Trimuter Project 1

[kehollyfield]

Hello group I am in the process of building a Trimuter from plans that I bought from Robert Q Riley I have been on a few yahoo web group sighs and a few members have told me that it has a high center of gravity and can roll over. I want to make a electric version which will weigh around 1550 pounds the gas version is around 900 pounds. Wheel base is 88 inches .length 146 inches. width is 67 inches. height 45 inches. road clearance is 6 inches. turning circle is 25 feet. any ideas would be great I thought about changing the turning circle by maybe changing it to 30 feet with a stering stop. or changing the suspension system. I like the tilting wheel system and I am in the process of reading up on it. I am getting starting a yahoo group about my build progress soon.

 

[GregLocock]

To put my answer in perspective I designed and developed the suspension on two three wheeler solar cars, which had a single wheel at the front for good reasons, but safety and handling weren't prime considerations in that choice.

OK, how much money have you spent so far? It might be a really good idea to throw the plans away and build something else instead, the potential instability of single front wheel vehicles is well known. If you fit the batteries low then that will minimise this problem.

However, if you are determined to proceed, then you need to be aware that limiting the turning circle won't really help except at very low speeds.

Typically when driving at normal speeeds, or even racing, you only use about 1/2 a turn of the steering wheel. Most cars have around 1.2 to 1.5 turns from centre to lock. So you don't actually need much lock to generate the full cornering capacity of the tire.

I'll split the next bit into two parts - answering your question directly, and then some theoretical stuff which may or may not lead anywhere.

1) Limiting the lock is best achieved at the steering wheel, as the forces are smaller, but in practice that is difficult to achieve, so most people just fit a mechanical stop to the steering arm. That's fine.

2) What you really need is a way of limiting the max lat acc of the vehicle. The usual approach is gobs full of understeer. You can do that at the front axle or the rear. At the front you could do this by shifting weight forward, and so try to overload the front tire, or fit a smaller tire. Neither approach is good, and shifting weight around can make things worse. I can't think off hand of anything else that you can do there, mechanically. So, if we work with the rear axle the trick is to make it steer into the curve, using either the suspension geometry, or the suspension bushes, or more likely both. The other option is to increase the roll stiffness at the back (well, the front has zero roll stiffness) and then try and saturate the outer tire. This is a bad idea, as it will cause the car to spin, which might be less alarming than a roll, or it might not be. (our solar cars would spin, even though they understeered most of the time).

The 'proper' solution these days is active intervention, but that is not practical for a homebuilt project.







Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

As a further thought - it seems to me the best way to see if this is a good approach would be to use say the educational version of car sim (see FAQ) and test the three wheeler versus a similar 4 wheeler in the following maneuvers:

elk test
double lane change
fishhook
brake in turn

Although the results would not be definitive (testing uncorrelated models is a black art, and the tire model is not really designed for this), I think the comparison with a 4 wheeler (or indeed a two wheels at the front trike) it would show up any gross deficiency in the basic cg location vs wheel location configuration of the car.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

Last further thought -

the half a turn of lock comes from two numbers, the slip angle at which the tyre generates maximum side force, typically 6-10 degrees, and the steering ratio, typically 15-20:1

so if we multiply those numbers together we get around 90-200 degrees of steering wheel angle to reach full cornering capacity of the tyre. It is more complicated than that, but not much.

One other option, but one that I really don't like at all, would be to add some compliance steer to the front wheel.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

For curiousity's sake I ran step steer on the generic car supplied with CarSim, modified with a single front wheel, and with the weight reduced to give the same corner weights as the original model. I doubled the rear roll stiffness. I had to stuff around a bit to get the model to run but it looks OK now.

With mu set to 1 (not unreasonable) a 20 degree step steer resulted in roll over at 100 kph. The original 4 wheeler was still well inside its linear range at this condition (roughly 0.4 g steady state), in fact it was fine at all speeds up to 140 kph.

Given that the Trimuter has a narrow track it will see this sort of problem in spades. Unless Riley has done something remarkably clever then the handling limits of this car are substantially lower than that of modern 4 wheelers. Say a factor of 2.

This is not rocket science, performance oriented three wheelers have usually had two wheels at the front, to reduce their self-inverting propensity, and set them as far apart as possible.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

If you can get back to me with specific dimensions I think there is a reasonable chance that this thing can be made reasonably safe, for lower speed operation.

I'd need: cg location, and Tire size

I'd like: spring rates, sta bar rates, motion ratios.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kehollyfield]

Thanks Greg I have not spent any money yet but I have located some of the items that I need. have square tubing for my frame looking for a rear suspension system that will best suite my needs looking into small cars and truck types

 

[kehollyfield]

Opps forgot to say something I will take some pictures of the plans showing what they have on them and scan some of the pages in the booklet that came with them

 

[kehollyfield]

I have 60 ft of metal for my frame in 10ft length it looks to be close to a 1/4in thick it is 2 inch steel tubing that I came across and 12 6 volt batteries at no cost to me. I posted links to some scans that I tried to scan to give you more info.

http://www.geocities.com/kehollyfield001/electricframe.jpg

http://www.geocities.com/kehollyfield001/electricframerearview.jpg

http://www.geocities.com/kehollyfield001/specsfrombook.jpg

http://www.geocities.com/kehollyfield001/tmuter_fv1.jpg

http://www.geocities.com/kehollyfield001/tmuter_rv1.jpg

a friend brought up the idea about just using a 4 point suspension going to explore that idea also

 

[Fabrico]

Better keep that tubing low, with a 1/4" wall it will make or break the propensity to self-invert.