Charlie Aarons, please read

[landcruiserbob]

Charlie, when you speak about an accident you should have your facts right.
-I was driving my u500 camper 21,000lbs @ 61mph on 15.5 20 michelins XL tires when I had a blowout. There are several reasons for the blowout & one of those reasons should concern you if you have CTIS.
-The 15.5's XL's were new & never mounted but spent a few years sitting around at 10,000ft.
-The CTIS system showed both rear tires having a pressure of 65 psi but it was incorrect. The inner bead lock had covered the CTIS inlet & was giving me a false tire reading. The tire that blew out probably had about 30 psi when it failed. It was 20 degrees when it failed & maybe 30 minutes from our previous nights camping spot.
-Tires were rated for 55mph & we were going 61mph. Since we were at half the rated load & it was cold this shouldn't be a problem unless the tire was under inflated.
-You are incorrect stating that the govenor had been removed; I never drive over 65 except for the time I tested the 16r 20 & hit 86 for a couple of miles. I used the 16r 20's for an extra ovedrive not to go faster. Get the facts correct before you post. I got 35k miles out of the XZl's without a problem.
-I have had four blow outs in Unimogs. Three have been at speeds under 20mph. Three have been Michelin's & one a MPT. I was four wheeling when I lost the MPT on a u1200.Two other blowouts were at 30mph & I had an implement on the u1200. All of the Michelin blowouts were on XL tires.
-I do not think Michelin has a problem with the XL tire but they do have a hop that is due to the tread pattern that could generate uneven heat.
-All of my unimogs are still running Michelins & I would never use anything else. Robert g

[Christian P.]

I must have missed something...

:)

[landcruiserbob]

It was an old thread from maybe January & a buddy of mine at Mercedes forwarded it to me tonight. A member of this forum was speaking without any facts on an accident caused by a blow out. I was just clearing up the facts.Robert

[Joaquin Suave]

Robert, Are you sure that Charlie was talking about you?

I've heard of several BAD Unimog / tire experiences! From sources that are truely "in the know" & could be "jobless" if some if what he shared with me got out.

Hopefully your Mog didn't damaged too badly.

Didn't you notice one tire was lower than the others before you started driving? The XL's (395/85's) on Libelula change profile drasticly between the different pre-programmed steps of the CTIS.

[landcruiserbob]

Hi Joaquin, I'm sure Charlie was speaking of me since we conversed during his buildup & I told him about testing the 1600-20's at 86 mph out on the saltflats of Utah. I built the first u500 in North America back in 2003 & my truck was oringinally outfitted with the 365-85 xzl's. They were recalled after a Unimog North America driver was involved in a blow out. I choose the 16r 20's to get better mileage. When my speedo showed 45mph I was running down the road between 58 & 60 @ about 1300rpm's. Perfect, & milage was great.

These are huge trucks & I know more people who have had blowouts on their mogs than people who haven't. It's inherent with large trucks. I work my mogs pulling loaders, plowing, blowing snow or whatever comes up. Large truck loose their tires all of the time.We are running super singles & the blowouts can end ugly. I remind my employees everytime they jump into one that it can get ugly.

To answer your question I did notice the tire looking a little low but I was sitting in deep sand. We four wheeled the day before & I could have damaged the tire; I will never know. The tire completely delaminated & took out the exhaust & rear bumper(1/2 steel plate). I had control for about 50yds & then I felt like I lost the other rear since there isn't a back flow preventer on the CTIS. I found the complete carcass 200yds away & the inner tire was still on the truck; maybe I traveled another 50yds before the final blow out? My son was driving behind us & thought both tires blew. The deep sand & the road being elevated 6ft put the truck on it's side. The camper saved us. It took the brunt of the accident & looked the best afterward. The cab (carbon fiber) was ripped up pretty good.

The truck needs new paint & a new cab. Insurance was great & accepted the appraised value. The truck was about to be sold & we were taking our last trip in it. I'm glad it happened to me & not the new buyer. I'm done with the U500 & I'm about to buy a 1300L crew Cab(Doka). I was never happy with the u500's size & off road abilities(really rough if not fully laden & no torque tube). Our SBU's out work the 500's & are much more capable.

*****One more quick point; THE Accuride RIMS Are Rated For only 90psi of air pressure & I know many Unimog u500 owners that run their air up to 100 plus psi.*****

In no way is Mercedes Benz or Michelin responsible for the incident above, they didn't twist my arm to go out & buy it & then convert it to a camper.Have a good day.robert g

[Chas Stricker]

Howdy Robert,
From your experience do you believe that most blow-outs are caused by heat and flex? I check my temps(by touch) after each fill up. Even driving at 60-65mph here in the desert I've never noticed them being hot to the touch. I run MPT81s. Any other advice you might give seeing your time behind the wheel? I have some trips this summer and always think about tires. Thanks, Chas

[oonimog]

Chas,
I've had two blow outs on my U1300L37, one in front and the other at the back. They were separate incidents but both were well used Michelin XS's and both occurred at ~60mph on the highway. The rear was no big deal, just bang, and I gradually slowed to an easy stop. The front was a lot more dramatic, bang and a sudden right turn, Clyde. It was all I could do to keep the truck from shooting off into the ditch. The rim slammed down onto the road so hard that it was bent and what remained of the tire bent up the fender. I think I was lucky in that I was going down a staight road, had I been in a left curve, it could have been disasterous.

I believe that heat, as a result of low tire pressure, likely contributed to the front tire failure. I have found that occasionally, mud or pebbles get lodged between the rim and tire (from off road driving at lower tire pressures), causing a slow leak. It's important to check the tire pressures often to ensure they haven't changed, especially after four wheeling.

I'm certain that low pressure was not the case for the rear as the inflation was correct and the truck was lightly loaded. Also, I do not believe speed was the killer, rather age. Tires have a shelf life, whether worn out or not. This is why the military sells off all those "new" surplus tires and why they sell tire covers for parked RVs. In my case, the tires were the ones that came on the truck from new and althought they looked to be ok, close inspection showed that there were small cracks all over them, a result of age (ozone, the sun, etc.)

Cheers,
Pete

[landcruiserbob]

Chas, Pete is correct regarding heat & pressure. Low tire pressure = heat. I have never seen a tire blowout if it was inflated correctly. All of my blow outs were operator error. Most have been surplus tires. If you buy surplus tires take a good look after about 1000 miles to see if you have any cracking especially after you have aired up & down. No cracks = a good carcas.

Pete, what rims are you running?rg

[kerry]

Everyone probably knows this, but an infrared thermometer is a good way to keep track of the heat in your tires. I keep one in my Bluebird Wanderlodge and check the temperature of the tires when I stop.
There's a good (Michelin?)video on YouTube about how to react to a blowout in an RV. It's counterintuitive but you need to floor the accelerator

[oonimog]

Bob,
I've got Sudrad standard disc brake rims (non-Super).

Pete

[Chas Stricker]

Thanks Guys,
I'm running the Sudrad Supers. I do watch the pressures, especially when travelling at speed. I hope to buy another set of MPT81s since they've been great tires, after I had them siped. Thanks again,
Chas

[Robthebrit]

Everyone probably knows this, but an infrared thermometer is a good way to keep track of the heat in your tires. I keep one in my Bluebird Wanderlodge and check the temperature of the tires when I stop.
There's a good (Michelin?)video on YouTube about how to react to a blowout in an RV. It's counterintuitive but you need to floor the accelerator

Its a mog and unless the tire takes a minute to burst then flooring it is not going to help!!

[Robthebrit]

I had an old XZL burst on the street, it was pretty worn and the outcome was fairly none dramatic but I was only doing 30 and the truck wasn't loaded. The most weird part is when the wheel drops a foot. Now the camper has worn XZLs which need replacing promto as it is always heavy and will do over 60mph.

All my other mogs run MPT81's I really like those tires, the camper will have a set soon. I have never had a problem with them other than them not being great in snow. I have 25,000 miles on the set on the 416, they have been over some knarly stuff and drive on hot desert roads, I'd say they are still good for a few more years.

Rob

[762X39]

-The 15.5's XL's were new & never mounted but spent a few years sitting around at 10,000ft.
-Tires were rated for 55mph & we were going 61mph. Since we were at half the rated load & it was cold this shouldn't be a problem unless the tire was under inflated.
I never drive over 65 except for the time I tested the 16r 20 & hit 86 for a couple of miles. I used the 16r 20's for an extra ovedrive not to go faster.
No disrespect to anyone but...for starters, tires are not new if they have been sitting around for a couple of years. In fact they may as well have been mounted and in use because they have aged and hardened a bit by then.
My 2nd point is why does anyone ever drive a mog at speeds over 90 kh (55mph)?
If tires are rated for 55mph then that is their speed rating, not higher regardless of load and air pressure.

[Chas Stricker]

Howdy 762X39,
I also do pay attention to speed ratings on tires, especially for this truck. The MPT-81s are rated for a top speed of 120kph and load rated at percentage of 110kph. I stay below that for cruising. It seems to use a gob(technical term) more fuel above 100kph also.
Chas

[Robthebrit]

Me too, I stay around 55 (2400rpm) not just because of fuel consumption but also noise level and oil consumption. You can talk in the camper at 55mph, if I get up to above this it starts to drink oil, at 3100 rpm its really loud and I'm doing ~70mph, i would not want a front blow out at that speed when fully loaded.

Rob

[landcruiserbob]

762 x 39, I agree to your above statment . I realize I was traveling 5mph over the recommended speed of the tires & they were in used condition. I also know that most tractor trailers running down the road have tires that are rated for 55mph to 65mph loaded. Most e rated tires for 1 ton trucks have max speed of 65mph. We have lots of time bombs rolling around out there. I just looked at the leased Kenworth out front with the bull dog trailer & the tires are rated for 62mph on the tractor & 55mph for the trailer. I live in a state where the speed limit is 75mph for all vehicles. I will always travel down the interstate between 50mph & 62mph. I will go up 70 if I get trapped in the tractor cruise control war that we all hate.rg

[Jeep]

Hi Robert, sorry about you mishap. You mentioned that you were taking your Mog for one last trip and was about to be sold, well a friend of mine Tim D from Calgary was supposed to be buying a Mog but it was in a roll over on it's last ride. Is this the same rig? It sure looked good in the pictures!

Regards,
Mark

[landcruiserbob]

That would be me Mark. I'm glad the blowout happened to me & not your friend. We all came out okay & the truck handled the accident well. Just needs a new cab & of course some 70mph 395 xzl's.thanks/robertg

[Jeep]

It's a small world isn't it. You might want to give Tim a call if your unit is going to be repaired as he is still actively searching for a Mog, even if the camper is not on it I'm sure he would still be interested.

Regards,
Mark

[landcruiserbob]

Mark, have your friend email me at landcruiserbob@hotmail.com I can send him photo's before & after the accident. The camper is in great shape & is attached to the truck. The cab sustained substaintial damage to the passenger side. As a result the cab has several structural tears in the A pillar. Yanni (unimog N.A.) wants me to put a new cab on it. A bare cab is 13k. The passenger side is the only part of the truck that made contact with tera firma. I wouldn't say it was a slow ride down but we didn't move more than 2ft once contact was made in the deep sand. The drive line checked out & so did the engine. There is a small crack in the manifold which is a common issue & probably didn't occur in the accident. I'm heading out friday to possibly throw a plexi glass wind shield on it & drive it 400 miles home. Low Low boy trailers get expensive. cheers/robertg

[wolf359]

Robert,

I am pleased to hear your mog mostly survived. I would be very interested in pictures and more details. Email wolf359@shaw.ca. Thanks in advance.

Tim

[charlieaarons]

I've been away from a computer for a while.
Robert, I'm very sorry if my posting caused you distress. I'm also very sorry about any injuries, and damage to your vehicle.
However, a couple of points:
1) The speed rating of 15.5/80R20 XLs is 50 mph, not 55. The thermal stress on a tire goes up as the square of the speed. At 61 mph the thermal stress is ~48.8%> than at 50 mph.
2) My stock U500 tops out on a level road at 70mph indicated (speedometer), but only 68 on the GPS. 16.00R20 XZLs are exactly 12.5% larger in rolling circumference (397 rev/mile vs. 447) compared to 395/85R20 XZLs. One can calculate that with 16.00s a U500 could go 76.5 mph; not 86. How does your U500 go 9.5 mph faster? You must have also been reading off a GPS, the speedometer only goes up to 75.
3) When was the last time you weighed the vehicle? Expedition trucks are like people, they gain weight as they age. My rear axle load gained 1100 kg over 2 years.
4) It's not at all clear from your comments how you knew that the tire only had 30 psi in it when it read 65 psi, since it and its' axle-mate were probably quite flat after the accident

Thank you for your warning about the beadlocks. I am aware of that possibility. I tried mounting one at a truck tire shop in Tucson and when they finally got it installed they couldn't fill it presumably due to inlet hole obstruction. When I got home I drilled 1" holes over the inlet fitting. But my local truck tire shop couldn't even get them installed. With that in mind (the impossibility of working on a tire in the field with beadlocks) I decided to go "bare" and not go below 22 psi in front and ~35 in the rear.
Lastly, I have derived a formula for calculating tire temperature from pressure change (assuming the indicated pressure is correct!):
Thot = Tcold X (Phot + 1 bar)/(Pcold + 1 bar). Temperatures are on absolute scale. 1 bar is added because a "flat" tire has 1 bar in it. This is of course only valid near sea level. That this means is that on a 80 deg morning in the desert, a 95 psi cold tire reaches 114 psi at 180 F.

Again, my apologies for any distress I caused you.

Charlie

[lowenbrau]

Robert,

I am pleased to hear your mog mostly survived. I would be very interested in pictures and more details. Email wolf359@shaw.ca. Thanks in advance.

Tim

Welcome, Tim

I suspected the quading would get old eventually. :roost:

[Joaquin Suave]

Good reply Charlie!

I'll be using your formula someday...When I get Libelula's transmission issues resolved.

[Bajaroad]

I have derived a formula for calculating tire temperature from pressure change (assuming the indicated pressure is correct!):
Thot = Tcold X (Phot + 1 bar)/(Pcold + 1 bar). Temperatures are on absolute scale. 1 bar is added because a "flat" tire has 1 bar in it. This is of course only valid near sea level. That this means is that on a 80 deg morning in the desert, a 95 psi cold tire reaches 114 psi at 180 F.

Charlie

Umm, I think that's called the ideal gas law. (PV=nRT)

The formula assumes no volume change as the pressure increases, so maybe the pressure change won't be quite that much when starting with a low pressure tire.
The infrared thermometer is a great idea since the black tire surface should give a very accurate indication of the air temperature inside. Gonna have to test it.

[charlieaarons]

Umm, I think that's called the ideal gas law. (PV=nRT)

The formula assumes no volume change as the pressure increases, so maybe the pressure change won't be quite that much when starting with a low pressure tire.



Of course it is the gas law.But the sneaky part is adding the ambient pressure to the indicated pressure, otherwise inaccuracies are introduced by just reading the pressures off a gauge and taking the ratio.
I've read that tires increase in volume 2% up to 30kph then don't expand any further with velocity. I'm not sure what the volume-pressure coefficient is.

Charlie

[landcruiserbob]

Charlie, I was fortunate to get the 365's not the 395's. I do remember that on my gps running the 365's I would be about 73mph when the govenor kicked on. Thus the 86mph run in the salt flats. The 15.5 were 55mph rated not 50mph; I took a long look at them this weekend in Utah. My truck ran a straight exhaust that's where the extra ponies came from.

Unimog never turned my truck down on the recall since I had already replaced the tires. No new tires = speedo being set for 365's @ 70mph.

I weighed my truck 2 weeks before the accident & it weighed 21,592 lbs full of water & gear. Unlike most my truck lost weight over the years.I removed roll cage(dumb).

The 30psi was just an assumption. The 30psi was a reading in a previous incident when I noticed the tire being low & the axle showing 70 psi. I put air into the rear axle & it went up to 80psi but the tire in question sat at 30psi. Hence a possible blockage. Pulled tire & found the blockage. 10 minutes after the accident we were inspecting the tires when we noticed we had lost pressure in both rears.You would think there would be a check valve somewhere.

We drove the truck this weekend & it ran great w/o the windshield. New owner picked up the u500 today & I finally get my new crew cab mog I've been looking for. Have fun on this web site I must start building my new camper.rg

[Chas Stricker]

Robert when do you get your DoKa? Do you have any pictures? Is it in country yet?
Chas