On corrugations, tyres and pressures

Submitted: Wednesday, Aug 20, 2003 at 08:03
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We encountered horrendous corrugations (horrogations) on the Mitchell Plateu. They were huge. It was like driving on the ocean with small statiionary waves. A lot of vehicles experienced trouble on the Mitchell Falls road. Most common problem was failed shock absorbers. Shocks became overheated ( actually too hot to touch) and leaked oil. Some vehicles lost control while driving a strait line. This occurred on all types of vehicles. There were other problems. One vehicle lost its aluminium bull bar. Trailers broke axles. Roof racks broke (ours did and the fridge as well) One family told us their children became car sick and the kids had to hold buckets in from of them! I measure the peak to peak length of some of these corrugation. They varried from about 400 to 600mm. Thats 2000 corrugations per kilometer for an average length of 500mm. The Mitchell Falls detour is about 160km return. This eqates to about 320,000 hits. I was surprised to find that the peaks of the corrugations consisted of sand. This could be easily scraped away by hand to reveal a smaller hard corrugation So why are they so hard on the vehicle. Different soil types produce different corrugations. Without exception red soil produces the worst and grey soil the best (smallest, nice and friendly) We soon learned to expect bad corrugations when we saw red soil in the disstance. The iside of a curve was less severly corrugated than the ouside. I also believe the peak shape is not symetrical. Often on an out and back journy driving times were different. I tried testing the theory by driving on the wrong side of the road. Diddnt seem to work. Corrugations became worse at the end of a tyring days drive, the fatigue factory. Or when there was something wrong with the vehicle , the worry factor. By the way the Mitchell Falls are simply superb. Better than the glossy magazine photos But i wont do it again unless the grader is in front of me. I played around with tyre pressures. I usually run 65psi or more on bitumen. I deflated to 50 psi on general dirt. On the sever corrugations i tried 45 psi then down to 38 psi with no noticeable difference. Maybe my 14 ply tyres are too rigid in the side walls ,i dont know. I prefer road tread tyres. They are less noisy, pick up fewer stones and last longer than lug tread tyres. They have more rubber on the road per contact patch, dig fewer holes in soft ground and steer better as well. In mud they clog in one revolution, the same as lug tyres. Regarding shock absorbers. Take spares and buy the best (Konis) Dont buy American, or the popular 4WD aftermaket yellow ones as they will fail. Take spare rubber bushes. These can get hot and fall apart. I fitted duel shocks (ie two per whell) on the OKA and they only just got warm from then on. Vidas
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Reply By: Deleted User - Wednesday, Aug 20, 2003 at 08:05

Wednesday, Aug 20, 2003 at 08:05
I haven't had the unfortunate experience (or pleasure?) of travelling in conditions as bad as that, but I do wonder whether your tyre pressures aren't too high? I run the Landcruiser on BF Goodrich A/T's at around 38-42lbs (Toyota recommendations) for highway cruising, and on rough dirt find that about 30-32lbs gives me a good compromise between ride and handling. Travelling on the corrugated section to Canarvon Gorge, we ran the BT at 30lbs without any significant 'bounce' or heating of the tyres (at a cruising speed of about 50-60kmh). Phil
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Reply By: Noosa Fox - Wednesday, Aug 20, 2003 at 08:06

Wednesday, Aug 20, 2003 at 08:06
On bitumen roads and short stretches of unmade roads we run the F250 on 45psi and Bushtracker on 40psi. While out around Birdsville and the Birdsville Track we had all tyres on 30 to 33psi and never had any problems. Our travel companions also had the same pressure and again no problems. Collyn says he runs his tyres (I think) about 30psi all the time on the gravel roads around his area without problems. With too much pressure there is no flex in the tread and rocks tend to penetrate rather than just push in a bit. On beach sand I let F250 down to 18psi. Brian
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Reply By: Deleted User - Wednesday, Aug 20, 2003 at 08:07

Wednesday, Aug 20, 2003 at 08:07
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Reply By: Deleted User - Wednesday, Aug 20, 2003 at 08:08

Wednesday, Aug 20, 2003 at 08:08
We normally ruin our 78-series Troopie and diesel Hilux 4WD at about 30 psi on our mostly dirt roads but keep speed down to <80 km/hr. Never have tyre blow outs.

We run the OKA at about 60 psi though. The 14 ply tyres are more or less bullet-proof anyway and letting them down does not improve the ride. The current 900 x 16s Michelins have done about 40,000 km ) including several Gibb River Rds, three Tanami crossings etc and are about 45% worn.

Like Vidas I normally run on road pattern tyres, but I do have a Detroit Locker rear diff and an ARB Air Locker up front.
Collyn Rivers
AnswerID: 559437

Reply By: Deleted User - Wednesday, Aug 20, 2003 at 08:09

Wednesday, Aug 20, 2003 at 08:09
I was really surprised that lowering tyre pressure had no affect. I was reluctant to go lower than 38 psi because our all up weight was about 5 tonne. We had no difficulty on any other terain including roads which had brick size stones on them. I ran 45 psi on these roads and we could hardly feel a bump. Similarly 45 psi on broken surfaces was OK as well. But big corrugations were the problem. I think that our vehicle doesnt like hitting them strait on. On the Ghann Heritage Trail i found a complete wheel and tyre, lost by a Finke Desert race competitor. I traced the owner who happened to have won the event this year. He told me his buggy has 23 inches of wheel travell. He has 3 inch diameter shock absorbers with remote canisters and reached speeds of 200 km/h on sections we were flat out at 40 km/h.. So i might have a look at reseting springs to improve corrugation performance. What suspension set up does the F 250 have Stay well Vidas
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Reply By: Deleted User - Wednesday, Aug 20, 2003 at 08:10

Wednesday, Aug 20, 2003 at 08:10
Vidas, F250 has that "Dont buy American" rubbish on it ....especially the Rancho RS5000 steering damper. The American RS9000 adjustable from the cabin shocks are a bit ordinary also .... they are only adjustable in dampening about 700% over 9 positions .... not good for corrogations Regards Anthony
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Reply By: Deleted User - Wednesday, Aug 20, 2003 at 08:11

Wednesday, Aug 20, 2003 at 08:11
Anthony. RS something shocks were failing on the Canning Stock route. I heard over the HF radio several reqests for replacements but i cant remember the number. I am running (horror) Monroe shocks which are pretty ordinary thats why i take spares. I am trying to find better ones which will fit the truck.Rancho steering damper on a mates vehicle failed in three months of on road driving. I like Koni shocks. They are adjustible and repareable but are expensive. They seem to cope well with rough conditions. I have them fitted to my 80 Series Toyo and they improve handeling signifficantly. I looked at the popular Oz aftermarket ones but they were like bicycle pumps by comparison. Does the F 250 have leaf or coil springs. Incidently my bro in law business partner is a huge F truck fan. I am trying to convince him to buy one as a flagship for his truck repare business. Vidas
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Reply By: Deleted User - Wednesday, Aug 20, 2003 at 08:12

Wednesday, Aug 20, 2003 at 08:12
G'day Vidas, Koni is the best thats for sure. I'm currently using them on one vehicle and as you say at least they are rebuildable. Corrugations are a big ask for any shock absorber. The PRIMARY purpose of the shock absorber is to dampen the release of energy stored in the springs by suspension movement. Therefore any effort to use really heavy springs with shocks valved for soft ones is doomed to fail. The higher the unsprung weight the greater the energy stored and released by the spring. Most people dont know that the spring is the most stressed part of a vehicle. Even the coil spring is still highly stressed. When a wheel moves upward to the bump position the stress in a typical spring can be 117,000 psi in torsion ...... almost the elastic limit for some !!!! Its a funny world you know .... as metallurgists come up with a spring steel able to handle higher stress or more cycles car designers take advantage of the expanded horizon and put them near their limit again ..... As you know shocks are fairly generic and just rebranded. I like the Ranchos because they can be adjusted on the fly with the incar kit and are certainly no worse than any other shock on the market except Koni. As they are reasonably priced for what they are spares are easy to carry. I've even seen two Bilsteins break their lower attachment ring on corrugations. Regards Anthony
AnswerID: 559441

Reply By: Deleted User - Wednesday, Aug 20, 2003 at 08:13

Wednesday, Aug 20, 2003 at 08:13
Woops ...almost forgot !!!! The F250 has leaf springs. Extensive testing in the outback revealed a shortcoming in the front shock valving and a big problem in the rear ( high unsprung weight, heavy spring rate with underdampened shock). Two more years of testing (fords words not mine) resulted in suspension geometry for Aust in the F trucks. They have heavier valved shocks and Aust springs in the front and to achieve the same in the rear have FOUR shocks fitted with Aust springs. Originally they had (I believe) a heavy rate spring and a soft damped shock ..... great for American freeways and ride comfort but doomed to fail here. I believe they got it right because Brian, Bob and Andy have seen some pretty rough stuff and have not had a suspension failure. I know of an F350 with around 100,000k with two trips to Gibb River Rd amongst many others with original shocks and springs...... time to replace shocks though ........ Anthony
AnswerID: 559442

Reply By: Deleted User - Wednesday, Aug 20, 2003 at 08:14

Wednesday, Aug 20, 2003 at 08:14
The important thing to remember with shock absorbers is that they are heat pumps. They convert the energy imparted to and stored by the spring into heat.

They more or less work by forcing oil (which is close enough to being non-compressible as not to matter) through small orifices, but if they get too hot the oil begins to bubble and the devices fade because gas compressible. Their effectiveness is thus to some considerable extent a function of their ability to dissipate heat.

There's more to than that of course - valving (orifice size etc) must be tailored for the vehicle's weight and conditions of use - and some are simply made a lot better than others. But by and large it's a function of adequate size - and there's plenty of good US-made units around.

The original single Monroe shocks fitted to OKAs were inadequate and most now have dual Monroes.

Personally I prefer the much larger (huge actually!) Gabriel shock absobers marketed here as 'Ralph'. They are actually produced as a Troopie replacement but have worked fine on my OKA for over seven years - and NEVER fade.

(Odd that a substantial part of the 'van industry still ignores several of Newton's more basic laws!).
Collyn Rivers



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