After coming up with the Bicycle Conveyance Contrivance, I needed to do a bit of angle measuring. Andy Ingham had posted asking if I had enough clearance for a steep incline… and I posted back I thought I’d have 35 to 40 degrees. After I’d posted that I started thinking…. and then the doubt set in. So, a few days later as the rain had stopped, I dragged Sue back to the storage compound, this time armed with some straight edges and a combination angle set.
We hooked up the caravan and using my modified jockey wheel handle raised up the jockey wheel to its fully wound in position and then raised it in the clamp to its highest position. The next step was to determine the maximum angle the vehicle could be at before the jockey wheel touched the bike rack.
WARNING: this post contains maths!
To find out what angle the Freelander could be at was fairly simple. All I needed to do was measure the angle between the top of the jockey wheel and the bottom of the bike rack. This would then be the maximum angle the vehicle could be at. The first step was to measure the angle between the top of the jockey wheel and a constant. It was no use using a spirit level, we just needed something that would not change. It was simple. A straight edge clamped on roughly the centre of the pivot point of the ball hitch and the clamping handle for the jockey wheel.
With the constant established, it was a simple matter to use another straight edge, from the pivot point up to the top of the jockey wheel. Keeping this lot from falling off was the hardest part of it! After a few attempts with Sue holding and clamps, I was able to get a reading….
29 degrees between the constant and the top of the jockey wheel. All I needed to do now was measure to the underside of the bike rack….
This time, I needed to use the top edge of the straight edge (spirit level in this case) and measure the angle between this and the constant….This time it was 36 degrees…. so 36 – 29 = 7 degrees…. what! only 7 degrees, that means the Freelander can only approach a 7 degree incline. Wow, this was not good.
I picked a good day to give up “thinking” cigars….
Hmm… 7 degrees… just over double the decent angle for an ILS… so just how high could the Freelander wheels be on a 7 degree incline?
Hammer Maths Time….. The wheelbase of the Freelander is 2565mm and the angle is 7 degrees, so
Rise = tan(7)x 2565 or better put 315mm.
Therefore the front wheels could rise up a 315 mm (12.5 inch) high speed bump and I’d not get into trouble… in theory. In practice, it will always slightly less than this as the pivot point for the Freelander is the rear axle point and the pivot point for the caravan hitch is rear of this, so will move downwards as the front of the vehicle moves up.
Now, a 315 mm high speed hump is unlikely but the ramp on a ferry might not be… a bit more calculation was required here. I needed to double the angle to get at least 600 mm of front wheel rise… so how much would I need to drop the jockey wheel by? Sharpening the old pencil and getting another envelope back…. carry the 1… divide by the cost of Sue’s shopping…. Ah Ha!….
I needed 600 mm at the front wheels so….
base angle = arctan(140/600) = 13.1 degrees
So I needed an angle of 13.1 degrees between the bottom of the bike rack and the top of the jockey wheel, measured at the pivot point… so how much would I need to drop the jockey wheel by?
The current distance between the top of the jockey wheel is…
Rise = tan(7)x 600 = 83mm
To find the new rise using the new base angle…..
Rise = tan(13.1)x 600 = 140mm
So if we take 83mm from 140mm it leaves us 57mm. All I have to do is drop the jockey wheel in the clamp by 57mm and that will give me the ability for the front wheels of the Freelander to rise to a height of 600mm. Fantastic, “Simples” as the Meerkat says! I’ll score a line on the jockey wheel at 57 mm above its maximum height and that will be all I need to drop it too if I encounter anything more than a speed bump. A quick check of this showed I will still have clearance for the jockey wheel above the ground at this level and angle.
13.1 is also slightly more than all the figures calculated for towing. Most manufacturers use a 12 degree incline as the mean for calculating what the tow vehicle can deal with when pulling away. I don’t think I’ll be attempting anything steeper than this!
So I can confidently say to Andy Ingham, that first got me pondering…. I can let the Freelander front wheels climb up to a 600 mm height, an equivalent of 13.1 degree incline. I also did a quick measurement if the jockey wheel was removed, and I could achieve a 28 degree incline, which would mean the front wheels would be 1360 mm higher than the rear’s. Off roading anyone? Cheers Andy.
Right, my pencil is still sharp and I have a bit of space on the back of this envelope… what’s next?
Thanks for reading
Andy Ingham said:
| think as the old joke went something about a constipated mathematician and a pencil and the working it out bit. Sorry but glad the pencil is still sharp and the front of the van will not have a couple of bikes as a figurehead.
Well, it was worth it, at least I now know I’ll be OK…. I hope!
Sometimes, it takes someone to stop and make you think.