2 replies to this topic

[mariner]

I hopr the professionals here will forgive me a bit of day dreaming which has resulted from the earlier new indycar concepts thread. The pictures on that thread made me start wondering about the potential dynamics and advantages/disadvantages of a five wheel car. By a five wheel car I mean a car consisting of two linked triangular chassis with both chassis having the pair of wheels at the rear. So the front of the rear chassis rests on the back of the front chassis at , or near the lateral plane of that chassis pair of rear wheels.

So it would look in plan view just like a three axle articulated truck but with one front wheel instead of two. Why might that layouit have any advantages?

1) the great benefit of a three wheel chassis is that it need no torsional rigidity since it rests on only three points. So it can be much lighter as only beam strength is required and lozenging is impossible.

The disadvantage is that a three wheeler can have no front/rear roll couple to control under/oversteer etc. However with two chassis each one can have its own roll stiffness at the plane of the two rear wheels.

2) So with a five wheeler you can tune front/rear roll stiffnes without torsional rigidity being needed.

3) I think the biggest POSSIBLE benefit of a five wheeler is that you can completely seperate stering behaviour from cornering behaviour because the only purpose of the front single wheel is to steer the front chassis into the turn. The forces neeeded to do this are minimal but the steering job since gets done.

4) It might be possible to acheive an overall vehicle weight below a four wheeler because there is no torsional rigidity cost and the exstra front wheel could be very lightwieght. Maybe just a Kart wheel on a simple centre pivot suspension strut.

5) There is no need to worry about Ackerman effects ( although equally you could not utilise anti ackerman.

6) the layout might give a better downforce configuration as you have two sets of stable platforms to play with.

7) By careful design and positioning of the middle pivot of the rear to front chassis it should be posible to control the front / rear roll couple by developing a speific roll centre height and movement of the connection point.


I am far from sure if teh five wheeler concept has any dynamic advantages but I could see it being built at a nil or positive weight penalty versus a four wheeler. I appreciate few formulae would permit five wheels but I was thinking of hillclims etc.

Any expert thoughts please



The only article on articualted truck handing is summarised here http://journals.pepu...h5853h2t71j436/

[mariner]

As nobody has yet pointed out a fallacy in my post above I thought I had better explain something I left out to keep the post short.

You can have separate roll stifness for reach chasis and no need for torsional rigidity. What you can't then do is to adjust the front rear roll stifness by tuning each end. That can only be done if you have some torsional rigidity betwen the chassis'. THere are two ways I can think of to do this.

1) Yout can restore the normal torsional stiffness in the rear chassis and fit an anti roll bar across the pivot centre. If I tried this concept out in practice that is what I would do since most single seaters modelled on an F1 car have a high,narrow nose with many linkage mounting points. So you can replace the conventional front suspension by the front tricycle chassis a above and find out just the benefits or otherwise of the fifth steering wheel at the front. If it is no good you can restore the chassis to a four wheeler and recover the costs.

2) There is another way of varying the front/rear roll couple without torsional stiffness. This involves putting the rear chassis pivot on the front chassis in a sliding fore and aft track of, say, 20% of the wheelbase. You then arrange for the pivot to move forward for braking and back for acceleration. this does two things. Firstly it varies the weight distribution to better suit braking and acceleration and secondly the front/rear roll couple is effectively changed by varying the share of rear chassis weight caried on rear wheels of the front chassis.

You could of course just adjust the pivot position fore and aft at rest to just vary roll couple.

I am sure this idea has been tried before but my googling to date has failed.

[Engineguy]

The trailer hitch height would determine the amount of weight transfer from the inner front wheel to the outer front wheel. If it's at ground level; no transfer, if it's up high, lots of transfer. So the front and rear are not as independent of one another as you might think. To eliminate the transfer completely, the hitch would have to be, essentially, at the front-front tire patch; yes? At that point the other two front tires are doing absolutely nothing.

I'm assuming a single spherical pivot somewhere within the triangle defined by the front three contact patches; I think any other location would be a nightmare to keep consistent weight on the steerable (front-front) tire. So its size may need to be same~ish as the other two front tires. I guess the pivot is best put at a point just far enough forward to get consistent weight on the front-front tire, and as close to the ground as you can package it.

You'll end up with a very short wheelbase 200lb three-wheeled race car pulling (more likely being pushed by) a 1000 lb trailer with a 300 tongue weight. I'm not sure what the point is.

Braking:
If only two of the front wheels have brakes you've got the mother of all scrub radiuseses giving steering kickback if there is a traction differential between the two. Also you're losing the braking potential of whatever weight the front-front wheel is carrying. Even if all three front wheels have brakes, the location of the hitch is going to have a major effect on the braking while steering stability, etc.

That's about as much as I want to think about the concept... not feelin' much love for it.