Weighed my Jeep

[V]

I took my Jeep to a public weighbridge today (fee £10) and got three measurements for how it is typically loaded day-to-day:

 

Weight on front axle = 1120Kg

Weight on rear axle = 1080Kg

Total weight = 2200Kg

 

I am actually quite surprised at only 40Kg difference between the front and rear. If I had a full petrol tank the difference would have been smaller.

[V]

It would have been nice to get a CofG measurement but the weighbridge I went to was an older 'in-the-ground' style. Some of the above ground bridge beam can tilt to measure the lateral weight shift for determining the height of CofG.

 

I have had a think about it and found a reasonably quick way to get an accurate measurement of CofG height at the same weighbridge. I need to make two sturdy wheel ramps that have a top platform deck 50cm high. If these are placed just beyond the edge of the weighbridge so I can reverse up them to get the rear axle in the air by a known amount (50cm). This should create a measurably greater load on the front axle. If the regular front axle weight is measured with the rear axle the same distance from the edge of the weighbridge, I can calculate the CofG height using the angle and weight differences.

 

I started to work out the maths on paper then I wondered if someone had already built a calculator... They have, this one!

 

So now I just have to make some easy to assemble 50cm tall wheel ramps that I can get in the back of my Cherokee, that can be temporarily anchored to the weighbridge railings to stop them kicking backwards. Or perhaps using my front winch with the cable running under the Jeep to stop them moving.

[digger]

Very interesting!  My simple  timber ramps already get me to 12" but their height can be varied with different layers .

Could one  just make a couple out of a piece of used bulk timber from a reclaimer.

[V]

My welding equipment is still in Wales so timber ramps will be easier to make here. I should be able to make them out of C16 framing pine. Each ramp has to support a minimum of 560Kg so if I design them for a minimum of 750Kg I will have some safety margin. C24 would increase the margin further.

 

Some quick calculations...

C16 has a compressive strength of 1.7N/mm2 to 2.2N/mm2

750Kg = 7355N

For a single piece of C16 timber 90mmx45mm

• Cross sectional area = 4050mm2
• Minimum compressive strength = 4050mm2 x 1.7N/mm2 = 6885N (703Kg)
• Maximum compressive strength = 4050mm2 x 2.2N/mm2 = 8910N (909Kg)

Two columns of C16 90x45 sharing a point load across the width of a 35x12.50R15 tyre would support 1406Kg to 1818Kg, only 750Kg needed.

 

Working backwards to find smallest timber size...

7355N / 1.7N/mm2 = 4326mm2 minimum cross sectional area of C16 to support 750Kg

The triangulated frames will be made in two parts, a left and right side for each ramp. Divide the area by two to get the value for one frame.

4326mm2 / 2 = 2163mm2

2163mm2 / 45mm = 48mm

Therefore a C16 2"x2" should be suitable for the ramp framing.

 

Using the BND TechSource calculator the length of the tyre contact patch for my tyres is 215mm.

Assuming that I would use 45mm wide timber column, the beam span between columns being 215mm, the load would be split equally across the columns so half over the first column and half over the second. The gap between columns would be 215mm - ((45mm/2) +(45mm/2)) = 170mm

 

If I set a maximum length for the ramps at 1.5m with the first 860mm being a 30 degree ramp and the last 640mm being a flat deck 500mm off the ground, I would need (1500mm / 215mm) = 7 columns in each side frame and allowing for the pointy bit of the ramp.

 

Each frame 'cell' would be triangulated to distribute the load into the base and. It makes sense to use a 9mm+ OSB3 skin on one side of each frame to act as a diaphragm panel to increase the frame rigidity.

 

Thinking a bit more about transporting the wooden ramps to the weighbridge in my Jeep, perhaps I should make each ramp in two parts that are bolted together. The 30 degree section being bolted to the deck section. Two 2-piece spacer bars between each ramp would constrain the width to match my axle and would help make a quick setup at the weighbridge.

 

Well that's the column calculations done, now I need to work out the beam strength for the ramp's surface. I wrote this post as I did the calculations. Some may be interested in why timber frame can be OK for ramps. The same maths are used if working in angle iron if you have the material strength data for the steel sections you will be using. I will do the rest of the calculations offline.

[digger]

Excellent calculations. very interesting indeed. Thanks

However , I'm not a professional engineer. I just reckoned if the ramps are solid wood which are dry and in good nick they won't compress and should be ok if sufficiently massive. Its worked for me. LOL

[V]

I have had some prior experience doing structural calculations. I converted a 200 year old oak barn into a residence. Half of the original oak structure was rotten. As the barn wasn't listed, I replaced all of the rotten oak that would have to be hidden behind insulation with softwood timber frames. The roof had nearly 4-tons of oak supported on the new softwood frame. Using the same principles for wheel ramps supporting a 1-ton load wasn't an imaginative leap.

 

Here's the initial sketch. More work needs to be done for ramp surface but it conveys the general idea. 

[digger]

I think it looks really good. However it is a lot of work even when one has all the tools.

Obviously my method is crude but its quick and simple. In concept. Take one bulk timber eg railway sleeper .Make one shallow angle saw cut. Two ready to go! Double up until height is required. If its of interest I will post a picture of the one's I use.

What I have learnt from my experience over the years that might help, however you do it , is do not make the slope angle too steep (45 degrees is too much) . I found that even in low range it is difficult to climb because the ramp is not fixed. I put a stop at the front end, its much better for piece of mind! A cross piece at 90 degrees can be used to stabilise as one goes higher. Its not necessary with the YJ but I have done this in the past by cutting a mortice in the base section.

[V]

If it was just for working at home then railway sleepers would have been fine. I have to fit these in the back of my Cherokee and transport them to the weighbridge. I need something relatively light compared to solid railway sleepers. I also need to be able to set them up quickly. The alternative is to find a weighbridge that can measure CofG height where the fuel for the journey is less than the cost of the wood.

 

This ramp has a 30 degree slope. I will put a chock at the high end and put some method of lashing them. My first design used a shallow 'V' depression for the tyre to settle in at the top of the ramp but this lost the ability to accurately determine the change in height for the weight shift calculation.

 

Thanks for the lateral stability idea. I may need to consider some sort of 'A' frame at the end.

[digger]

Yep 30 degrees is about what mine are.

I appreciate your point about weight. Sorry that's my fault. I often speak conceptually, when I'm churning out ideas, which is where the sleeper came from! The bulk timber I've used was soft wood.  I've also chopped up  scaffold boards and doubled  them in twos using screws (could glue as well but I've never had a problem). They are easy to cut with a chop saw. I've raised the height by adding these sandwiches together with longer screws,so adjustment is easy. Even new scaffold boards are cheap but strong. Like you I'm lucky I have materials left over from my building days!

I used the above  when I was measuring the prop angle etc to fit the 8.25. It was necessary to get the Yj dead level and raised more than usual . Not easy ,when a lunatic  prior house owner put in a floor well  out of level in every way!