Tesla Model 3 Towing Dinghy

I wanted to get an idea of energy usage involved in towing Custard – a 14′ Wanderer sailing dinghy – so I could work out our range when towing the dinghy.

A Better Route Planner provides an easy way to work out how to get somewhere with charging stops along the way. You can also tweak your car’s energy usage. Their base figure is nominal usage at 65mph.

Custard’s trailer has higher drag that I would expect – in my Mazda 6 it looks like petrol consumption goes up by 25%. This doesn’t appear great for something that is fairly aerodynamic (boat shaped) and not particularly heavy (I guess around 340kg). When I used to tow a Jaguar 21 – 1100kg plus trailer – it seemed to double petrol consumption so for something much smaller 25% seems high.

Anyway, first guess was that the energy consumption in the Tesla would go up by 25 – 33%. The nominal energy consumption per mile is around 255Wh/mile at 65mph according to A Better Route Planner. This seems close to what we get on a normal local journey. So this would mean consumption of around 320 – 340 Wh/mile.

We hitched up the trailer and set off for a tour of some of the faster roads round here. With a trailer max speed was 50mph most of the time, but there were a couple of dual-carriageways where we could go 60mph.

Overall on the trip we did 20.9 miles at 305Wh/mile in 35 minutes. At times the energy graph on the screen showed very high power usage, generally when going up a slight incline. The average speed of the test drive was 34mph – a long way below 65mph used by A Better Route Planner. From eyeballing the Tesla’s energy screen I think the guess of 340Wh/mile is reasonable. At any rate we should be able to get to where we need to go on holiday in the summer without problems.

The test run did show up a few problems with the trailer that needed to be sorted out:

  • The trailer was very jerky, particularly over bumps. I suspect this was due to the boat bouncing on the flexible trailer and launch trolley. I’ve added more straps, replaced some old tubing that was used as a cushion between bits and generally tried to make everything more rigid.
  • Some of the bolts holding the trailer together weren’t as tight as they should have been. Ooops. Now tight.
  • I still couldn’t see why the trailer was so draggy. We’ve got a box trailer with lid that you really don’t notice towing. Ok, so it is rigid so doesn’t bounce, has much bigger wheels and a hard top. However I couldn’t see where the difference could be.

Toe-in

So I had a look at the trailer wheel alignment with the aid of a couple of bits of aluminium angle, bungy hooks and a tape measure.

Aluminium angle for measuring wheel alignment

The aluminium is across the tyre. I think this is valid as the tyres should closely reflect the axis of the hub as long as the aluminium isn’t across any writing on the tyre.

The tape measure can then be clamped onto the bar and read on the aluminium angle on the other wheel, fore and aft of the axle.

Reading off the distance between the aluminium angle bars

This showed that the difference was about 6mm or 1/4″ – rather a lot, although the right toe-in is hard to define.

Aside: toe-in is where the wheels are closer together in front of the axle than behind it. There is always some play in a system so the toe-in means that when moving the wheels end up parallel. What we don’t want to happen is the wheels ending up toe-out as this could be very unstable, making the trailer swerve across the road.

Front-wheel-drive cars often have toe-out when stationary as the force of the wheels pulls them forwards to parallel, but undriven wheels are toe-in when stationary so the drag pulls them parallel.

So the conclusion for this trailer – toe-in is good as it keeps the trailer stable. We definitely don’t want toe-out under any circumstances, but we want the wheels as close as possible to parallel when moving as this minimises drag.

However, this trailer has rubber torsion suspension where the pivot pin is located by rubber that also acts as spring and shock absorber. So the pivot and wheel will move around a bit. Plus the hubs are taper-roller bearings and these must not be too tight, which in practice means there is some movement of the wheel on the bearings. So with a flexible system like this how much toe-in is correct?

The suspension units on this trailer are bolted on, so I tried loosening the bolts to see how much toe-in control I could get. The answer – not very much but slightly better than before. I think the toe-in is now about 3-4mm, which given the slop in the system is about as low as I’d want to go.

Wheel weight and tyre pressure

Torsion rubber suspension isn’t refined so a lot of the spring has to come from the tyres. My tyres are rated at 268kg when pumped up to 60psi. However, I don’t think the boat & trailer weighs that much.

If the tyres are too hard then the boat could be damaged. If too soft then the tyres could get too hot. If the tyres have inner tubes then excess movement due to low pressure could cause a blow-out which would be dramatic. Fortunately these tyres are tubeless so I can run them at lower pressure without worrying too much.

I did try measuring the weight at each wheel with bathroom scales but the scales top out at 120kg and the jack was still taking some weight. So I’m going to guess and say that the load at each wheel is 160kg. This figure could be way off – hard to know – but it seems a reasonable guess.

Doing the maths,

Thus I’m trying the tyres at 35psi. The key is to check the tyres after a few miles – they should get slightly warm but not hot. If they are heating up too much then I’ll pump them up a bit more.

Hitch weight

I measured the hitch weight with my bathroom scales – about 20kg. This could probably be a bit higher and means I must only load more stuff into the boat ahead of the axle.

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