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Scooter & rack weight on back axle


bob

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Hi all. I am fitting a scooter rack to the back of my motorhome. Because of the distance between the back axle and the scooter the increased weight on the axle will be more than the actual weight of the scooter. Does anyone know how I can calculate the extra weight that will be put on the back axle? Thanks for any help given. Bob.
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MMM August 2004 carried a comprehensive article on this. Back issue? Also calculate how much weight you will be lifting off the front axle or you could end up having both grip and steering problems.
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Calculators out 1! Weight on back axle Measure the distance from the centre of the rear axle to the enter of the Rack -- DAR Now add together the weights of Rack and Scooter/cycles -- WS/C Weight to add to the rear axle -- W+A W+A = DAR x WS/C
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Thanks RonB. Does anyone have a copy of the mag. please? I didn't have a motorhomes back then therefore wasn't buying the mag. I will buy a back issue if necessary but if some-one could email (or type) a copy of the article that would be great. Bob.
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You will not be far out if you double the weight of the scooter and add the weight of the rack. Its all based on "force X distance" from the fulcrum if you want to do the maths. The back of the motorhome is likely to be 60% of the distance between the axles, plus the scooter will be further out with some clearance. The local weighbridge comes into the equasion if you want to be sure. Back issues of MMM covers it all in an article or two by Mel Eastburn. But X 2 is not far out.
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Guest David Powell
Hi Bob,,,Dig out your old school books and look up "MOMENTS ABOUT A POINT" or put moments about a point on Google, and it will all pop up on there. On a standard British coach built, Clives last statement:- 'Times two' is not far out, and he should know he has been working on his back axle weight for quite a while, and is used to carrying motor bikes.
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Hi why not just use the formula that I gave you?????????????????? It is based on "Moments of Forces Around a Point" I have been teaching and using them for some 25 years No 2 times the weight of the scooter plus the weight of the rack is not correct - it all depends upon the "Distance between the rear axle and the center of the rack The back axle is the Fulcrum or pivot The distance between the back axle and the center of the raclk is the length of the lever Not all motor homes are built to the formula that Clive gives - only SOME of them BA(Hons0.; TEng.; HNC.;FTC.; CGLI Adv.; Cert Ed.
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Guest David Powell
Yes PeteC, I appreciate your concern about precision in your calculations, as during my working life on cargo ships, and later in the marine division of the dept of transport, I had to work moments of force about a point using centre of bouyancy and centre of gravity, both of which had to be precisely in the correct place before we sailed, so I go along with your line of thought regarding all the measurements should be correct. I was just saying that Clives way ers on the safe side, is simple so no mistakes, and no chance of dangerous errors in calculations creeping in.
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Guest Bill Ord
It's highly unlikely that a motorhome with a single back axle,ie not an Alko twin axle or a van with twin rear wheels, Ford or Mercedes will have enough capacity to carry even a light weight scooter. Even with my present van a Rapido 924F with 3850 maximum weight there was only 160kg available at the rear axle when I weighed it in loaded trim.Not much choice of scooters there! Most vans carrying a scooter on the rear on a rack just ignore the limit and hope they don't get stopped this is why most vans fit Michelin camping tyres as they will stand the abuse, check at a weigh bridge first if you're unsure but I think you'll find you're short of capacity.I notice that most garage models have a warning in the garage about maximum weight, the most I've seen is 150kg and remember this is inboard of where you will mount a rack!! Bill Ord
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No it os not in the pursuit of pressision Clive's formula of having 40% or whatever overhang does not hold for quite a lot of vans On mine I only have 1 meter rear overhang I normally work with a safety factor also I also make simple calculations using rule of thumb But as most people just ignor any rules As one ex transport manager who claimed that as he had a Max Train Load of 5500kg it meant that he could load up to 5500kg You meet loadsa people who "think" they knoe better
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Guest David Powell
Well Bob, have you gone off the idea of a scooter on the back? My little old coach built has a 60% overhang so even a push bike on the back is not such a good idea. Have you thought of conditioning your mind to thinking "trailer?" or a rear doored 'van so you can put a scooter inside between all the wheels? Or is that even more off putting? Good luck with your two wheel venture.
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Guest Derek Uzzell
Petec: Now, I've confessed in the past to being a mathematical thicko, but isn't there something wrong (though "simplistic" is possibly a better word) with your formulae? As far as I can see there are 2 discrete downward forces that need to be taken into account - the force relating to the weight of the scooter rack and the force relating to the weight of the scooter. Unless the rack is attached directly to the motorhome's rear end (rarely the case) a good percentage of its hefty framework will extend well forwards beneath the vehicle towards its rear axle. If you just wanted to calculate the extra load being placed on the rear axle by the addition of such a scooter rack, then using the distance between the axle-line and the longitudinal centre of the rack's frame seems a fair compromise. But the weight of the scooter itself acts directly downwards where its wheels meet the 'tray' fitted to the rack, and the moment arm (I believe that's the correct term) will be the longitudinal distance between the scooter-wheels' point of contact on the rack and the rear axle-line. As this distance is bound to be greater than the axle to centre-of-rack figure, for any sort of accuracy one needs to do 2 separate calculations and add the results, otherwise there's likely to be a significant underestimate of the additional axle load. Does this seem right, or am I further revealing my ignorance? (Presumably, if a motorhome has an Al-Ko chassis with trailing-arm rear suspension, one still needs to take measurements to the axle centre-line? Or is it to a lateral line taken through the wheel centres? Or perhaps to a point midway between wheels and axle?)
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Bob Wished you had never asked? If you would like to Email me your address to 'called@btinternet.com' I will send you a copy of the article. It will be comprehensive enough for your needs without giving you a migraine attack. PS. Cannot scan you a copy as said machine is up the creek.
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Hi all! Sorry it's taken so long to get back, been too busy/tired to get to the computer since the week-end. A friend came up with the article RonB mentioned and the wife (my calculating tool!) did the math. She used PeteC's formula first (it was there). They came out roughly the same. We have enough weight space to put the rack/scooter on, and just enough length space to stay within the 60% ruling. Bill Ord, my van does have twin rear wheels. I've had the thing to the local weigh bridge to make sure. I'm not keen on ignoring the law and risking it, it'd be my luck to get caught. I'm now convinced that we can do it, just so long as 'she who must be obeyed' got it right. (She's not often wrong in the calculator department, just neede the formula.) Thanks all Bob.
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Bob:= Glad to find out that my memory is still OK Its some years since I was teaching that subject You did right to ask Just take another check once it is fitted and the van is loaded for the off to see that - then you can move stufff around - Heavy goods forwards Derek:- You seem to want to go to the far end of a XXXXXXXX So you have a mor complex arrangement than a simple lever system where your fixing point is to th chassis somewhere in the center of the vans wheel base Well you COULD use "Beams, Trusses and Struts" to solve the problem You simply apply the force around the fixing point - the force is now greater at the fixing point (more strength required) You then take a moment of force as applied to the chassis from the fixing point to the rear axle - This amount is deducted from that applied to the fixing point The net result is the same as if it were to be connected as in the simple "Moment of Force" calculation that was originally given We used tocarry out caculations on the forces applied tp the groundwhen lifting heavy objects via cranes (both on trucks and make up from tripod legs) also on roof trusses etc The art of being a good teacher is to be able to explain complex problems in such a way as it is easy to understand by your students Little knowledge can dangerous
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Guest Derek Uzzell
PeteC: For a number of years I used to earn my living as a professional IT trainer, so perhaps that's why I'm still a touch finicky about advice being accurate. We used to joke that a concept difficult for a 10-year old to grasp would be well beyond the reach of an adult over the age of 30. Your latest missive has left me totally high and dry but, as you are an expert teacher, clearly this must be due to my moribund powers of comprehension rather than any shortcomings on your part. (Presumably nobody else has trouble with PeteC's explanation?) Loved your last sentence - is this a Zen mantra?
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to peter c I admit to being plain thick. but I,M happy. I need to put an invalid scooter rack on the back of our motorhome.can you help. if it is 6.0 feet from the centre of my back axle to the centre of the scooter rack and the scooter weighs 216lbs. the rack weighs 55 lbs what is the load on the axle. regards phil
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Guest Derek Uzzell
phil: I would be very helpful if you said what make (2006 model-year Auto-Trail?) and exact model of motorhome you own. Given that information a calculation can be made that will provide a reasonably accurate figure not only for the extra weight on your 'van's rear axle but also the 'unloading' that your scooter + rack would produce on the vehicle's front axle.
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Hi Phil I make the total weigh you are adding 270lb 216+55 6x270 = 1620lb or approx 800 KG Now you require the Actual load on your rear axle when loaded fro the off Add the 800kg to that This must not be more than the max rear axle load Derek willwork out the reduction in load fro your front wheels In our case you need to keep load there to both Drive the van AND to Steer How about a trailer for the scooter??? They are not difficult to drive with & you can reverse modern ones with a bit of practise
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Guest Derek Uzzell
PeteC: Surely that can't be right? 800kg is a huge amount. My own calculation based on a notional motorhome with a 3700mm wheelbase produced the following figures: Extra load at rear axle = 184kg (405lbs) Reduction in front axle loading = 61kg (134lbs)
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Derek, It is a autocruise Pioneer renoir 2006 max weight 3500. we got it because it has less rear overhang than our previous one so we could put a scooter rack on the back. when in towns we hope to take scooter of the back and park in normal disabled bays. regards phil
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Guest Derek Uzzell
Ah!!! I find that I haven't got a current Autocruise brochure or a test report of the Pioneer Renoir and the Autocruise website's no help. I also note from the MMM Buyers'Guide that the Renoir is built on an Al-Ko chassis which makes it trickier for me to guess at the wheelbase dimension. MMM says the Renoir's length is 20' 2" and you confirm that the rear overhang is relatively short. I've measured the rear overhang of my slightly longer overall Hobby for a guide and, based on your originally quoted axle-to-rack-centre distance of 6', I shall take an educated guess that a Renoir's wheelbase will be around 3700mm. If this is correct then you can use the weight figures I mentioned previously. If the Renoir's wheelbase is longer than 3700mm then the extra load on the rear axle will reduce, and if the wheelbase is shorter then it will increase. I've recalculated using alternative wheelbase lengths of 4000mm or 3500mm and the results were: 4000mm wheelbase: Extra load at rear axle = 179kg (394lbs) Reduction in front axle loading = 56kg (123lbs) 3500mm wheelbase: Extra load at rear axle = 187kg (411lbs) Reduction in front axle loading = 64kg (141lbs) It can be seen from these figures that even quite a large change in the wheelbase does not produce a major alteration in the axle loadings. As I said earlier, for a really accurate measurement of a rack/scooter scenario one needs to do two calculations. The first would relate to the effect of the weight of the rack itself and this would involve knowing the distance from a (notional) line that runs across the motorhome and connects the centres of the rear wheels (because the Renoir has an Al-Ko chassis with the axle well forwards of the rear wheels) to the 'centre of balance' of the rack. (This is where I think PeteC and I were at cross purposes originally.) Basically, you'd need to shove something under each side of the rack's framework (rather like the pivot of a see-saw) then move the rack backwards or forwards until it's in balance. The 'pivot point' is where the rack's weight is concentrated and, when the rack has been mounted on the motorhome, it's the distance between that pivot point and the wheel-centre to wheel-centre line that needs measuring. (Obviously this presents problems when the rack is already fixed to the motorhome, but, as long as one knows the overall weight of the rack and can see where the heavier parts of its construction are, it should be possible to estimate roughly where the pivot point is. The second datum required would also be based on the line that connects the centres of the rear wheels and you'd need to find the point on the rack's 'support tray' through which the scooter's weight acts. Assuming your scooter is pretty much symmetrical side-to-side, if you placed the scooter on the rack, then dropped an imaginary plumb-line through the scooter's centre line, where the end of the plumb-line meets the rack is the point you'd be interested in. Knowing the distance from that point to the line connecting the rear-wheel centres allows the weight changes caused by the scooter to be calculated accurately and these figures can then be added to the rack-related data to produce overall results. However, as the weight of your rack is not excessive, I reckon that, even if you bothered to do all this extra arithmetic, you'd end up pretty close to the figures I've given above and in my previous reply. (If I had to guess I'd say at around 5% below those figures.) Hope this helps. As PeteC suggests, it would be a good idea to have the overall weight of the motorhome and the weight on each axle checked when the vehicle is in full 'holiday mode' and with the scooter in place. This would allow you to know how much latitude you have with respect to your 'van's maximum permitted weight limits.
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  • 2 weeks later...
Hi all........ Went to Towtal in Stafford, got one fitted, asked him to check the length before allowing him to start. Wasn't cheap at £650 but it will take a bike 'cos that's what I'll get before we go, decided the scooter idea was a bit 'modish' for us (remember mods????)(and they're not much smaller than a bike). Thanks for the help, enjoy yourselves and your vans. Will, no doubt, be back. Bob
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