Where to put a mounted spare wheel on my Merc 1823?

[nick disjunkt]

Hi all,

I am in the process of converting my truck into a motorhome and am wrestling with ideas regarding the storage of a mounted spare tyre.

The truck is a Merc 1823 and so has a typical flat fronted European cab sitting over the engine. Below is an old photo showing the cab and box arrangement pretty much how it looked when I brought the truck.

There is absolutely no space along the chassis rails to fit a spare (batteries, air tanks, generator, fuel tanks, and exhaust silencer fill the space) and the back of the truck doesn’t lend itself well. There is no gap between the cab and the box so this option is also out.

The options I feel like I’m left with are:

1. inside the truck
2. On a rack over the roof of the cab
3 On a rack over the box
4. On a bull-bar mounted to the chassis/bumper at the front.

I don’t like option 1, I have limited space in the garage area under the bed at the back of the truck and the spare tyre would occupy most of it.

Option 2 seems reasonable but my cab already has significant excess weight over stock (it used to be a cash-in-transit vehicle) and I was worried about how the cab frame and mounts would deal with the additional weight of the rack, wheel and tyre (probably 250kg/500lb). It would be easy for me to mount a swing hoist to the front of the box. I see this option a lot on the expensive European 4x4 motorhomes from action mobil, unicat, ormocar, etc so I guess it works.

Option 3 seems like a bad idea as all that extra weight would be very high, it would also become the highest point on the truck. The hoist arm would have to be removable or it would stick up

Option 4 seems like the best idea to me but I don’t recall ever having seen it done. Any ideas why this is? The chassis rails are a strong fixing point for a suitable bull-bar mount and the weight would be very low. The wheel would block a little air flow to the radiator but I can’t see this being a problem. I’d have to fit a small winch to lower the wheel and bull-bar before I could tilt the cab but this is less of an engineering challenge than having a hoist to get the wheel and tyre onto a rack. I guess it would extend the length of the truck but I don’t plan on doing much shipping so it won’t be a huge concern.

Any reason I shouldn’t go for Option 4?

Anyone else been through the same design process and settled on a different option?

Thanks

Nick

 

[Bluewater Australia]

Option 2 for me...

I don't know that you'd hit 250kg; maybe 100kg wheel/tyre and 50kg for a tough aluminum rack and mountings. My swing hoist (also aluminum) mounts in the short tube on the front left, just a drop-in when required, otherwise stored deep inside. A small boat trailer winch, Dyneema rope and a decent tip pulley are fitted to it.

The other things up there are 4xMaxTrax and a HiLift in a case. No complaints.

 

[westyss]

Is there something wrong with the back?

 

[FusoFG]

mounting on the front might be okay if it doesn't exceed the front axle GWAR.
-
And it's not just the weight of the spare, but the weight on the front axle will be greater depending on the distance you mount it in front of the axle center.
-
Remember, any weight on the truck that's located in front of the rear axle is adding some proportion of it's weight to the front axle. The farther in front of the rear axle, the larger proportion of weight is added to the front axle.
-
Until it's mounted in front of the front axle, then it adds more than it's weight to the front axle and lightens the load on the rear axle by the extra amount.
-
That's why it's very easy to overload the front axle of a cab over truck. The cab, seats, passengers are much farther forward than a conventional truck.
-
A better spot is at the back, behind the rear axle. Then the weight will help reduce the load on the front axle.
-
If the spare really weighs 500lb pounds I'd be very concered about keeping the weight as low as possible. If you have to move something else higher in the truck to be able to store the spare tyre as low as possible make sure it's something lighter.

 

[nick disjunkt]

Option 2 for me...
I don't know that you'd hit 250kg; maybe 100kg wheel/tyre and 50kg for a tough aluminum rack and mountings.

Maybe you're right. My mounted tyre weighs 120kg (315/80 on a 22.5x9.00 steel rim) and I had planned on making the rack from stainless or mild. I guess making it from aluminium is cheaper (than stainless) but is it strong enough to take 120kg bouncing around on it without massive tubes and loads of gussets?

What truck is yours on? Do you have any photos showing how you tied the rack into the cab structure?

Is there something wrong with the back?

yes. The top half of the back is a fold-up flap to access the bed area and is largely covered with a window anyway. The bottom half is covered by the tail-lift. In a push I could mount the wheel to the underside of the tail lift (the side facing backwards when the tail-lift is up) but I would have to add in new torsion bars to allow me to use the tail lift and the tyre would mean the platform could only get within 315mm of the floor. I'm also not conviced that adding too much weight to the platform is a good idea as it'll stress the hinges and columns but the proportional increase in load is small. It would make raising and lowering the tyre easy as the lift is rated to 1500kg.

Here's the back-end:

and with the tail-lift up when I brought the truck (it is less rusty now!):

mounting on the front might be okay if it doesn't exceed the front axle GWAR.
-
If the spare really weighs 500lb pounds I'd be very concered about keeping the weight as low as possible. If you have to move something else higher in the truck to be able to store the spare tyre as low as possible make sure it's something lighter.

The front axle is 8 tonne and last time I weighed the truck it had 5.5tonne on the front axle so I have a fair bit of capacity left. The mounted tyre is about 120kg but I factored in the weight of the rack.

I'm nowhere near GVW or max weight on the front or rear axle so I'm not rearly concerned about wheight distribution except in terms of keeping the centre of gravity low.

 

[Bluewater Australia]

What truck is yours on? Do you have any photos showing how you tied the rack into the cab structure?

That's on an Australian-made OKA. Fortunately they come provisioned with tapped roofrack holes in the bodywork, so it was a simple mount.

I couldn't see from the photo how I'd do it on your MB, but there would be a way. Aluminium was used to keep the weight down and corrosion away (it doesn't get washed much up there); and it's powdercoated. If you got hold of somebody who knows their stuff with building aluminium roofracks I'd reckon design would be no problem - some 50x25 tube and adequate mounting support. You do have a heavy wheel but I'd be surprised if it ever gets thrown around as much as mine does on a regular basis (check http://www.youtube.com/watch?v=6urG6vRcHTU for some recent stuff) and I've had no problems over the years. But you might need a winch geared a little lower!

 

[westyss]

looking at that rear lift in your picture, I would say 'that' would be the ideal location, unless the lift is used often?? Like you said the spare could be lowered easily to a usable height, and it would be plenty strong to support the weight, easy to make a solid mount for it, keeps it in the trucks slipstream (drag). Not sure if the cab tilts on these trucks, but a spare on the roof would make it much more difficult to tilt. Roof mounted makes for a complicated lift system to get it off there and back on.
I am not sure what you are talking about with a torsion bar?
If its a democracy and we are voting,,, I say on the back.

 

[Bluewater Australia]

I just came across another pic of an MB with a roof-mounted spare - if it provides any inspiration on how to do the roof rack! Not exactly underdone... click to enlarge

 

[nick disjunkt]

looking at that rear lift in your picture, I would say 'that' would be the ideal location, unless the lift is used often?? Like you said the spare could be lowered easily to a usable height, and it would be plenty strong to support the weight, easy to make a solid mount for it, keeps it in the trucks slipstream (drag). Not sure if the cab tilts on these trucks, but a spare on the roof would make it much more difficult to tilt. Roof mounted makes for a complicated lift system to get it off there and back on.

I am not sure what you are talking about with a torsion bar?
If its a democracy and we are voting,,, I say on the back.

Maybe you are right. I had written the tail-lift option off too early but it is the simplest solution. I do use the tail-lift a lot at the moment but that is largely because the conversion isn't complete and I have been using the truck to move equipment around. When the conversion is finished the storage area under the bed will be too small for serious equipment and so I don't think the tail lift stopping a foot off the floor will be a huge problem. The heaviest thing I plan to store under there is my welder and I can lift that onto the platform if necessary.

The platform of the tail-lift is extremely heavy and so there are torsion bars fitted to assist in moving the platform from vertical to horizontal and back. They are basically just bars of steel which twist as you drop the platform, resisting the urge of the platform to slam downwards and snap the hinges. They work in the same way as torsion springs fitted to some vans (Iveco daily front axle). It was previously fitted with three but I removed one as the force was too great and the platform tried to spring back up unless you had some load on it. With an extra 120kg (or 240kg with two tyres) I'll need to refit the one I removed and possibly make them thicker.

I plan to use the platform like a veranda to eat meals on and provide a deck outside the bed area, but provided I get the torsion right, the platform shouldn't be any harder to raise and lower than before.

With regard to the cab tilt, the cab on my truck is fitted with a hydraulic pump (operated using the wheel brace) and so tilting the cab with the extra weight shouldn't be a problem.

I just came across another pic of an MB with a roof-mounted spare - if it provides any inspiration on how to do the roof rack! Not exactly underdone... click to enlarge

The truck is visually hideous but at least I can see how they mounted the rack! The cab is the older SK style rather than the atego/axor cab I have but I'm sure it is constructed in much the same way. The bull bar looks easily capable of mounting a tyre, I'm not sure why poeple don't use them for this purpose?


Thanks for all the help.

 

[westyss]

After you wrote "deck" I thought of Jay Shapiro's build, dunno if you have seen it.

http://www.expeditionportal.com/forum/threads/8936-Eco-Roamer-F650-based-Expedition-Vehicle/page71

 

[charlieaarons]

The bull bar looks easily capable of mounting a tyre, I'm not sure why poeple don't use them for this purpose?


Thanks for all the help.

Increased difficulty tilting the cab??? Since the bar is frame mounted.

Charlie

 

[nick disjunkt]

Increased difficulty tilting the cab??? Since the bar is frame mounted.
Charlie

Most of the big rig bars are mounted on hinges with retaining pins preventing them from folding forward when you brake. You'd just need to incorporate a small winch to help you lower the bar or even use a small hydraulic pump and ram like the Euro trucks use to tilt the cab. It's surely not any more complicated than a swinging hoist used to get the tyre on the cab roof?

 

[ExpoMike]

I plan to use the platform like a veranda to eat meals on and provide a deck outside the bed area, but provided I get the torsion right, the platform shouldn't be any harder to raise and lower than before.

With that idea in mind, I would make a "hinged" mount for the spare that when you have the deck "stored" the tire would be flat against it but when you put the deck down, you would rotate the spare so it would be standing up at the rear of the deck. This way you get almost full use of the deck when you wanted to but the spare would be out of the way the rest of the time.

Just an idea.

 

[RoosterBooster]

... I'm not rearly concerned about wheight distribution except in terms of keeping the centre of gravity low.

you dont state the intended purpose of the truck or where you plan to take it ...
but IMHO after owning and driving several 2WD/RWD trucks in the sandy West i would strongly advise you to keep a watchful eye on the front axle weight.
try to keep the steer as light as possible or it will plow badly in soft ground.
usually a lot of peeps end up with too much weight on the steer tires and too little weight on the drivers.
i would definitely recommend to mount the spare to the rear.
i think that a front "bumper" mounted spare would also block valuable radiator- and (even more important) intercooler air flow.

interesting box ... how heavy is it ??? ... is it armored ? ... bulletproof glass ?
any plans to convert to SRW ? (IMHO dually`s are a pain in the a.. on rocky roads :Wow1: ... wrestling rocks out of between the tires gets old quickly )

 

[nick disjunkt]

With that idea in mind, I would make a "hinged" mount for the spare that when you have the deck "stored" the tire would be flat against it but when you put the deck down, you would rotate the spare so it would be standing up at the rear of the deck. This way you get almost full use of the deck when you wanted to but the spare would be out of the way the rest of the time.

Just an idea.

I think you have the wrong idea of how it would work. There are a few type of hydraulic tail-lifts fitted to cargo trucks in europe but the type fitted to my truck is a standard column lift. The platform can be raised and lowered in both the stowed position or the the horizontal position, and the platform is hinged at the bottom edge to get thr platform from vertical to horizontal. To use the platform as a deck, or to get equipment in and out of the garge I'd use it as it was intended shown below:

To lower the tyre in the event of a puncture I'd keep the platform stowed and would lower it to the ground as shown below:

you dont state the intended purpose of the truck or where you plan to take it ...
but IMHO after owning and driving several 2WD/RWD trucks in the sandy West i would strongly advise you to keep a watchful eye on the front axle weight.
try to keep the steer as light as possible or it will plow badly in soft ground.
usually a lot of peeps end up with too much weight on the steer tires and too little weight on the drivers.

I take your point, although this truck is not an offroading toy, nor is it for expeditions. It is intended to be a home for me, my girlfriend and my dog whilst we travel around the world. Our first trip is to visit my brother in Shanghai (from London, UK) but if finances allow we intend to continue travelling indefinately. We plan to drive through Europe to Italy, from there through turkey to Iran, Pakistan and India, and onwards to China. If we have enough money we will carry on through cambodia and Thailand to Singapore and hopefully ship to Australia. I understand that not all the roads we will encounter will be like those in Europe but tough 4x2 rigid are used in every country I have visited to carry goods and usually more loaded than we will be.

I do not expect to have to drive through much sand although it is not unlikely when we are looking for places to camp, but if I find the front end is ploughing in and getting bogged too often I will fit some 385/65 tyres to the front. mercedes fitted these tyres as standard to the rigids with a 9ton front axle so I know they will fit without any problems. I wanted to avoid it so that I can used 315/80 throughout and not have two different spares but it's a fallback option.

i would definitely recommend to mount the spare to the rear.
i think that a front "bumper" mounted spare would also block valuable radiator- and (even more important) intercooler air flow.

Perhaps you are right. I have not been anywhere hotter than 30deg in the truck yet but even long hills in these conditions have not raised the temperature significantly. The cooling system on most large trucks are totally over engineered these days as the efficiency and emissions are only within spec in a small temperature band. I'm leaning towards the tail-lift option at the moment so I may not have to deal with this issue.

interesting box ... how heavy is it ??? ... is it armored ? ... bulletproof glass ?
any plans to convert to SRW ? (IMHO dually`s are a pain in the a.. on rocky roads :Wow1: ... wrestling rocks out of between the tires gets old quickly )

The box was a cash-in-transit body but I removed most of the heavy armouring on the inside and have recently replaced the heavy steel doors with glass coated plywood units. The box construction itself is nothing unusual, it is a skin of 18mm grp coated plywood over a 50mm steel box section frame. The inside is a 3mm skin of aluminium and a 3mm skin of steel. The steel is rather surplus to requirements but it would be a pain in the *** to remove and doesn't add a huge amount of weight.

All the windows are bullet proof with the exception of the new double glazed windows I have added.

No plans to go single rear wheel. The only places I have ever got debris wedged between the tyres is on building sites and I wont be visiting many of them. The rocks are too small on gravel roads to get stuck. I also like the fact that a rear blowout isn't so dangerous. I may take a look at some 395/85r20 tyres at some point but I'm not certain that they will fit over my disc brakes. They will also derate the 13tonne rear axle. Using a supersingle 22.5 is a bad idea as the profile will be to low and will give a horrible ride.


Thanks for all the advice!