Dingmark Jim
2nd February 2015, 06:13 PM
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/ob68bsqzhy2az3b/20150201%20D4%20Tyre%20Deflation%20%26%20Loadings. xlsx'dl=0
[[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/ob68bsqzhy2az3b/20150201%20D4%20Tyre%20Deflation%20%26%20Loadings. xlsx'dl=0