Aragorn Posted June 19, 2009 Share Posted June 19, 2009 Following on from a thread in the disco forum about not using a roller brake tester for an MOT, i'm trying to get my head round exactly why its a bad thing.... The centre diff is open, so putting one axle into the brake tester while the other axle is being held stationary (ie its on the garage floor) will simply mean diff spins its input side. If the TXB is in neutral, then nothing else happens, if its in gear then the brake tester would feed the power thru the centre diff to the gearbox output shaft, but at this point its no different to the entire vehicle rolling along the road in neutral, bar the fact that one side of the diff is stationary. A similar thing happens regularly on each axle when one wheel loses traction so i really cant see what the issue was. Clearly if difflock was engaged, or the centre diff was a Torsen or Viscous coupled type then it would be a different story, but with the centre diff in "open" mode it should be fine. Am i missing something here? Quote Link to comment Share on other sites More sharing options...
martinhollings Posted June 19, 2009 Share Posted June 19, 2009 i to dont see the big problem yes it will make the center diff work hard but no more than getting stuck in mud when i had my last mot done just as he was putting the guage thin in the pass foot well i said you can stick it on the rolling road for me he would not do it Quote Link to comment Share on other sites More sharing options...
western Posted June 19, 2009 Share Posted June 19, 2009 my 110 always gets brake tested on a single axle roller tester, not caused any problems for me or my 110. read the warning notice on your fuse box cover. Quote Link to comment Share on other sites More sharing options...
missingsid Posted June 19, 2009 Share Posted June 19, 2009 Same as. My Hybred was always put on a singlr roller system and the tester did loads of LR with full time 4x4. The risk must come from a locked or limited slip centre diff? If it grabs it will push the LR off the roller, thing is how fast? Marc. Quote Link to comment Share on other sites More sharing options...
pat_pending Posted June 19, 2009 Share Posted June 19, 2009 Thought we'd done this one to death. It's all about the speed differential across the centre diff. Land Rover recommend that if single axle roller rigs are used, then the speed of the rollers is no more than 5kph, about 3mph. I'm not sure what speed the brake rollers run at, but the vehicle specific information issued by VOSA for permanent 4wd Land Rovers says don't use rollers. Whether or not the centre diff is capable of being run at over this speed or not, IF damage were to occur then the testing station would be liable. That's why we use a decelerometer. Some testers don't, that's their look out. In all likely hood I doubt if it would cause any damage, in fact so long as the diff was in good nick I'm sure it wouldn't. But I can't take the risk with someone else's vehicle and my bosses money. Oh, and putting the T/box in neutral has absolutely no effect on any of this. Quote Link to comment Share on other sites More sharing options...
gsr341 Posted June 19, 2009 Share Posted June 19, 2009 mine allways tested on the rollers too , even my evo 6 goes on the roller tester without any probs if it where to grip and get spat out of the rollers would probably stall it , thats what i was told when the car was on a 4wd rolling road @ 100mph Quote Link to comment Share on other sites More sharing options...
Turbocharger Posted June 19, 2009 Share Posted June 19, 2009 It's a risk assessment thing. If your tester isn't familiar with every type of 4WD system he could inadvertently shoot your truck through the end wall of his garage. If he can get a satisfactory brake test by another method, why take the risk? Those who think they understand the systems can use the RBT if they want. My rear axle has a TruTrac and I always forget to mention it at MOT time. It once tried to climb out of the rollers which caused that garage's power to trip out. He was particularly miffed because the workshop radio had lost all its presets. Quote Link to comment Share on other sites More sharing options...
Dave W Posted June 19, 2009 Share Posted June 19, 2009 What Pat said.... The centre diff isn't designed to sustain a speed difference across it for any period of time and it can be damaged by doing so. It's not the same as getting stuck in mud because you'd have the centre diff locked in that scenario. You CAN test a vehicle this way but eventually you will kill one and when you do there is every chance that it will lock up, launching the vehicle out of the rollers. Quote Link to comment Share on other sites More sharing options...
Hybrid_From_Hell Posted June 19, 2009 Share Posted June 19, 2009 I love the fact that the system says "No Roller test" use Gyroscope My possiblity to get my own back on an MOT Tester by braking hard and having him and Gyro in footwell, .................and then a "Chat" re my MOT ? Nige Quote Link to comment Share on other sites More sharing options...
martinhollings Posted June 19, 2009 Share Posted June 19, 2009 how many disco 2s do you no with difflock not many i no well other than mine (((will agree to disagree on this one))) Quote Link to comment Share on other sites More sharing options...
cipx2 Posted June 20, 2009 Share Posted June 20, 2009 LR stated the 5 km/h value not necessarily because over that speed (say starting with 6 km/h) damage may occur in the diff. They simply used the average max value at which the roller brake testers work and they knew that speed is safe enough for the tests on a RBT. The speed at which these testers work is less than 6 km/h. Have a look at a range of RBTs from SnapOn for example: http://cartec.snaponequipment.eu/category/33684 , the speed is between 3.3 and 5.5 km/h. Others run as low as 2.2 km/h. In my neck of the woods the MOT book doesn't say anything about 4x4s and all of them get tested on RBTs with no exceptions. There's no option of using accelerometers. IMO this specification in MOT manual is a bit obsolete. I don't know of any 4x4, 4WD, AWD etc that can't handle a 5 km/h difference between the front and rear axles no matter the configuration (except locked diffs). Same for the method to test the dampers - ancient practice that doesn't tell much. But given the fact that the dampers are called "shock absorbers" in a MOT book, it's not that surprising You'll be surprised to see how many dampers don't pass the dynamic test on the stand but appear perfectly OK when you push a corner down. The way this should be done is also on a RBT (which would be a combined brake and damper testing equipment). How about the hand brake, does its efficiency get tested? If you can't put a LR on a RBT and you can't test it in motion (as per the handbook - "DO NOT apply the handbrake while the vehicle is in motion as this could result in loss of vehicle control and damage to the transmission") then how do they do it? Quote Link to comment Share on other sites More sharing options...
errol209 Posted June 20, 2009 Share Posted June 20, 2009 How about the hand brake, does its efficiency get tested? If you can't put a LR on a RBT and you can't test it in motion (as per the handbook - "DO NOT apply the handbrake while the vehicle is in motion as this could result in loss of vehicle control and damage to the transmission") then how do they do it? After a heated conversation with a MOT inspector at a certain chain of car accessory shop (that also does parts, servicing and MOTs) who was about to follow the MOT H/B to the letter, he found a small hill ... Quote Link to comment Share on other sites More sharing options...
tychoS Posted June 20, 2009 Share Posted June 20, 2009 After a heated conversation with a MOT inspector at a certain chain of car accessory shop (that also does parts, servicing and MOTs) who was about to follow the MOT H/B to the letter, he found a small hill ... In Denmark the MOT testers are instructed to never put a vehicle with permanent four wheel drive on a brake tester. It's written in their publicly available MOT inspection manual. Instead they take the vehicle for a drive and if they feel the brakes worked ok when they braked hard they will pass it. I have never heard of any measurement device like an accelerometer being used for road testing the brakes over here. Quote Link to comment Share on other sites More sharing options...
Les Henson Posted June 21, 2009 Share Posted June 21, 2009 Turbocharger is right - not only is it a risk/familiarity with all variables of 4wd, but also it's because it's not the recommended brake test for the vehicle. If the tester is using a method not recommended, and then issues a test certificate based on an incorrect test, then it's possible your MOT is not valid So Ralph, you could have been driving a dodgy motor for years Les. Quote Link to comment Share on other sites More sharing options...
Aragorn Posted June 21, 2009 Author Share Posted June 21, 2009 Thanks for the pointers. I knew the MOT manual did say that vehicles with permenant 4wd dont get put on the tester, and assumed this was just a blanket thing so the testers didnt have to have intricate knowledge of wether they can or cant be run on the roller brake tester. My question was more is there any reason why they cant be in this particular case. It seems from this thread, and the quotes from the handbook, show that the centre diff can perfectly well handle the low speeds the roller brake tester puts thru it, which confirmed what i thaught! Cheers. Quote Link to comment Share on other sites More sharing options...
Les Henson Posted June 21, 2009 Share Posted June 21, 2009 This is why MOT stations have a taplow meter - to avoid possible damage to the drivetrain of a 4x4. Les. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted June 21, 2009 Share Posted June 21, 2009 You're forgetting LR's with the BW or Ferguson transfer box, or indeed the Freelander with its VCU. I'm not sure what later models with ETC would do but I can imagine the ETC interfereing with the test if nothing else. Although all of this should technically make no difference on truly open or part-time 4x4, it doesn't take much for something to drag, catch, or jump in the box and you have two tonnes of customer's pride and joy heading off across the workshop. Far easier and safer (and idiot proof) to say that nothing with a green oval should be tested on the road. Quote Link to comment Share on other sites More sharing options...
calle-fas Posted June 22, 2009 Share Posted June 22, 2009 The Swedish MOT has the same policy as the Danish. No single roller brake tests on permanent four wheel drives. I once had a Defender with ETC tested on a single axle roller during the yearly inspection in the military though. No problem whatsoever. ETC on a Defender won't be a problem imo since it only works sideways on the individual axle iirc. Quote Link to comment Share on other sites More sharing options...
Escape Posted June 23, 2009 Share Posted June 23, 2009 I'm not sure what later models with ETC would do but I can imagine the ETC interfereing with the test if nothing else. Back in the days when my Defender still had ETC, it was put on a rolling road a couple of times, without problems. When the wheels aren't driven, the ETC isn't active, instead the ABS is, which at most could mean the wheels not on the tester would get less braking. So I never objected to the car being put on a normal RBT. Different for the Rangies with their visco. And I think my dad wouldn't be comfortable putting his ML on a RBT either. Most (if not all) Belgian technical inspections have at least 1 special RBT, mainly used for multiaxled trucks, which tests one wheel at a time and allows the other to turn freely. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted June 23, 2009 Share Posted June 23, 2009 I was reading the manual of a Nissan Skyline the other day and funnily enough they specify very similar conditions on towing & brake testing to LR. Quote Link to comment Share on other sites More sharing options...
lrfarmer Posted June 23, 2009 Share Posted June 23, 2009 my moter has 2 mot bays one thay test on the roller the other thay use the little black box Quote Link to comment Share on other sites More sharing options...
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