Benefits of going from 19" to 18" Tyres on Sand

[Dingmark Jim]

There's been a few threads discussing how much benefit is really gained by going from 19" to 18" tyres, which gives only an extra 1/2 inch (12mm) more sidewall. I attempted to do some experiments and calculations to show this and the answer to the question appears to be ?quite a lot?.

[[OTT: While I'm here working and living in Northern Iraq, my transport is one of those monstrous Nissan Patrols. In this situation, I'm in the back seat and there's plenty of room for 2 of us + the requisite flak jackets (and, frankly, room to dive and hide). The "hired help" in the front seats find that there is room for 2 off AK-47s. Both the driver and offsider are able to quickly reach these tools of their trade at the same time without hitting shoulders or elbows. See, there's a reason Nissan made something so big and ungainly!]]

Okay, back to the topic. My wonderful Kurdish neighbor caught a security spike on a sidewall of her D4 (19" wheels). I took the opportunity to assist in changing the tyre. Her husband is of the "sell the vehicle if it needs anything" school. However, like us she really likes the D4 and wants to keep it - like us. This is despite Da'ish/ISIS/ISIL knowing that D4 drivers are discerning and therefore are targets. One thing led to another and the husband and I did some in-street experiments of tyre footprint at various levels of deflation. What we did was push the deflated/ruined tyre into a shallow mud puddle in the street. One person measured the sidewall deflection while the other pushed down on the tyre. We then lifted the tyre and measured the length of the tyre print in the mud. This drew quite a crowd and we ended with a full platoon of Peshmerga assisting in measurements and pushing (I live next door is an embassy so there's always "excellent" security in place). I decided to call it a day when I counted 4 AK-47's inadvertently pointing at me while their owners were pushing the tyre, measuring footprints, or re-smoothing the mud. (No, there's no photos of this as security people in Iraq take a dim view of people photographing them and I prefer to continue living.)

Having trained as an engineer, I couldn't help but try and do some calculations on sidewall height vs footprint. Lo and behold, theoretical trigonometric calculations reasonably matched what we measured. Note our mud puddle was only about 1mm thick, so we didn't measure any benefit for sidewall bulging, etc. The calculations are (hopefully) attached and accessible below.

The results show that "all else being equal" the differences between a 255/55R-19 and 255/60R-18 tyre are significant. If, for example, you have deflated to the point where only 2 1/2" inches of sidewall remains, then the differences are:

* The 19" tyre's footprint is 18 inches long, but on the 18" tyre it's 19.26 inches long (on a flat, unyielding surface, with a 3400 kg K4 that's perfectly balanced, etc);
* The pressure on the ground by this footprint goes from 10 psi to 9.3 psi. This is a 7.5% increase in flotation, which is key in sand.

In other words, the benefit of going from stock 19" tyres to stock 18" is about the same as reducing the weight by 215 kg. Anyone who's been on sand knows that having 3-4 passengers out of the vehicle makes a big difference. Another good way to understand this is to put around 5-7 kg of lead weights on your waist and see how much more effort it takes to maintain flotation. People trying to float is water is pretty much like my D4 trying to maintain floatation on soft WA beaches and dunes.

The benefit of having a bigger tyre on 18" wheels is calculated to be huge. I chose a 265/65R-18 as my example. In this case, deflating this tyre so that only 2 1/2 inches of sidewall remains increases the flotation by 19%. All of these flotation calculations disregard the fact that the tyre is sinking down into sand (introduces other effects like varying sand support strength, non-vertical forces, sidewall support, etc), but it would be very difficult to calculate the benefit of these, so I've ignored it.

Many of us have seen how airing the tyres down another 1/2 or 3/4 inch makes all the difference in getting un-stuck. In mathematical terms that provides only an additional 4%-5% more flotation, so that puts the benefit of bigger tyres into context.

The overall lesson is that that small extra 1/2 inch of sidewall gained by going from 19" to 18" tyres can make about the same difference as reducing the payload nearly in half. The benefit of going from stock 19" tyres to a very large 18" is more flotation than that gained magically reducing the weight of a D4 by 900kg.

So, you ask, what do I do? Alas, I am the accidental owner of fifteen 19" wheels (don't ask, my wife no longer asks) and am out of Australia for 90% of the time. Seven of these have Goodyear MTRs on them which have lots of tread but are getting on in years. In other words, I'm happy to deflate down to the rim if need be and if a wheel breaks or tyre is shredded, it doesn't matter much. But once I am able to retire back in Perth, I'll be ordering a set of Gordon's 18's (already have his extension arms), moving a/c vents and will be melting plastic in the wheel wells to get a much bigger tyre in place. This assumes I've had enough fun to break a few 19" rims in the meantime and survive Iraq for a couple more years.

Link to the spreadsheet calculations is: https://www.dropbox.com/s/ob68bsqzhy....xlsx'dl=0

[Boggs]

Sounds like where I work in the highlands of PNG.. difference being the police paramilitary mobile squads don't know what a safety catch is...

[ScottD4]

Our regular off the track haunt is Redhead beach, or some might know it as Nine Mile Beach in Newcastle/Lake Macquarie. Years ago, the sand on the beach was mined (which means had all the metal taken out of it, and the remaining sand dumped back on the beach). Nevertheless, this reportedly makes the Redhead stretch very soft from all reports.

We have a MY14 TDV6 with regular 19" rims and standard tyres. We let these down to 18 psi typically.

To be honest we have not got in trouble once. I normally have my wife, myself and the two kids (both under 5). We also normally have a fully loaded fridge for the day, as well as normal beach crap.

We have had to help retrieve a number of vehicles but have not noticed the Disco getting in trouble. I find it hard to warrant a $3000 exercise on new tyres when the original rims and tyres seem to be coping; at least for us anyway.

[Boggs]

I would agree that if it were just for sand work, I wouldn't bother changing down to 18", however 18" gets you into true LT (light truck) tyres and far more aggressive, hard wearing treads for really nasty stuff like gibber and limestone...

[Dingmark Jim]

Ah, driving over areas that are mined - we find ourselves doing that too, but ours are more of the anti-tank variety Where our gas facilities are there are about 5,000 people involved in demining. However, our perception of the problem changed as the mines have proven very beneficial in keeping ISIS/ISIL/Da'ish out of the area.

I agree for most beaches, particularly if there's any moisture in the sand the 19" tyres are okay. They may have to be deflated to the point where the rims are at risk, which is a risk I'm willing to take. WA beaches can have a combination of soft sand and fist-sizes chunks of limestone, which don't do anything good to sidewalls or rims. I found that 19" rims can be found inexpensively as there are urbanites wanting larger sizes. In winter in WA, the beaches become exceedingly soft in some areas and it is needs 18" wheels and probably oversize tyres to drive these.