weather conditions. The sipes allow the tread blocks to spread on contact with the ground, which helps expel water from the tread and provides better bite. Practically every all-season, mud+snow, and all terrain tire marketed today is siped in the factory mold, yet few mud terrain tires are siped. The traction benefits of siping, along with the purported increase in tire life due to improved heat dissipation from siped lugs, is something that can easily (and quickly) be done at home.

The simplest way to sipe tires is to take them to a local tire shop and have them cut. However, many of the siping machines used at shops do not account for all tread designs, may cut the tread blocks close to the leading or trailing edges of the tread blocks (causing the lugs to chunk easily), or don't allow for anything other than cuts straight across the tread block.

The Ideal Heated Knife includes the gun, cutting head, and 12 blades.
The blade is secured on the head with a screw clamp.
The heated knife with the cutting head is mounted, ready to begin siping.

A utility knife can also be used to sipe tires at home. I, however, elected to use the Ideal Heated Knife, which has a 250 watt heating element which heats the tread lug ahead of the cut, thereby making the cutting almost effortless. And at a cost of around $60, the knife is nearly equivalent in cost to having a shop sipe your tires once, while allowing the owner to cut dozens of additional tires at no additional cost.

This knife, which I found at a local speed shop, features a head with clamps for the blade. The blade ends are doubled over the head so two sipes are made with a single pass. The depth of cut is adjustable, but a good rule of thumb is to cut half way into the tread block, and never get near the actual carcass of the tire: in my case, I adjusted the blade so the sipes were 1/4" deep into my 1/2" lugs. Cutting heads are available in various widths from 0.05" up to 0.375", and the blade can be flipped so the tool can be used to groove tires as well. The manufacturer advises to always use the proper blade designed specifically for the cutting head provided: in this case, I received a #5 head which spaces the blade ends 0.29" (#5 blade) apart when cutting.

I did some informal research on siping patterns by looking at truck, MT, M+S, and all-terrain tires in parking lots and on tire web sites. My research showed that every tire maker has a different philosophy on patterns, and patterns differ within a maker's tire range by tread design. So I just decided to accomodate the 3 predominant suggestions about how to make the actual cuts on the big lugs: cut straight across, cut at an angle to the direction of tire travel, and cut away from the direction of tire travel. On the smaller lugs, I simply cut straight across the block. As many suggested, I did not cut the outer lugs due to the risk of chunking them offroad.

Lay the cutting surface flat on the lug, and guide it across.

I did the siping on yet another cold Cleveland winter day - outside temperature was near 25 degrees with a pleasant breeze, and the tires were cold after sitting all night. After letting the knife warm up for 15 minutes, I started making the cuts.

The cuts are made by placing the head tip flat on the tread lug, and pushing the knife across the block at a moderate pace. The head will heat the rubber and make the cuts quickly, while producing only a slight amount of burnt rubber stench. Surprisingly even in the cold, the cutting required little effort: the heat and razor-sharp blades literally cut through the rubber with minimal effort. Because the blade head configuration allows two cuts to be made with each pass, the entire process took 15 minutes per tire with 3 passes (6 sipes) per big lug and one pass (2 sipes) per small lug.

Onroad results

The siped Grabber MT's. Upon initial cutting, the sipes are hardly visible.

After a hundred miles or so, the sipes really opened up.

After cutting the tires, I stared at them and wondered what I had done wrong (see picture at left). The cuts in the tread were hardly visible, and I thought the thin blade may not have been adequate for the job. However, within a hundred miles or so, the sipes really started to open up as the tires went through several cycles of heating and cooling, including a long jaunt on the freeway.

I tested the siped tires in the wet, snow, ice, and on dry pavement. After driving with them for several weeks, traction in our wintry conditions has improved dramatically. In my Toyota with a rear Detroit locker, launches from a stop on the slick stuff are now straight and absent of any drama. The truck is well-planted while taking corners and sweeping curves on crowned roads, whereas in the past it was prone to slide in snow and ice. And while I can't measure it, braking performance feels more confident and predictable. As the sipes opened up, noise from the tires did increase, but not enough to be objectionable.

Offroad results

Not much change in the mud, but on hardpack, ice, and snow, they gripped well when aired down.

My expectations for improvements in offroad traction were minimal. I have heard anecdotal accounts of siped tires providing better grip in rocks, but since rocks are minimal around here, I didn't have a chance to run them in those conditions. In the mud and loose trails, the tires performed the same as they had prior to cutting them. I also had the opportunity to test them on snow- and ice-covered trails, and the additional traction was a significant improvement for maintaining control and momentum.

While wheeling, I checked the tires at every opportunity for anything abnormal. I never found any rocks or debris in the sipes, which could lead to chunking. And, the tread blocks retained their integrity after a good day-long thrashing over small snow-covered rocks and after clawing and spinning through ruts and ice.


Warranty Concerns:
If you have any type of warranty on your tires, it is recommended that you check with the warranty issuer (manufacturer or tire retailer) prior to siping. Policies on tire warranty work may be affected or the warranty could be voided if the tires are altered.

The onroad results of siping my MT's are very satisfying. The additional traction and brake performance were more than worth the time, effort, and cost of the heated knife: that I can resipe or groove whenever necessary makes this even more gratifying. Time will tell whether this modification provides additional tire life (or reduces longevity), but even as the tires wear I now have the means to quickly and easily cut them to restore offroad and onroad performance. While the offroad benefits are likely limited by the terrain we wheel on, I will always have siped tires on trails where snow or ice may be encountered. Overall, this is one of the more gratifying - and quick, easy, and inexpensive - upgrades I've done.


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