[astro]

Guide position has been a mystery to me and caused me to ponder.

I have some cars with the pivot point on the back of the guide, seen one on a website with the pivot near the middle, and most pivot at the front. So this is the first puzzle.

The second point is where the guide and or pivot point is positioned. I am guessing the pivot point has the most effect on handling; scalextric seem to put this 5mm or so in front of the front axle (assuming this is the best reference point!) Carrera F1 ferrari has it about 5mm behind the axle, and people seem to say this has an adverse affect. Some ninco and others have the guide much further in front etc etc.

Is there an optimum position? what difference does it make and why?

Just curious, I like to know these things,

thanks

Dave

 

[GRAH1]

Yeah astro basicaly a slot car runs as a tripod with the three points of contact being the guide and the rear wheels . I the gide pivot is further forward the car will seem as if it is accualy longer and will respond smoother and be less twitchy
A shorter length betwen the back axle and guide pivot will give a twitchyer car wich changes direction quicker probably more suitable for the shorter tighter tracks .
There is a train of thought that says slot cars should have all 4 wheels on the deck but weve argued to death over the past 40 yrs so I say run what suits your own track or what works for you. but the guide lead thing aplys to both styles of building

 

[Doug]

Take a look at the Fly Racing Series article that Swissracer produced.

There are issues there regarding the placement of the guide pin and it does seem to make a difference.

 

[Russell Sheldon]

The "guide-lead", i.e. the distance measured from the centre-line of the rear axle to the centre of the guide pivot post is a critical dimension. Just moving the pivot point one or two millimetres forward or backwards has a profound affect on handling.

Here is an extract of an interesting article that was published in Model Cars magazine, October 1969, which probably actually explains why tripods handle better!

How far in front of the front axle should the pivot of the guide-flag be?

A simple experiment is worth doing to get the 'feel' of this. Fig. 2 shows a split-axle complete with wheels soldered to a strip of Meccano.

Using a nail as a pivot, as shown, if the nail is in position (1) close up to the axle, you will be able to steer the axle round the nail quite easily, but note in doing so that one wheel rotates in the opposite direction to the other; hence the use of a split-axle in the experiment. If the nail is moved to the dotted position (2) well away from the front axle, you will have difficulty trying to steer the axle around the nail; you will have to force the tyres to skid in a sideways direction. To give them a chance, round their edges with sandpaper if they are not already rounded.

If the nail is in some intermediate position, you will feel a medium resistance to steer-ability and you are getting close to a useful position for the pivot point.
The moral of all this is that (i) if the pivot is too close to the front axle, the car will tail-slide a lot wagging side to side up the straight and broad siding round the bends; (ii) if the pivot is too far in front of the front axle, the car will tend to tip over round the bends.

In getting the 'feel' of the resistance exerted by the front wheels against drift when cornering, remember that a 6 oz. car in a fast corner is subject to a centrifugal force of about half a pound. So push around quite a bit while doing this simple experiment and try to judge the anti-slide, anti-overturn control exerted by the front tyres.

In general, the more weight distribution you have towards the back of the car, the further the guidepost will need to be in front of the rear-axle; in 1/24th scale, you may have as much as 1" for the fast commercial tracks. But do remember to round off the edges of the rear tyres, to prevent 'tripping over' and erratic behaviour on corners.

Kind regards,

Russell

 

[Mecoprop]

The distance from center guide pin to center back axle is commonly known as the "guide lead" in USA, although it's not a very obviously descriptive expression.

Where the guide pin is situated in relationship to the blade depends to a large degree on what convenient space is available under the body to accommodate the pin and its attachments, including the all-important wires. There has to be room for them to swivel without snagging. Some modern guides eliminate the wires with metal tracks of various kinds substituting for them, but most of these provide other problems in the process.

Nose overhang forward of the front axle will obviously have a large impact on where the guide must be pivoted and the need to not have any part of the guide extending forwards of the car's nose also may reduce the choice as to where to mount it and what type it must be, whether trailing, leading, or something in between those two.

 

[astro]

great info everyone, thanks! As I suspected, there is a lot to it... and I am still digesting this info

 

[cheater]

I have a different theory about guide lead and rather than type a lot (it's late here in the States), let me post a link to a thread on the Old Weird Herald forum where I posted my theory a few years ago:

Slot Car Dynamics

Simply stated, a slot car is basically a damped pendulum oscillating around the guide flag pivot axis and guide lead is one parameter that affects the period of the pendulum.

 

[bwaminispeed]

QUOTE Simply stated, a slot car is basically a damped pendulum oscillating around the guide flag pivot axis and guide lead is one parameter that affects the period of the pendulum.
Now that,is brilliant,and exactly what I have allways asserted,but never quite was able to express it that way.

I never did,nor still do not hold with the standard tripod theory that most believe.

I allways build my cars as Quadrupeds,with the front wheels fully suporting the weight of the front end.I never let the guide suport the car,and use it for guiding the car only.

And,I guess this car is totally doomed,as the guide pivot point is about 5/16"of an inch BEHIND the front axle.

Swissracer's Bently Chassis

You will have to ask Swissy how this car handles.It was an absolute rocket on my track.

 

[cheater]

Al, you do simply beautiful work and I am very sorry that you could not make it to Vegas.

Please take the time to read my posts in the thread that I posted the link to, as there is more info regarding my thoughts on the pendulum topic. It has been a very useful concept for me, in terms of understanding what factors can be used to achieve a better handling slot car. In my view, the front wheels act primarily as anti-roll devices, though their contact with the track will also have an effect on the pendulum motion.

Now, I simply HAVE to hit the rack. It's 1:00 am here and the alarm is set for 5:00 am . . .

 

[Swissracer]

It is one beautiful chassis to behold AND run ! It prefers smooth track and I have not tried it on routed yet - looks abit wierd running with just the chassis and I have to admit (might as well do it sometime) that I screwed up that 1960s Pyro body

I couldn't get the bodywork clean enough and the paint would not stick well.

However the good news is I finally found an Airfix Bentley Kit so will cobble the two together and finally do the car justice.
I am going to keep my promise to Al and get it into THIS years Marconi.

 

[howmet tx]

That all makes wonderful sense, but I can't help but say cars with the guide behind the front axle look very weird going through the corners! Generally try and avoid it myself, but ANYTHING is permissable when it's a Bentley! Keep us posted with that one, Swiss!

 

[astro]

bwa - fabulous chassis! Am I missing something tho, cos in the pictures it looks like the pivot is in front of the front axle?

Cheater - fabulous thread-link, thanks. Tho the dr starting the thread says you can't have understeer on a slot car, and by his definition (oversteer=rear wheels sliding out, understeer=front wheels sliding out) this is true (apart from deslots!). However, I have noticed negative oversteer - ie rear wheels tracing a smaller arc than the curve of the road, as opposed to a wider arc when the back end is flung out.

Just found the pendulum bit,... reading!

cheers

Dave

 

[bwaminispeed]

QUOTE Am I missing something tho, cos in the pictures it looks like the pivot is in front of the front axle?
That is because most of the pics are with the guide plate removed to show the chassis details better, the guide is just hanging loose.

Look at the very last pic at the bottom and you can clearly see the guide pivot is well behind the front axle.

 

[astro]

ah yes, missed that! goodness me

 

[cheater]

Astro wrote:

"I have noticed negative oversteer - ie rear wheels tracing a smaller arc than the curve of the road, as opposed to a wider arc when the back end is flung out."

First of all, we need to agree that this discussion about oversteer/understeer applies only to non-magnet cars.

My suspicion is that the behavior you describe above would have been caused by a chassis that was not in basic alignment. Basic alignment means that the rear axle is parallel to the bottom of the chassis and at 90-degrees to the chassis longitudinal centerline, with the rear tires of equal diameter.

In general, if these three conditions are not present, the car will not move directly down the slot when power is applied on the straight and in the corners you will see odd behavior as well.

 

[astro]

this behaviour (negative oversteer, or something a bit like understeer) occurs in a lot of my magnet cars when driving around corners at VERY slow speeds, slower than cautious.

A magnet car can have oversteer, positive or negative, this is a geometrical principle. But as you imply, very different factors may be invlolved compared to non magnet cars

 

[Tropi]

I'd suggest that the rear wheels must always trace smaller circles than the corresponding fronts - unless the rear end slides some. This would apply whether mag-assisted or not, although obviously MUCH more likely to occur without that assistance than with it.

 

[bwaminispeed]

This effect is very noticable with almost all cars when trying to negitiate 180 deg of Scaley inner curves.If you don't have the inner aprons,the rear inside tire will drop right down the hole so to speek.

To get around these tight iner corners,you really have to dive in nose first,and give a good shot of throttle,to make the back end slide round,if properly done,the car will be perfectly straight on exit,and you can shoot straight off into the distance.

All very spectacular looking with the long wheelbase Scaley F1s with no magnets.