Formula Vee Ackermann Angle

[Kronos1]

Hi all,

I've just bought a Formula Vee racing car, and one of the previous owners has moved the steering rack rearwards from its original position.

I'm not sure if his intention was to straighten the steering rods from a loading point of view, and whether or not he considered the ackermann angle when doing so.

According to Race Car Vehicle Dynamics (Milliken), moving the rack rearwards like this will increase the ackermann angle. Could somebody verify that this is true, and perhaps give their opinion on whether or not increased ackermann effect is beneficial (or otherwise) to a Vee?

Please find an image of the steering system at the attached link.

Thanks,

- Kronos1

 

[GregLocock]

Ackerman is a tradeoff. An esteemed contributor here sets his cars to parallel steer on the basis that life is too short.

There are sound arguments in favour of:

negative ackerman (outer wheel steers more than inner wheel)
parallel steer
some %age of the theoretical ackerman steer
100% ackerman steer (if you drive a coach on gravel driveways)
Over ackerman steer.

So you have at least 4 things to think about. If you use the search function you will find previous discussions. Read them and get back to us.



Cheers

Greg Locock


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[patprimmer]

I would be more worried about bump steer than Ackerman on a race car with some real suspension travel.

I would also be more concerned to find out where the opposition was cheating interpreting rules very loosely to find an advantage with their engines and match their efforts

Regards
Pat
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[evelrod]

First thing I would look at is the rule book. FV is a very restrictive class and, in my experience, cheating is pretty much held in ck by the competitors in the class...In the issue of 'cheating', you have NO friends in the class. If you want a competitive class where most entries have a shot at the top, this is the class. Walk away from the competition a couple times and you will find yourself in the 'teardown' bay soon enough. The FV and FF classes are the only ones in vintage that I see this phenomenon (sadly).

Second, the class has been around for over 40 years with rules that have changed little in that time. Not a great deal of difference between a new model and older pre '70 model. Anything and everything you could try, has been...to death!

Something else in the Vee classes in vintage racing, there is no question you can ask that your fellow racers are not willing to answer, loan you parts and tools or, even do the job for you.

http://www.vararacing.com/info/?id=5300

Give them a call. FV # 291, Kim M. is the class representative.

Rod

 

[patprimmer]

Rod

My observations in FV here are that they all cheat, but to the same degree so no one stands out.

They have sealed engines, but I believe that most have managed to duplicate the seals.

Regards
Pat
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[Kronos1]

Thank you for your replies.

Greg, I've had a read of the previous posts regarding ackerman - but none I came across discussed it in the context of a racing car. (Drifting was the closest, but still very different)

Perhaps you could provide some of the arguments for and against negative ackerman, parallel steer and some % of full ackerman.

I don't think that full or over ackerman would be beneficial because of the drag of the inside wheel on fast corners - but perhaps I'm mistaken.

Pat, you're not the first person to highlight the importance of bump steer on a Vee! - it's true they have a LOT of suspension travel. Would you have any advice on how I could produce a bump steer curve for the car?

Rod, where I'm racing the engines aren't always sealed - so I imagine it's a regional thing. The engines also have different specifications around the world - I know the UK engines are more highly-tuned than in some other areas.

Regards,

- Kronos1

 

[patprimmer]

The ends of the parallel trailing arms scribes an arc in one plane during travel, but the tie rod ends scribe an arc in another plane. simply drawing it up to scale at different points of travel will get you pretty close, that is presuming they still use the old link pin, dual transverse torsion bars and dual trailing arm front ends.

The rear ends also bump steer quite a lot, presuming they still use the swing axle rear end.

Regards
Pat
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[Tmoose]

One or several of the Carroll Smith books talk about the different Ackermans' "advantages" and "disadvantages". Some folks consider that info dated.

 

[GregLocock]

You've identified that different ack values might effect different parts of the circuit. Just think it through.

Also bear in mind that toe out is fake ackerman, and that ackerman varies as you turn the wheel.



Cheers

Greg Locock


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[patprimmer]

Also if you are getting roll, the roll or bump steer on a VW design might be more substantial than the Ackerman.
Just say it toes in on bump and toes out on droop, what does that do to each wheels angle relative to direction of travel.

At least on a FV you have equal length tie rods. A beetle sedan had a short and a very long tie rod. With left and right fronts changing toe at substantially different rates and with the rear also changing toe and camber substantially, drifting through undulating corners was ummmm, interesting and required some unique skills to be developed.

I expect FV has some of the same tendencies but to a significantly lesser extent due to equal length tie rods and much less roll and actual travel.

Regards
Pat
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[evelrod]

Greg put his finger on this issue. Some cars need ack to function on a competitive level. My Mini Cooper is one of them. I even use toe out at the front. Turning in a corner under power would require lots of steering input to work without lots of ack built into the front.

As I've said before, I really don't like tuning with ackerman in the front steering on most RWD cars. I've just never found it necessary to have a good handling race car. I have parallel steer in out Lotus Cortina...had it for years...no problems. Much less difficult to set up the car for different tracks...just one more thing to worry over is my opinion.

If you need it, as the Mini does...just deal with it. No matter, you need to test, test, test...In the very particular case of FV, why re invent the wheel if you don't know what want or need?

Rod

 

[GregLocock]

Incidentally when I said toe out is fake ackermann I was serious. A successful race engineer says that if he has time he does a swing on toe, and they decide how much toe a fast, medium and slow corner needs. In an ideal world then they adjust the ackerman to suit, more often just use the best toe for that track.


Cheers

Greg Locock


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