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#1 Fred_J

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Posted 23 May 2020 - 12:01 AM

The idea behind the weighted guide flag is ingenious. But if you took the idea to an absurd extreme, could you lower the center-of-gravity of a car to below the track surface of you made the blade of the guide flag heavy enough? You would need to use something like depleted uranium in place of tungsten to accomplish that! Assuming you could get a car like that moving, how does a car with CG below the track surface handle? Could it ever deslot?
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#2 bbr

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Posted 23 May 2020 - 01:07 AM

might depend on how much traction the tires have, you can have so much bite, the car just rolls out of the slot


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#3 zipper

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Posted 23 May 2020 - 04:17 AM

Uranium is lighter than tungsten  ... :crazy:


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#4 Ramcatlarry

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Posted 23 May 2020 - 06:58 AM

That is why magnets are on many type of slot cars. Artificial gravity without adding mass.  If you research the concept of automotive suspension designs, you find it is an attempt to lower  the roll center to improve handling in spite of the center of gravity.


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#5 boxerdog

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Posted 23 May 2020 - 05:21 PM

It would be virtually impossible to get the CG that low by weighting the guide. I also don't think you can move the roll center or roll axis on a slot car although the roll stiffness is a very important thing to consider when building a chassis. JMO. 


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#6 Upfront slot cars

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Posted 23 May 2020 - 06:57 PM

A slot car doesnt have a roll center. Just get the weight as low as possible. Pay attention to polar moment.
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#7 boxerdog

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Posted 23 May 2020 - 07:12 PM

Why wouldn't it have a roll center at the axle centerline, and a roll axis from there to the guide?  It is going to be restricted by an chassis "ears" dragging on the track, sure. 

 

I also think you can move the roll axis horizontally in the rear based on the centering of the motor weight between the rear tires. 

 

But I agree, I don't think the whole concept is particularly useful information when applied to slot cars. 


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#8 Ramcatlarry

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Posted 23 May 2020 - 07:31 PM

It is actually the center of MASS at either axle.  The motor end is usually closer to the axle centerline than the front axle and the weighted flag does help lower that end.  The chassis and its lower mass lowers the roll center the heavier it is. Just basic physics.


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#9 Jay Guard

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Posted 23 May 2020 - 08:06 PM

A slot car doesnt have a roll center. Just get the weight as low as possible. Pay attention to polar moment.

 Yup!

 

...But I agree, I don't think the whole concept is particularly useful information when applied to slot cars. 

Correct, it really only applies to a vehicle with a body separated from the track surface by a suspension.  Take your Retro slot car and place it on a tech block with the guide in the slot.  Now try to roll the car by picking it up from one side or the other.  Guess where the "Roll Center" (for all practical purposes is), it's where the outside edges if the tires meet the track surface opposite from the side you're lifting.


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#10 MSwiss

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Posted 23 May 2020 - 08:23 PM

The idea behind the weighted guide flag is ingenious. But if you took the idea to an absurd extreme, could you lower the center-of-gravity of a car to below the track surface of you made the blade of the guide flag heavy enough? You would need to use something like depleted uranium in place of tungsten to accomplish that! Assuming you could get a car like that moving, how does a car with CG below the track surface handle? Could it ever deslot?

Not close to being possible with existing slot car tracks.

Maybe if you built a track, with no bottom to the slot, and had a blade, a foot or two, deep.

Regardless, I don't see the point.

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#11 boxerdog

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Posted 23 May 2020 - 10:50 PM

Well, I don't agree on some of these points, but it is interesting to hear other opinions. So I'm not throwing rocks at anyone, just putting my notions out there because I enjoy the discussion. So don't get offended or call me names. And I don't think it has much to do with slot cars, nor is it really scientific. If it were, you wouldn't have the amazing variety and innovation that is obvious in almost all current slot car chassis. To me, you guys that design and build the retro cars (over and over) are amazing innovators and thinkers. It's also amazing that radically different cars can perform very similarly in some cases. So I don't think there is just one solution. They just keep getting incrementally faster. So...

 

1. A car, any car, does not need a suspension to have a roll center, it simply makes it adjustable more easily. Weight is transferred from inside to outside around some point. The relationship of that point to the CG creates a simple lever if the CG is higher than the roll center as in many vehicles. A slot car might well have the CG below the roll center, which gives the effect of no roll. It basically wants to slide and/or flex and lift the inside tire. 

 

2. Lowering the chassis mass lowers the CG, but has nothing to do with the roll center. Just like a "real" car. I'm ignoring front axles and wheels and I shouldn't because they are movement restrictors like chassis ears. So if I cut a big hole in a JK 21 and make it a drop-through chassis, I have simply moved the CG down. 

 

3. Somebody talk about tire sizes! What are the obvious differences in 2 identical cars, 1 jigged for .700 tires, 1 for .750??  Assuming all of the gear compensation has been made ant they tech at the same height??

 

Anyway, please chime in. 


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#12 Fred_J

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Posted 24 May 2020 - 12:45 AM

A lot of interesting points! But, if you want to drop the CG below the track surface using a weighted guide flag, wouldnt it work better if the chassis and the rest of the car was as ultra light as possible?
Fred Jespersen

#13 Kim Lander

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Posted 24 May 2020 - 06:15 AM

HHHHHHmmmmmm......very interesting.



#14 tonyp

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Posted 24 May 2020 - 06:53 AM

You guys have been home too long. Lol
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#15 Jay Guard

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Posted 24 May 2020 - 09:14 AM

Well, I don't agree on some of these points,...

 

1. A car, any car, does not need a suspension to have a roll center, it simply makes it adjustable more easily. Weight is transferred from inside to outside around some point. The relationship of that point to the CG creates a simple lever if the CG is higher than the roll center as in many vehicles. A slot car might well have the CG below the roll center, which gives the effect of no roll. It basically wants to slide and/or flex and lift the inside tire. ....

 

Anyway, please chime in. 

 

Dave, et al;

Check out the following link concerning how roll center and suspension are specifically connected in every case.  

 

Roll Center Understood - The Car Techthecartech.com › subjects › auto_eng2 › Roll_Center

 

Nonetheless we both agree that when it comes to slot cars it's at best theoretical and really has no bearing on how they handle.


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#16 boxerdog

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Posted 24 May 2020 - 09:52 AM

Jay, I appreciate and fully understand the info you posted.

The only area where we disagree is that a rigid car still has a roll center. It doesn't just disappear if you stiffen or eliminate the springs, IMO. Not a big deal.

 

Fred, I just don't think you could ever get that much weight on the guide to offset the "high" weight of the motor. Anything you could do to reduce mass above the CG would help, but I don't see the point?

 

Tony, you are right 100%. Time to go work on real cars, the fleet is calling. And some of them need 1/1 suspension work!!!

 

And nobody talked about tire sizes. 


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#17 Eddie Fleming

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Posted 24 May 2020 - 10:37 AM

OK David I will throw out some thoughts on tire size.

 

I am not a trained engineer so you can consider this the babbling of a simpleton.

 

If you change tire size I see two or three things that are effected. center of gravity. gear ratio and weight (rolling weight specifically)

 

The gear ratio can be changed to match the tire roll out and is easy to compensate for.

 

The center of gravity can be adjusted for in many cases with axle bushing placement to get the same ground clearance as another size tire. This does leave the axle at a higher point with larger tires. so you do raise the CG slightly with larger tires even after lowering the car back to the desired ride height.

 

The larger tire will have more weight given equal hub size and the like. Is this enough to be a large difference? I would say it depends but probably not a huge difference.

 

So I guess that big or small tires could work just fine if you make the correct adjustments for ride height and gearing. That is as long as you stay within the neighborhood of what the total car is designed around.

 

I suspect others opinion may differ. 


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#18 Danny Zona

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Posted 24 May 2020 - 11:04 AM

OK David I will throw out some thoughts on tire size.
 
I am not a trained engineer so you can consider this the babbling of a simpleton.
 
If you change tire size I see two or three things that are effected. center of gravity. gear ratio and weight (rolling weight specifically)
 
The gear ratio can be changed to match the tire roll out and is easy to compensate for.
 
The center of gravity can be adjusted for in many cases with axle bushing placement to get the same ground clearance as another size tire. This does leave the axle at a higher point with larger tires. so you do raise the CG slightly with larger tires even after lowering the car back to the desired ride height.
 
The larger tire will have more weight given equal hub size and the like. Is this enough to be a large difference? I would say it depends but probably not a huge difference.
 
So I guess that big or small tires could work just fine if you make the correct adjustments for ride height and gearing. That is as long as you stay within the neighborhood of what the total car is designed around.
 
I suspect others opinion may differ. 

It's off topic but just to add a bigger tire has less wear during a race compared to a smaller one. Just another factor in the formula of a race set-up.
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#19 tonyp

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Posted 24 May 2020 - 12:44 PM

Smaller tire also has a smaller contact patch so less bite all things being equal.


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#20 Upfront slot cars

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Posted 24 May 2020 - 03:20 PM

No need to be upset if you dont agree. You are free to come up with your own ideas with your own testing. Roll Center Actually Moment Center only happens through a series Ive moving parts creating invisible instant centers that then in turn create whats called Jacking Force . For your roll axis theory to make sense to me, you need to have your frame riding on springs on motor of a solid axle to create an axis for the car to roll on. Again this axis is imaginary. A word of advice is to research Go cart racing. This will be more beneficial to you as they deal with a solid frame as slot cars for the most part. Of course this is all just my opinion. My opinions come from a long and successful back ground in stock car racing. Focus on COG and Polar Moment.
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#21 SpeedyNH

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Posted 24 May 2020 - 05:26 PM

we used to spend a lot of time on this back when i used to race 1:1.

i always looked at slotcars as having a roll center at the track surface because that was where the tires and the flag hit. slotcars are front-limited, which kind of overrides any suspension effect, IMO.  and yes, i tried. 

we used to inverse-taper our flags to try to get them to move the center down into the slot, but it didn't seem to do a lot of good because as soon as the car tipped, you were back to the track surface again. the slot width wasn't a whole lot greater than the flag's, so you didn't have a lot of angle to work with. and when the roll angle is zero (as in when sliding), roll center is DGAF.   

lowering the CG below the track as with Mike's weighted guides has to help. 


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#22 Ramcatlarry

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Posted 25 May 2020 - 01:19 AM

Part of the 'roll center' issue only applies when independent swing arms are employed.  With solid axles for most slot cars, the go-kart rules are more consistent.

 

A book somewhere in my library is "Costin and Phips(sp) Racing and sports car chassis design" from about 1960.  It explained all or the early designs to that era.


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