Driving over corrugations

Got some advice to share or a question to ask? Heres the place to do it!
RASPUTIN
Been here a while
Posts: 176
Joined: March 14th, 2010, 1:26 pm
Location: Thirlmere

Unread post by RASPUTIN » July 13th, 2010, 3:10 pm

Depends on the road but as fast as I need to until the vibrations die down to a comfortable level. Usually around 60 - 80 km/hr. I have done a lot higher speeds but that was for fun with a car designed for doing speed over rough roads. 120k's in a top heavy loaded up 4x4 is nuts. Hell 1 patch of bulldust and they could have rolled. Next time if I was you i'd say i'll meet you there :).

bigpig
Avid Poster
Posts: 725
Joined: June 18th, 2009, 1:09 pm

Unread post by bigpig » September 3rd, 2010, 6:23 pm

Smoothing out the corrigations has a lot to do with sprung and unsprung weight and the speed of travel. Once you get going the frequencys of the two will go through a stage where they add to each other making the cagin bounce the highest. As your speed increases to a speed that seems smooth its because the weight of your undercarriage is bouncing against the weight of your vehicle holding you fairly still. Any faster and your shockies are just doing all the work and the harder the shock absorber the more movement will be transfered to the cabin aswell. More speed means more heat at your shockies. I find 80kms a good spd. sure i can justify the risk at 100km/h on some roads but at another 20 i think the risk outweighs the advantage of saving an hour driving every 5.
Mick

DJR96
Need to get out more
Posts: 3153
Joined: May 25th, 2007, 10:54 pm
Location: Beerwah, QLD

Re: Driving over corrugations

Unread post by DJR96 » September 9th, 2010, 11:48 am

You're on the right track re: unsprung weight.

The aim is to go at a speed (where safe to do so) that is too fast for the suspension to react to the corrugations and thus forces the tyres to absorb them instead. So you don't want tyres too hard, and they're also not just skipping over the tops either so it's not as dangerous as it sounds.

There's an awful lot of unsealed dirt roads in this country, and many others, that can easily be travelled on at essentially highway speeds. You may not like to and that's fine, but please don't automatically criticise those that are comfortable doing so.

But like all driving, choice of speed is determined by assessing all the conditions you're in at the time. And that varies continuously.


It's a topic discussed here:- viewtopic.php?f=7&t=48775

This is what I wrote then:-
DJR96 wrote:Something to point out here. It doesn't matter what springs you use. So long as they are stiff enough to carry the weight of your vehicle at a ride height somewhere in the middle of the suspensions range of movement (preferably 35-55% compressed). Aftermarket springs may be more durable, sometimes not. You only change springs to change load carrying capacity and/or ride height. This is where spring rates come into it- higher spring rates for higher loads etc) And the great thing about airbags - adjustable!

After that, it is a case of getting dampers (shock absorbers) that match the sprung and unsprung weight of the vehicle. Buying aftermarket ones to handle the weights of your vehicle, and better durability too.

And getting a speed that effectively negates/offsets the frequency of the corrurations and the natural frequency of the vehicles axles. This is a hard one to explain........


By driving at a speed where the corrugation bumps pass under the tyres at a frequency significantly faster than the natural frequency of the axle (but not at 2nd or 3rd order harmonics), you avoid the destructive harmonics (when the frequencies match). It's the big slow bounces (at the axles natural frequency), that are almost impossible to control and can do so much damage to vehicles, including skipping you right off the road.

To do this, the tyres will have to absorb much of the bumps themselves without the suspensions help. Thus the importance of tyres with relatively high sidewalls. That's why manufacturers used good old 7.50-16 cheese cutters for so long! To make up for the pathetic dampers they used. Low profile tyres just can't do it and make the suspension do the work (which is the whole idea in race cars on tarmac).
This is where tyre pressure comes into it. The softer the better, BUT you still have to have enough to allow the vehicle to handle safely AND to retain durability of the tyres. ie. too soft = too much flex = too high temps. = potential tyre delaminations and blowouts. Note too that the pressure used on sealed roads is higher mainly to extend tyre life from wear on the unyielding sealed surface, and to improve handling that can be acheived on a sealed surface. Not to improve ride comfort.

The heavier an axle is, (it's unsprung weight) the slower it will want bounce (it's natural frequency). Solid axles being heavier unsprung weight will be at a slower frequency, that we can drive much faster than. An independantly sprung wheel has lighter unsprung weight and therefore it has a higher natural frequency. You would therefore have to travel even faster again to avoid the bad harmonics, but that may not be practical (safe)- just too fast. So therefore it is even more important to get the dampers just right, because they will have to do more of the work (because we can't avoid it by driving faster) Adjustables recommended!!! And if the sprung weight is not heavy enough for the dampers to work properly, you and the vehicle are going to get all shook up.:p


This is all a very good reason to have solid axles on a tourer or workhorse vehicle. Independant suspension often just doesn't work well enough at practical speeds (80kmh+) on dirt roads - of which most are corrugated.
Manufacturers take note!!!

Sorry about such a long post, but I hope this helps explain why driving faster can work.:thumb:
Cheers, Dave.
RCV Supertourer build:- [url]http://www.4wdaction.com.au/forum/viewtopic.php?f=22&t=36370[/url]

chikoroll
Need to get out more
Posts: 2544
Joined: August 4th, 2007, 7:12 pm

Re: Driving over corrugations

Unread post by chikoroll » September 9th, 2010, 8:01 pm

a few years back, i was driving at 60km/h, i had the tyres aired down to 17psi, it was the speed the corrugations were nice at and the right pressure for them

came over a minor hump, to find a large puddle immediately in front, 2 wheels in, vehicle going sideways... had it been "flooring" it, would have ended up on its side

DJR96
Need to get out more
Posts: 3153
Joined: May 25th, 2007, 10:54 pm
Location: Beerwah, QLD

Re: Driving over corrugations

Unread post by DJR96 » September 9th, 2010, 8:59 pm

As with ALL driving, appropriate speed for the circumstances. Period. :D
Cheers, Dave.
RCV Supertourer build:- [url]http://www.4wdaction.com.au/forum/viewtopic.php?f=22&t=36370[/url]

BMKal
Part of the furniture
Posts: 1387
Joined: February 8th, 2007, 2:50 pm

Re: Driving over corrugations

Unread post by BMKal » September 11th, 2010, 3:15 am

I used to have a maintenance foreman up at Yandi who always sat on AT LEAST 120 km/h in a 75 series trayback on dirt roads when off the mine site (BHP would shoot him if they caught him doing that sort of speed on the site).

I only ever travelled in a vehicle off the lease with him ONCE. Scared the hell out of me. :eek:

I generally find that about 80 is not a bad speed - but depends on the conditions and the vehicle you're driving. As others have said - drive to conditions.

Having said that - I've driven a 2WD Hilux ute from Alice out to the Granites on the Tanami road a few times (well before it was sealed nearly all the way to Yuendumu). The ute was generally pretty well loaded, but never excessively and never a high load. When the road was in reasonable condition, the Hilux was quite comfortable at about 120 km/h. Would NEVER attempt the same speed on a dirt road in a 4WD Hilux though. Have seen way too many of them on their roof.

Mr Guzzi
Avid Poster
Posts: 595
Joined: January 31st, 2010, 11:28 pm
Location: WA

Re:

Unread post by Mr Guzzi » September 23rd, 2010, 11:27 pm

RASPUTIN wrote:Depends on the road but as fast as I need to until the vibrations die down to a comfortable level. Usually around 60 - 80 km/hr. I have done a lot higher speeds but that was for fun with a car designed for doing speed over rough roads. 120k's in a top heavy loaded up 4x4 is nuts. Hell 1 patch of bulldust and they could have rolled. Next time if I was you i'd say i'll meet you there :).
Actually one effect of driving fast is that soft stuff like sand and bulldust effectively becomes harder, as you aren't on top of it long enough to sink into it. Mate of mine used to go fishing at Wedge Island in their HQ panelvan. They just floored it across the sand and didn't get bogged

I'm not an experianced driver in these conditions, but I Am an experienced rider on big road bikes on dirt roads in these kind of conditions. I spent many years touring in the outback on dirt roads on my old R-series BM. If the road was rough, go faster and it'd smooth out? Sand or bulldust (of course a bigger problem on a bike than in a 4WD) - just go faster and the bike floats on top - 120-140 kmh no problem.

I add a caveat tho' - I'd only do this on a flat, open road where I could see for miles and could stop in plenty of time if needs be. Second caveat - bikes become intrinsically more stable at higher speeds due to the gyroscopic effect and are less likely to be thrown off line, but in a car going fast on dirt they become less stable at high speed (ie could start drifting/sliding)

NissanMadness
Here and there
Posts: 93
Joined: June 25th, 2010, 2:32 pm

Re: Driving over corrugations

Unread post by NissanMadness » October 7th, 2010, 8:05 pm

Fast sure smooths it out but those speeds with those weights and top heavy just sounds dangerous.

Phil G
Getting to know the place
Posts: 19
Joined: September 29th, 2010, 6:00 pm

Re: Driving over corrugations

Unread post by Phil G » October 10th, 2010, 6:24 pm

I travel on the Anne Beadell Hwy most years. My first move is to soften the tyres with pressures down to low 20's. Speed usually 40-50 kph. Much safer, softer and won't trash the suspension and everything else inside the vehicle. Seen a few vehicles with bullbars broken off and being carried on roofracks by driving fast to smooth out the corrugations. Seen the odd overturned vehicle as well.

Aaron Schubert
Need to get out more
Posts: 4682
Joined: November 9th, 2009, 7:32 pm

Re: Driving over corrugations

Unread post by Aaron Schubert » October 10th, 2010, 7:15 pm

As it has been mentioned, drive for the conditions. For you to be doing 120km/h you would have to be able to see a long way ahead, and even then you wouldn't be able to see a pothole, which would be enough to make some serious havoc.

I wouldn't be going over 70km/h (and even slower if the conditions were not 100%) unless I had done the track several times over!

Aaron

rodw
Avid Poster
Posts: 833
Joined: April 3rd, 2008, 7:40 pm
Location: Brisbane QLD

Re: Driving over corrugations

Unread post by rodw » October 10th, 2010, 9:01 pm

I think it is all about your experience on driving on the dirt and the conditions on the day. IMHO, 70 km/hr is a ridiculous speed for dirt road driving. Far too slow. We are not talking tracks here, but roads!

I would favour 80-100 myself maybe the lower end if heavilly loaded but you have to have your wits about you and know how to read the road ahead and react to changes in the conditions well before you get to the washout/gutter or the like as we don't want to brake hard on the dirt.

You can't learn how to drive like that on the odd 100 km section of dirt road. You have to live on them day in day out but it is a skill you never forget. On a lot of dirt roads, 120-140 is quite doable. Some dirt roads, are every bit as good as bitumen...

Corindi
Moderator
Posts: 3515
Joined: December 3rd, 2006, 4:17 pm

Re: Driving over corrugations

Unread post by Corindi » October 10th, 2010, 9:15 pm

On a recent trip in western NSW, rarely could I get over 60. The first pic shows the road conditions and the second shows what happens when you get it wrong. I am guessing this was a local as there was a 200L drum in the back and no camping gear.
DSC_0924.JPG
DSC_1099.JPG
You do not have the required permissions to view the files attached to this post.
www.BillabongFurniture.com.au

Aaron Schubert
Need to get out more
Posts: 4682
Joined: November 9th, 2009, 7:32 pm

Re: Driving over corrugations

Unread post by Aaron Schubert » October 10th, 2010, 10:52 pm

Again, it comes down to the conditions of the road or track that you are driving on. If you are confident at driving at 100km/h then that's fine, assuming you aren't risking anyone else's lives!

Aaron
rodw wrote:I think it is all about your experience on driving on the dirt and the conditions on the day. IMHO, 70 km/hr is a ridiculous speed for dirt road driving. Far too slow. We are not talking tracks here, but roads!

I would favour 80-100 myself maybe the lower end if heavilly loaded but you have to have your wits about you and know how to read the road ahead and react to changes in the conditions well before you get to the washout/gutter or the like as we don't want to brake hard on the dirt.

You can't learn how to drive like that on the odd 100 km section of dirt road. You have to live on them day in day out but it is a skill you never forget. On a lot of dirt roads, 120-140 is quite doable. Some dirt roads, are every bit as good as bitumen...

zerky
Getting to know the place
Posts: 19
Joined: August 23rd, 2010, 7:28 pm

Re: Driving over corrugations

Unread post by zerky » October 13th, 2010, 9:40 pm

I think a lot of this is common sense, (don't know if that's spelt right for that fella who is always up people for their spelling mistakes, but then again I'm not going for a job ;-) ) (EDIT: It is now! Spellcheck is good!)
I had a really bad/sorta good experience a while back. I'm only 16 now but can comfortably drive as I'm from the bush. The roads I was traveling on I knew very well because I drive them when I work for the neighbours. However, I was in dads mates Prado and had his son Matty with me (Matty is about 8). As I pulled into a neighbour's gate where another mate was working 2 say g'day, I slid out just in the gate around a turn. Now I wouldn't have been doing anymore than 40k's and I was dam lucky not to roll it; that was a eye opening experience for me let me tell you! The corner was very corrugated and had that red marbley antbed pebbles on it. I think that I will always be cautious now days on dirt.

cheers zerky.

steve.j
Been here a while
Posts: 360
Joined: February 15th, 2010, 9:43 pm

Re: Driving over corrugations

Unread post by steve.j » October 16th, 2010, 9:42 pm

I find about 80-90ks is quick enough on bad roads. If conditions permit i will travel on the right hand side too, seems to make a bit of a difference going against the corrugations. Generaly you will see dust coming long before you see an oncoming vehicle.

Return to “Driving Tips and Techniques”