Friction drive, tyre against tyre

[JohnAh]

If you want to experiment with hybrid systems, extra ICE's etc easily, one major problem may be how to transfer power mechanically. I've been thinking a long time for a spring-loaded 5:th wheel behind the car, but then it must not only transfer enough power to (from) the road, it must also follow in every pot-hole and bump. It the 5:th wheel should double as a clutch, the swing arm travel must be even longer.

So why not press that 5:th wheel against one (or both) rear wheels of the car instead of the street? If placed right behind the rear wheel, it will be almost independent of how the car's suspension moves. If the 5:th wheel is to double as a clutch (by an electric motor or mechanical link) the movement only needs to be a few centimeters or an inch.

This kind of friction drive will probably slip a lot easier than a 5:th wheel directly to the ground, but fir longer distances in higher speeds the power is transfered more by speed than by torque, so perhaps it can work great?

The 5:th wheel can also be smaller without need for a sophisticated suspension. A smaller wheel means less torque transfer and/or perhaps more deformation/friction losses?

Will there be a hopeless slip in the slightest of rain, mud and snow?
Will there be more friction heat than with a 5:th wheel directly to the street?
Will there be increased and strange tyre wear or even damage? (something chemical with rubber against rubber perhaps)

If this sort of friction drive can be used it will make experimenting a bit easier, and the 5:th wheel may even be hidded inside the fender discretely. It can make a front-drive car a 4x4 (or 4x3) that MAY not get stucked just as easily in the winters.

I intend to use this type of drive system for a tiny Kubota diesel engine with barely enough power to keep my car at a steady cruising speed. My idea is that such an engine kan be kept perfectly within the BSFC sweet-spot for long distances. (brake specific fuel consumption), being way more efficient than a larger engine at part-load. If the tyre-tyre friction drive can have marginal losses, the rest of the transmission can be kept very simple. The soft friction drive will also help reducing vibrations from the quite rough diesel engine.

With a second friction wheel, a 6:th wheel to the car's other rear wheel and a lower gear ration, I can also get a simple backup system that will get my car back home after a main engine failure. Perhaps I'll waste some extra weitht on installing a hydraulic CVT on that 6:th wheel, then I may get a perfect drivetrain for rush hour traffic as well.

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I've been doing this insane Burn & Glide (burn & coast, pulse & glide) driving for two years now, saving incredible ammounts of fuel. This efficiency comes with a price though, in shape of starting/stopping the engine, accelerations and coasting, in cycles of about a minute. Since there is so much variations in speed, I have to stay away from other traffic as much as possible, or someone may kill me in pure road rage one day... This benefits of this crazy driving comes from the fact that most car engines are way oversized for keeping the cruising speed but the extra power is surely needed quite often in short bursts. By covering the two needs by two separate ICE's I think it may be possible to get the best of both.

[JohnAh]

If the tyre-tyre friction drive has a potential to work good, it is off course well suited for ICE-electric hybrids as well. With an efficient electric motor/generator of proper size and a quite small but efficient battery pack it may be possible to make a semi-hybrid that can be combined with the good old Pulse & Glide driving method without getting these annoying variations in speed.

Such a system only needs to keep the car at a steady cruising speed for perhaps 5-10 minutes while the ICE is shut down. When the batteries are nearly drained the ICE is started and the 5:th wheel motor is used to charge the batteries again. This will increase the load of the ICE, helping it to reach BSFC sweet spot without too much acceleration or light but inefficient throttling.

[MobilOne]

I like these ideas!

[oldtamiyaphile]

I think putting your friction drive behind the tyre won't work, it will only take a small amount of suspension movement before contact pressure is lost, or it will take a lot of excess pressure to prevent this. Think about a steep hill start, suspension compresses, drive looses friction, you go no where. You also have to factor in the extra rolling resistance and possible bearing/ suspension wear from fore/aft loads it wasn't originally designed for. You could design all sorts of mechanical methods to compensate for wheel travel, but then it gets pretty complex. Plus in the EU everything you do has to be type approved, no?

The sprung separate wheel on the road is a far simpler idea to pull off, it's been done at least a couple of times.

Far easier to buy something like a Fiat Panda 4x4, remove the drive shaft and add a motor to the back.

I'd consider a small powered aero trailer, you'd gain hybrid capacity, improve aero and perhaps even have a bit more cargo space. Maybe even a swivel wheel single wheel type unit.

[Frank Lee]

Re: Fore/aft bearing wear: forgetaboutit.

I have a BikeBug friction drive assist engine; there are pros and cons to the friction drive. I've found it to be rather sensitive to proper tire pressure and also the pressure of the drive roller against the tire, but some of that was probably just me always wanting the minimum roller pressure that wouldn't suffer noticeable slip. And slip it would if going through rain and puddles. I've thought about car friction drives too and three ideas I had re: wet slip was to run a soft "wicking/wiping roller" ahead of the drive roller to help dry the tire, and also to have the drive roller on the front rather than the rear part of the driven tire as that gives the driven another 180 degrees of rotation to fling contaminants off. Also having the friction drive on a rear wheel instead of front (like my bike) allows for a somewhat cleaner, drier driven tire because it follows in the front tire's track.

[JohnAh]

I don't think keeping constant force between the tyres will be difficult or complicated. The 5:th wheel must off course be spring loaded to follow the car's tyre, but the movement will be much smaller than if the 5:th wheel has to follow the ground.

Mounting the 5:th wheel in FRONT of the car's own rear wheel like Frank Lee suggested is probably good, but such a location is almost impossible on most passenger cars without some serious body modifications (and sacrificing some of the rear seat). Placing the 5:th wheel BEHIND the rear wheel may be possible without any modifications that may upset the authorities/MOT/car approval. The extra engine/motor can be located in the trunk as cargo or on the hitch-ball. Drilling a hole or two for suspension and drive-shaft in the fender will probably not upset any approvals.

If built in a clever way, the 5:th wheel will be difficult to discover along the road and if the entire hybrid system is installed as cargo (hooks and straps instead of bolts and no need for tools) there's not much to say about it for either the police or MOT/yearly test and approval.

For accelerating a car from stand-still through the friction drive will take a LOT of traction in the start, but as the speed goes up the torque will decrease to transfer the same amount of power. -Just like any car powerful enough to spin the tyres in first gear, it will probably not spin in higher gears. If powerful acceleration is needed, the car's own engine is used, not the auxiliary drive.

To minimize friction losses and tyre wear the 5:th wheel can have a variable spring tension, by an electric motor or air bellow. If the swing arm is designed carefully it may even control the tyre-tyre force by transferred torque.

If I get some spare time I will definitely try this 5:th wheel idea out in my 40 year old scrappy Saab. To keep things as simple as possible, the first test will have a fixed gear ratio for highway cruise, where the need to transfer torque is relatively low. I will either try my little 12hp diesel (quite heavy thing) and go for maximum efficiency, or put the friction drive to a more demanding test by the 30hp Yamaha two-stroke snomobile engine with CVT.

[oldtamiyaphile]

Originally Posted by JohnAh

. If powerful acceleration is needed, the car's own engine is used, not the auxiliary drive.

I thought the idea was to downsize the engine so it cruises at BSFC, so it will barely be capable of accelerating at all.

The hardest part will be actually driving the thing, especially with a manual gearbox.

[Frank Lee]

The idea is to use the stock drivetrain to get up to cruise speed then throw it in neutral and shut it down, then activate the little cruising engine. Makes for low HP requirements and low torque transfer requirements via friction drive.

[sandychain]

You could replace one side rear tire and suspension with a scooter power unit. The whole thing is already complete.
Maybe a burgman 400 0r something like that.
Probably only need about 25 to 30 H.P. to keep a small car going.
Post pics when you get it running.

[JohnAh]

Originally Posted by Frank Lee

The idea is to use the stock drivetrain to get up to cruise speed then throw it in neutral and shut it down, then activate the little cruising engine. Makes for low HP requirements and low torque transfer requirements via friction drive.

Exactly! By adding a second engine, barely powerful enough to keep a steady cruising speed, I can run THAT engine at the BSFC sweet-spot most of the time, without having to work engine and transmission as a madman, as I have done now for two years. (with great result but to an awful lot of work)

Any kind of transmission has some friction losses, some more than others. A standard gearbox designed for 4x the power usually needed to keep a steady speed, with all gears rotating soaked in oil, will probably give more losses than just a simple chain-drive or belt with a single fixed gear ratio. When designed for high speed/low torque i also THINK that a simple friction drive with one tyre pressing against another without too much deformation, may have quite low friction. If such a drivetrain should be used from stand-still with multiple gears I don't think it would work very well.

A huge benefit of the friction drive compared to making a more "normal" transmission is that almost nothing needs to be changed at the car itself. The extra engine is attached as "cargo" and it's wheels are never touching the ground so the attachment cannot be called a trailer either.