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  1. #1
    Join Date
    Dec 2013
    Location
    Sydney
    Posts
    201

    Default Narrow trailer stability

    Hello,

    I'm building a towable bandsaw mill, which is in effect a long thin trailer. The bandsaw head runs on inverted Vee rails which sit on top of the main 100x50x3 mm RHS chassis rails. So the "trailer" frame width is set by my saw head track width. This means I've got 950 mm outside dimensions for the trailer frame width. The mill is 6.5 m long (5 m rails plus a 1.5 m drawbar). The "trailer" parts including the main frame will weigh ~290 kg. The saw head will weigh 260 kg. The plan is to register the trailer for an ATM of 1000 kg, leaving 450 kg of payload (cut timber or a log). The trailer will have mechanical over-ride disc brakes. The springs are 5 leaf, 45mm, slipper springs (rated for 1000kg)

    The frame width of 950mm, and a standard 50 mm gap from frame to inside of tyre, is dictating an axle length of 1340 mm or 53 inches. For comparison, a small 6x4 box trailer uses an ~ 1650 mm or 65 inch axle. So I'm 150 mm, 6 inches narrower on each side than a 6x4 box trailer.

    My question is will this narrower width lead to stability issues when towing at freeway speeds? A moderate proportion of the weight; trailer main frame, cross members, timber load, etc. is low down right on top of the springs. The saw head has a 45 kg engine and heavy bandwheels that will sit about 600 mm above the trailer frame. Most of the time I'd be towing it without any timber loaded, so the weight would be ~550 kg.

    Any thoughts on stability? Should I be looking at welding sister frames outside the main frames to increase the axle width?

    Thanks,
    Graham.

  2. #2
    Join Date
    Jun 2010
    Location
    Wimmera
    Posts
    96

    Default

    "Should I be looking at welding sister frames outside the main frames to increase the axle width?"

    Graham, I am certainly not an engineer, but through experience, I would suggest a wider frame.
    That is a long trailer for the width you propose.
    What about making a separate frame with axle, guards etc that can be attached to the main frame with "U" bolts, like you see on a lot of boat trailers. That way you can adjust the position of the axle to suit the weight distribution. No guessing before you weld.
    I can take a photograph if you are desperate, but I will have to drive to where my boat trailer is stored.
    Let me know.

    Hooroo.

    John

  3. #3
    Join Date
    Feb 2003
    Location
    formerly from Sydney (north of The Harbour), NSW, Oz
    Age
    68
    Posts
    306

    Default

    Like John, I'm not a vehicle engineer either, but I suppose you have read these requirements:

    Rear overhang (ROH): No more than the length of the front load carrying area, or body, ahead of the rear overhang line, or 3.7 metres, whichever is the lesser.

    Drawbar length (DBL): Measured from the centre of the axle/axle group to the centre of the drawbar coupling pivot point with the drawbar level, must not be more than 8.5 metres.


    As far as I know, the longitudinal weight distribution in respect to the axle is as important, if not more important, than the total trailer mass and its height relative to the axle. I'm sure there's a youTube vid demonstrating that the centre of mass should be in front of the trailer axle.
    regards from Canmore

    ian

  4. #4
    Join Date
    May 2011
    Location
    Murray Bridge S Aust.
    Age
    71
    Posts
    5,942

    Default

    I like Johns idea to make it a little wider, you need to keep the weight down low, as at highway speeds side winds can cause problems. Ideally there should be a weight of approx 50 - 60 kg weight on the drawbar, so you'd need to work out the correct placement of everything to achieve this goal.
    A sketch of your thoughts/proposal would be good, preferably with measurements.
    Kryn
    To grow old is mandatory, growing up is optional.

  5. #5
    Join Date
    Aug 2009
    Location
    East Warburton, Vic
    Posts
    357

    Default

    Rather than using springs, use a torsion axle, they can be wider than the frame. Have a look at the Woodmizer sawmill, these are setup exactly the same as you’re proposing
    Cheers

    DJ

  6. #6
    Join Date
    Feb 2003
    Location
    formerly from Sydney (north of The Harbour), NSW, Oz
    Age
    68
    Posts
    306

    Default

    regards from Canmore

    ian

  7. #7
    Join Date
    Dec 2013
    Location
    Sydney
    Posts
    201

    Default

    Thanks for your replies everyone.

    John, at this stage I'm committed to building the trailer parts directly on to the saw bed frame because I've oversized that frame so it can take all the spring loads etc. and also so it's stiff enough to take log loads and keep things flat when I'm sawing. Adding a secondary frame would add too much additional weight. I'm not concerned with getting the balance right because I can adjust where I place the sawhead to get the optimum towbar down load. The towbar on my vehicle is rated for a 75 kg down load, and I'll adjust the sawhead position to achieve close to that to give me the best possible stability.

    Ian, thanks, I'm aware of the trailer design regulations, I romp it in on those overhangs etc. I'm acutely aware of the potential stability issues with long trailers, hence my questions here.

    Kryn, yes, sidewind stability, and sufficient margin from getting the sways up, definitely need to be considered for this trailer. I'm stuck with the long dimension because of what it is. But I can easily place the axle where I need to, and also adjust the sawhead position, to get the weight distribution at optimum. It's whether a wider track will be needed that is concerning me.

    Several commercial mills do what I'm doing, ie. ending up with a relatively narrow track, with apparently no adverse effects. I guess I'm just double checking, and wondering if anyone has any direct experience with narrow and long trailers??

    Graham.

  8. #8
    Join Date
    Dec 2013
    Location
    Sydney
    Posts
    201

    Default

    Acco,

    That's a good idea, thanks. I've used Flexitors in the past. Does anyone know where to get rubber torsion suspensions in Australia? A google search brings up importers and wholesalers, but no retailers. I'll keep looking.

    Graham.

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