| Motoring Discussion > Axle tramp | Miscellaneous |
| Thread Author: Cliff Pope | Replies: 28 |
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New Wed 8 May 13 08:27
Axle tramp - Cliff Pope
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I again witnessed a lorry this morning, in difficulties getting up a steep section of road. There are two such sections on my regular commute, made much worse by the recent reduction of the speed limit to 30 mph so that lorries can't get a run-up. What technically happens when an axle tramps? I can see that it is not the same as merely poor grip, as on a slippery road, but seems to be something to do with the suspension. Artics seem much more prone. |
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New Wed 8 May 13 08:55
Axle tramp - Dog
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| "A form of wheel hop that occurs on cars with live axles, caused by the axle repeatedly rotating slightly with the wheels and then springing back". |
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New Wed 8 May 13 09:04
Axle tramp - Cliff Pope
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| Yes, but why? |
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New Wed 8 May 13 09:12
Axle tramp - Dog
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"Typically when used in automobile suspension the leaf both supports an axle and locates/ partially locates the axle. This can lead to handling issues (such as 'axle tramp'), as the flexible nature of the spring makes precise control of the unsprung mass of the axle difficult". en.wikipedia.org/wiki/Leaf_spring |
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New Wed 8 May 13 09:23
Axle tramp - sherlock47
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For RWD cars, Historically leaf springs provided for both for vertical movement and horizontal location - but the horizontal location was compromised by the need for vertical movement! The spring leaf would 'wind-up'. Hence the need for anti-tramp location bars on such super cars as the Morris 1000. illustrated here www.britishracecar.com/MikeAdams-MG-MGB/MikeAdams-MG-MGB-DA.jpg but with telescopic dampers Beaten to it by the NC comprehensive explanation! Last edited by: pmh on Wed 8 May 13 at 09:25
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New Wed 8 May 13 09:12
Axle tramp - Number_Cruncher
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One way to see what's going on is to imagine replacing the tyres by a coil spring between the hub and the road, and imagining that the vehicle's chassis remains absolutely fixed. Temporarily, ignore the twist about the axle which tends to make the pinion rise or fall. The position of the heavy driven axle would then be fully defined by two dimensions - the height of its centre of gravity with respect to the ground, and the angle formed by the line of action of the half shafts and the ground. As the axle's position is expressed as two co-ordinates, there are two possible modes of vibration. Owing to symmetry, these are a pure bounce mode, where the axle moves up and down but doesn't roll, and a pure roll mode where the axle's centre of gravity doesn't move up or down. The last ingredient required is the excitation, which comes from the torque applied around the axle's roll axis at the pinion. Back with tyres in place, (which have their own vertical stiffness, and do behave a bit like springs) as the torque is increased, one side of the axle lifts, and that wheel is the one which loses grip. At this point, the axle begins to roll back to the equilibrium position, AND as the wheel has been slipping, the torque on the driveshaft at the pinion has also fallen. The cycle then repeats. In this way, there is a mechanism whereby energy can be directed from the vehicle's driveshaft into one of the modes of vibration of the axle assembly. As the axle is heavy, and has a large roll inertia, the axle can possess significant kinetic energy in roll even though the motion itself may not be very large. The cures for tramp include increasing the amount of roll damping - a function of the damper strength and the installed width of the dampers - for a tandem drive axle, it's difficult to get the dampers far enough away from the vehicle's centreline to be really helpful. As independent suspensions don't have this mode of vibration, so, don't suffer from the problem. For a more complete description of the dynamics, the twist of the axle about the half shaft axis needs to be considered, and also the gyroscopic moments resulting from the spinning wheels. |
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New Wed 8 May 13 09:49
Axle tramp - Manatee
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Interesting, I have been labouring under a misconception. I've certainly experienced one or another sort of hopping/stick/slip vibration which I understand to be axle tramp, with live axle cars such as the Morris Minor and Oxford I owned at one time. I always visualised it as a sort of fore and aft twisting of the axle in the opposite direction to wheel rotation, twisting and winding up the leaf spring. I dare say there's some of that going on as well in those lightly built vehicles. The mode you describe NC presumably means that the wheel loading on the grippy side is momentarily quite a bit higher than its fair share? |
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New Wed 8 May 13 10:54
Axle tramp - Number_Cruncher
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>>I always visualised it as a sort of fore and aft twisting of the axle in the opposite direction to wheel rotation, twisting and winding up the leaf spring. I dare say there's some of that going on as well in those lightly built vehicles. Yes, absolutely, there will be some twisting along the length of the spring, and the pinion will be moving up and down as the axle rotates when looked at from the side. >>The mode you describe NC presumably means that the wheel loading on the grippy side is momentarily quite a bit higher than its fair share? Yes. However, as the axle falls again, the grippy side will become temporarily light, as owing to the concentration of mass in the diff, the axle rotates about a point somewhere between the diff and the wheel. GB's explanation, including traction control is undoubtedly correct - there will be an interplay between these factors. Although not seen on heavy vehicles, for independent suspensions, the torque which makes one wheel go light with a live axle would instead be reacted by the chassis. |
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New Wed 8 May 13 09:46
Axle tramp - -
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Probably won't be axle tramp as such though looks and sounds like it, many years since lorries rear axles were located by being clamped in the centre of a cart spring. It is IMO basically a grip problem caused by much increased weights of the whole vehicle gradually outweighing the usual single standard drive axle. Its going to be a long post so switch off if you're bored already..;) Take a 6 axle 44 ton fully loaded artic for example, drive axle usually around 10.5 tons max (in practice on an axle weigher that will be about 9 to 9.5) whilst pushing a 7 ton steerer (6.2) and a 5.5 ton tag (5.2) on a tractor unit. Plus 3 x 8 ton max trailer axles usually around 7.2 to 7.8 tons in practice. These weights are from my own vehicle (we aim for 43t gross) though i usually impose max weight on the drive axle by ''braking'' the load forward a bit to help prevent wheelspin and promote overall stabilty, no tail wagging the dog if we can help it. Those weights show just how much work the drive axle has to do to overcome the drag of all those other axles. Average lorry 9.5 to 10 ton imposed on drive axle with fully loaded lorry having to pull and push a total of up to 33 tons for the combined weights of the rest. The thing you see that looks like axle tramp is just a momentary loss of traction which will in most cases be cut immediately by TC, which then causes all sort of problems if the driver hasn't pre planned a steep hill climb and is unfortunate enough to have satans automated manual gearbox, for that gearbox will in all likelihood trigger a gearchange just when its not wanted. Also some uphill wheel slips will cease immediately as the weight transfer changes and imposes more weight on the drive axle again, at one time it would have been a full tyre smoking wheelspin but modern traction systems (not perfect by any stretch) have all but rid that. Some lorries have the 5th wheel (trailer hitch) too far forward not helping the problem for it takes weight from the drive axle and shoves it forward onto the steerer, this also makes for uncomfortable road handling. There is a lot to steep hill climbing properly in modern lorries and you are absolutely right Cliff in that a run up is the best start, but there are several things a driver can do to assist the smooth progress, and this applies especially in poor conditions. 1. make sure as much weight as possible is legally loaded on the drive axle, this may include moving the 5th wheel to the best position, i experimented over several weeks with my new lorry to get the prime spot for legal loading, stabilty and traction. 2. be in manual override before the hill, most automated manual boxes (Volvo excluded who got it right) cannot cope with severe hills, in auto they down change too soon and don't necessarily block change enough gears and are slow doing all of the above, hence the power resumes in the wrong (too high) gear as speed has dropped faster than gears. The driver must be familiar with knowing what gear covers what speeds, and to use his noddle to judge deceleration taking into account engine torque at what revs, weight, road surface and load, and to know how many gears to block change down to allow the engine to keep up with the fast dropping speed This is where most automated manuals fail spectacularly...its no good a driver trying to learn this on a steep hill when accuracy and speed make or break the climb, it must be instinctive from use. Usefully some makers are programming manual override out of their gearboxes in fleet form, my new lorry tries to dissuade constant manual but i have learned how to switch it so it stays there, but expect lots of hassle on those hills in the coming years. New drivers are not taught any of this by the way, and the opportunities of learning are lessening. 3. pre plan the dumping of air in the tag axle, the one between steer and drive, all lorries** (except bizzarely some Iveco as far as i know) have this facility which works below about 25kph IIRC, if as speed drops the driver triggers air dump then the resultant weight transfer to the drive axle will often make the difference bertween grip and slip...this dump facility is mostly used by experinced drivers in low grip situations such as pulling out on a wet busy junction where every millisecond counts. Sorry i've gone on a bit, but its wasn't a simple answer....the above is only my opinion by the way of the practicalites involved in preventing and coping with it. **the company i worked for at the time bought half a fleet of transporters without dump valves for the aftermarket tag axles, they were dangerous to pull out on junctions with due to serious wheelspin triggering TC power cuts, a letter informing of the danger to management from a group of us soon saw retro fit of valves pronto. edit...proper explanation of whats really happening from N-C Last edited by: gordonbennet on Wed 8 May 13 at 09:50
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New Wed 8 May 13 09:57
Axle tramp - Runfer D'Hills
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Blimey GB, you can do all that while dunking a Custard Cream and you still can't park a Mondeo without stalling it? :-) |
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New Wed 8 May 13 10:03
Axle tramp - -
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Ran out of hands for phoning though so bluetooth it is..:-) Its all rubbish really, no one else only us old fools bother these days they just stick their clog down on the throttle and let the lorry do it all, if it comes to a shuddering stall they apply the handbrake and set off again kangarooing and revving the nuts off the thing. |
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New Wed 8 May 13 10:35
Axle tramp - Cliff Pope
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Many thanks NC and GB. I was watching the lorry ahead of me very closely this morning, as on each attempt to restart it slowly slithered backwards or sideways about a foot . There was no apparent wheel spin, as when trying to start on a slippery surface. That's what prompted my question - clearly something more complicated was happening. There was a lot of "grunting" from the lorry, which was jumping up and down, but it was as if it gave up before managing to apply any forward traction. A bit reminiscent of bungling a hill start on a Morris 1100, and keeping the handbrake on too much so the bonnet rose up and down but the car stalled. Was that the driver sensing a loss of grip and de-clutching, or is there some automatic process? I would imagine that there is a lot of acquired skill involved in avoiding this situation, and in extricating the lorry once stuck? |
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New Wed 8 May 13 10:57
Axle tramp - -
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''Was that the driver sensing a loss of grip and de-clutching, or is there some automatic process?'' If it was a fairly new lorry which i suspect then chances are it was an automated manual, as soon as slip detected then power would automatically be cut by TC, whcih would most likely trigger a gearchange and automatic declutch. Ha i forgot number 4 of the list in my daft post, theres a number 5 too..:-) 4. approaching a steep hill driver should turn off TC, that way a momentary one wheel wheelslip won't trigger the power to be cut leading to automatic clutch disengagement. 5. if a very steep hill is slippery its sometimes prudent to engage diff lock before attempt, this together with TC off and the above air dumps etc will give as much traction as possible without the vehicles systems interfering. In ice and snow depending on whats happening and how things develop it can be better to switch diff lock off, for two locked wheels spinning together will see the drive axle break out of line and slip sideways leading to what looks like but isn't a jack knife. ''I would imagine that there is a lot of acquired skill involved in avoiding this situation, and in extricating the lorry once stuck?'' Forunately what your saw Cliff doesn't happen too often these days, the seat of the pants driving skills are not being learned and they haven't been taught for many years (its now brakes to slow gears to go and thats your lot). Too much reliance on the vehicles stability and traction systems these days IMO, and to be fair in 99% of circumstances these systems work well and help prevent problems developing in the first place, its knowing where and exactly when to take over thats the art IMO. The difficult part being that unless the newer drivers learn to control things themselves by regularly overriding these systems so its second nature, the rare times such actions are needed is not the time to start. Last edited by: gordonbennet on Wed 8 May 13 at 10:58
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New Wed 8 May 13 11:09
Axle tramp - -
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Will have missed the edit. If a heavily loaded artic comes to a halt on such a hill as you describe Cliff, then the driver has a couple of options. If he hasn't already he should dump all tag axle air, thereby loading as much weight as possible onto the drive axle. If lucky enough to have a lift front axle on the trailer that will lift whilst loaded (an old trailer is most likely to be able to do this) then doing so would again increase the weight imposed on the drive axle. Driver should already have TC turned off at this point, and have manually selected 1st or crawler or extra low crawler depending on box, maybe even selected Manouever function in order to apply just enough to let the lorry get moving again. In practice all the above may not be enough and the combined weights of the non drive axles simply too much for the grip of the driven to overcome, the only option then is a careful reverse back down the hill to a point where start off is poosible and to use judgement as to which gear to let it climb the whole hill in. As i said above with good anticipation and pre applying the correct measures before the hill became serious then coming to a shuddering stop should not have happened, but it doesn't matter what you do sometimes things just go wrong. Last edited by: gordonbennet on Wed 8 May 13 at 11:10
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New Wed 8 May 13 11:22
Axle tramp - -
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By the way there is another problem, some lorries are fitted with single stage underperforming compressors, my new one is. Once a couple of failed starts have occured then its highly likely the air pressure will have dropped to the point brakes are starting to automatically apply, that might have been a contrinutory cause on the lorry you saw Cliff. If my lorry drops enough to apply the brakes it takes about 3 minutes at 1000rpm fast idle to get full pressure back again, a coupld of shunts across a busy road can see this happening with predictable results...no it hasn't happened to me yet but one of my oppos blocked the approach road to a custmers premises for several minutes recently when this happened twice to him during the 400 yard steep sharp curve descent down to their temporary unloading point. |
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New Wed 8 May 13 11:18
Axle tramp - Harleyman
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>> Probably won't be axle tramp as such though looks and sounds like it, many years >> since lorries rear axles were located by being clamped in the centre of a cart >> spring. >> Not for artics, gb, but many rigid tippers and bulk blowers still rely on the old-fashioned method. My last Volvo FM was a 6x4 on leaf springs, for whose demise I am eternally grateful even if I did end up with a Renault instead. ;-) |
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New Wed 8 May 13 11:25
Axle tramp - -
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My last Volvo FM was a 6x4 on leaf springs, for whose demise I am eternally grateful even if I did end up with a Renault instead. ;-) >> Yeah, but i bet you were thankful to have that during the bad weather, takes weather worse than we ever get to halt a double drive rigid..:-) wish i had an 8 wheeler instead of me artic blower. Last edited by: gordonbennet on Wed 8 May 13 at 11:26
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New Wed 8 May 13 11:36
Axle tramp - Zero
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How would you like to drive this lot over that bridge with an automated gearbox! www.youtube.com/watch?v=K4VSfzBqbXM About 5 mins in - where the bridge starts to flatten out. |
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New Wed 8 May 13 11:48
Axle tramp - -
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>> How would you like to drive this lot over that bridge with an automated gearbox! >> I'd like to be a fly on the wall listening to the conversation when a smooth salesman tries to push the virtues of an automated manual box to that operator..;) Actually what gearbox would that lorry have? Allison TC auto or a very old fashioned crash box or what? |
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New Wed 8 May 13 21:56
Axle tramp - Harleyman
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>> Actually what gearbox would that lorry have? Allison TC auto or a very old fashioned >> crash box or what? >> I'd suspect an Allison; it's pretty much the transmission of choice for many STGO outfits. Last edited by: Harleyman on Wed 8 May 13 at 21:56
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New Wed 8 May 13 11:31
Axle tramp - Armel Coussine
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| Cart-sprung rear drive Transits suffered from bad axle tramp when driven violently. I wonder how I know that? They can draw a dotted line in rubber down the road, I kid you not. |
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New Wed 8 May 13 17:02
Axle tramp - Slidingpillar
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| Axle tramp can break axles! I fitted the Morgan 4/4 with anti tramp bars to stop the tramp on very fast getaways - my first 60 feet as timed at Prescott was once the fastest of all cars - racing ones included! |
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New Wed 8 May 13 21:47
Axle tramp - Dave_
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>> Cart-sprung rear drive Transits suffered from bad axle tramp when driven violently. LDV Convoys too. Flimsy, flexible body shells to blame - can't coax that kind of behaviour out of a Sprinter. |
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New Wed 8 May 13 22:30
Axle tramp - swiss tony
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>> - can't coax that kind of behaviour out of a Sprinter. >> I beg to differ... the old 3.0ltr turbo LWB can be made to tramp... ;-p |
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New Thu 9 May 13 11:40
Axle tramp - Boxsterboy
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| All I know is that it used to be fun getting my first car (Morris Minor) to axle tramp when I drove it like an idiot. |
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New Thu 9 May 13 12:30
Axle tramp - DP
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>> >> - can't coax that kind of behaviour out of a Sprinter. >> >> >> >> I beg to differ... the old 3.0ltr turbo LWB can be made to tramp... ;-p >> As can a 2008, MWB 313CDi when unladen. Makes a remarkably good job of doughnuts as well. So I'm told. :-) I haven't been in a van that felt more terrifyingly quick than this. |
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New Thu 9 May 13 13:19
Axle tramp - madf
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| I had axle tramp on a Rover 16 - both front and rear axles.. although not at the same time.. Scary at the front. |
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New Thu 9 May 13 14:20
Axle tramp - Cliff Pope
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There seem to be two different kinds of tramp: 1) The kind I have noticed with lorries. The lorry is stuck on a hill. The axle tramps up and down, but the vehicle makes no progress forwards, and may well slowly slip backwards. The wheels don't spin - in fact more commonly I notice the engine stalls as if up against a brick wall. 2) Cars and light vans reportedly tramp while moving, leaving a trail of rubber ridges on the road. This seems to be either scary or fun. The second sounds more like ordinary wheelspin with intermittant grip, not the total lack of progress of the first kind. Any rear-wheel drive car will do that in certain circumstances. |
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New Thu 9 May 13 14:48
Axle tramp - Slidingpillar
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To get axle tramp, you need two conditions to be satisfied. 1) A twisting force on the axle, that actual does twist the whole axle - such as the application of power. 2) Something that releases the twisting force - such as the tyres losing traction If 1 and 2 are satisfied, the loss of traction will enable the axle to be less twisted, and the process of undoing the twist will greatly reduce item 1, consequently item 2 cease to be an issue, the tyres regain traction, the axle twisting starts again and the whole process continues until either the force ceases to be applied, or something breaks. The action of traction control systems will confuse the issue, and cannot really address the issue since it is not a loss of traction that is the real cause, but an uncontrolled suspension system/design. If the axle can't twist and move, axle tramp won't happen. I've seen axle tramp on an older lorry, and the releasing force is wheel spin, but only a very small amount and you would probably write off the wheel spin as being a factor. But, without something to release the twisting force, axle tramp won't happen. The fitting of anti tramp bars to the Morgan 4/4 not only dramatically improve competitive starts (where you actually want some wheel spin) but also stopped tramp at speed when the car went light over a bump (say the top of East Brae at Doune hillclimb). Last edited by: Slidingpillar on Thu 9 May 13 at 14:51
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