39 replies to this topic

[MSwiss]

8-27 should be good.

Probably balanced would be a safer bet, but, of course, feel free to try it both ways.

[Eddie Fleming]

My stock cars are all 8-27 or 28.

[DavidR]

Thanks to all , I appreciate the help, been back in slot cars a little over a year after 50 year layoff, when my track closed i was racing a classic manta ray and using a cox controller. Big learning curve getting competitive again.

[DavidR]

Hope to make some big races in the future

[Dominator]

Definitely start with 8/28 then try 8/27. A 9 pinion is too much for a hillclimb unless the power is really low (12 volts or lower). Try adding weight in fro t of the motor and at the rear of the rear of the pans equally. Also most importantly, try different bodies. VFC Charger and Daytona are usually the go to bodies. If the cat is chattering it's either a tire rub issue or the body being too stiff.

[tonyp]

In most cases the charger will have more bite.

[DavidR]

Thanks, power at track is 14v, body used is a legal at our track/ not at IRRA 71 merc cyclone.

[DavidR]

Well here are the results, 8 tooth pinion may work on shorter tracks but slowed the car down too much, it would wind out 2/3 down the straight, went to 8-27 same then to 8-26 and it was better but still not fast enough to be competitive, faster motor may do better as to weight 10 grams was way over what was needed. 4 gr spread to outer corners helped a lot. So back to testing,

[A. J. Hoyt]

It's very good that this thread was bumped and updated with new questions and responses. This is responding to the previous posts and is not intended to create any thread drift.

 

My process for selecting all JK true FK can motors including Falcon 7, Hawk 7 and RH motors (pre 7R7R...) involved an on-track break-in for 30 laps (never exceeding half throttle - only do this on an empty track), a full cool-down and then race it and a confirming check on the dyno after its first race. If it got hot during the half throttle break-in, it was not looking very good for that motor.

 

If it was not fast in Can Am, I would try it in a Stock Car using 9-28 gears. If it still wasn't fast, I would try a 8-26 in the Stock Car. I have found several RH's that liked that a lot; one has 9 competitive races on it and is still going strong with lots of motor brush still left!

 

Moral of the story; some motors like pulling a lot of weight with a "taller" gear ratio better than the lighter Can Ams. Oh, and this motor has spectacular numbers on the dyno SINCE moving it to a Stock Car and it definitely is NOT a 7R7R... motor. Just something else to try for those who run Stock Cars semi-regularly.

[A. J. Hoyt]

The way I see it weight distribution and chassis design go hand in hand? If you design/evolve a chassis you design it with just enough weight in all the "right" places ,making it as light as possible and as close to either side of any specific rules regarding weight.

Any extra you have to add is often in the form of lead weight and that is normally stuck ontop of pans,nose pieces etc which is all above the center of gravity.

Get the weight distribution right, build that into your design, so no extra added weight raising the center of gravity , keeping the overall weight down , any "extra" weight is gonna slow you down .

 

Back to the subject matter of this thread:

 

My measurement of a wide variety of Retro Can Am chassis (others and mine) showed a CG location of 1.950 - 2.050 (2.000 +/- .050) inch forward of the rear axle centerline at our local Hillclimb track. As a matter of reference, a recent Bartos Can Am chassis tuned (weighted with lead for the Tom Thumb Hobbies King Track) to be on the long (forward biased) side of the above stated limit, at 2.070 in.

 

My most recent chassis was designed in a CAD system that has very good mass calculations of assemblies to determine the vertical CG location off the ground (as reliable as any method of measuring it such as the tilted chassis cosine methods used on 1:1 cars). With all components (completed ready-to-race - no added lead - seems to run very well without it) the CAD model showed the vertical height to be 0.230 in at 96 grams. (This design used .025 thick full side pans to drop the CG just a small bit over .031 thick brass).

 

Curious, I took the opportunity to weigh a painted GT Coupe body with the interior at 8 grams, estimating the CG location as .656 inches up (in Z) and at 2.156 ahead of the rear axle centerline. I added this component to the assembly and it shifted the CG upward by 0.035 in (0.265 with body) and forward by 0.026 (negligible).

 

Adding lead on top of the pans will lower the CG significantly because it is about as low as you can add it. The alternative (heavier pans at the same total chassis weight) leaves the CG even lower but not by a lot. 

 

If you run at a track regularly and are happy with the way it runs (do not typically add or remove lead), then you could shift the weight down lower by taking the lead off and adding the mass to the pans to lower the CG (arguably, the better solution for better handling) and keep some of the punched-out (approx 1/4 in diameter) pieces of lead around for finer tuning.

 

I may add a post to this thread later after measuring a Stock Car body mass with interior and make a similar edit to an older Stock Car CAD design so we can all get a better sense of why Stock Cars handle like tanks - even at 150 grams running weight!

[Jay Guard]

David:

I've run (and won) with my IRRA Stockcar many many times on Eddie's track and I've almost always run 9/28 or 9/27.

FYI...I run my Stockcar around 130 grams and due to their top-heavy nature I like to run the car a little on the loose side.  If natural tires are too tight and treated are too loose you might want to try one of each.  I've done that quite a few times to loosen the car up just a bit and it's worked well for me.

[A. J. Hoyt]

As promised, the Retro Stock Car numbers (for those who care, also compare to Can Am numbers above):

 

Ready to race chassis without body is 129 grams that brings the CofG lower than a Can Am car in Z axis above ground plane at 0.149 (recall, front clearance is 0.050 in vs 0.015 for Can Am so half of the total mass goes up half the difference, or, 0.018 yet the CofG of the chassis is STILL lower than the Can Am chassis) and the X axis is 2.106 forward of rear axle centerline.

 

Chassis as raced with a 13 gram painted body and interior (assumed 0.956 in vertical and measured 2.056 forward of rear axle centerline) contributes to an aggregate CofG location in the Z axis is 0.224 (about where the ready-to-race Can Am car with body because, frankly, that's what works) and an X axis value of 2.130 forward of the rear axle centerline.

 

In summary, this confirms what we already knew but puts (in my feeble but educated mind) some numbers to it. The CofG of the stock car body is quite a bit higher than the Can Am body. Adding very low weight (at the bat pan level) quickly and significantly lowers the CofG. The surprise might be that the stock car chassis CofG is quite a bit lower than the Can Am chassis. Armed with this information, I am going to try mounting a Can Am body on a Stock Car chassis and see how it handles! Adding lead on top of the pans is just for getting the balance right.

 

Has anyone tried this?

[JimF]

I am going to try mounting a Can-Am body on a Stock Car chassis and see how it handles! Adding lead on top of the pans is just for getting the balance right.

 

Has anyone tried this?

 

 

Yes, I have tried it. Fairly often. I even raced a Stock Car chassis once with a Can Am body on it. If you try it yourself, I think you'll find that the biggest detriment to handling in these things is not at all the 4.5" WB.....it's the body. I also believe that the better weight placement on these cars is aft. Here are some examples currently being worked on and tuned for a couple of flat track races.

 

A pretty std frame for me, 125 gr as shown with no weight. This is too light for a flat track. This chassis has two brass weight tabs (~~ 5.0 gr. ea.) attached to the frame just ahead of the rear axle.

 

 

 

Same frame showing weight placement of about 9.5 gr. The two felt pen lines show the different balance points. The addition of this weight moved the balance point about .130" rearward. This is a good starting point for the flat track and works pretty well. Eventually the 4 gr. bit at the rear of the center plate will probably get split in two and moved to the rear weight tabs which currently hold 2.7 gr ea.

 

 

Here is a different approach. This car weighs 123 gr as shown and unweighted, the balance point is almost identical to the weighted example above. This was done by removing some weight from the nose assembly and reducing weight in the front part of the center weight. This chassis also uses brass weight tabs at the rear (5gr ea.) but in this case, they are on the pans not on the center section. At some point, I'll probably remove them from the pans and flip them to the center section to see what the difference is.

 

Given the effect of the heavy, stiff and tall body, I'm wondering if the weight on the pans might dampen the effect of the body? (don't know, but I'm about to find out I guess) This chassis also has some springs to further dampen the pan movement. I think I probably overdid it with the springs but they'll come off easily enough. This chassis has yet to roll.

 

[DavidR]

Well after taking all the tips on weight placement and switching back to 9/28 gears. I got the cars (4) of them all handling fairly well, then I started fine tuning the guide shims found that a little on the fronts worked best with all of them. Managed to get 2 of the cars turning 5.8 laps in green and two doing 5.7 this was the best times ive had and not that far away from the best times for this track ive seen with stock cars of 5.5. Anyway i was able to use one of the 5.8 cars to win our Saturday night series race tonight with a 4 lap lead over Second place car . Thanks for all the tips here and from between the lanes pod cast. I hope this topic continues lots of good info being shared.

[Eddie Fleming]

Good work David.