88 replies to this topic

[Shruska55]

Chris,

 

You read my mind. A buddy gave me an old F! chassis he had and I've, as they say, 'decomposed' it. The design was simple and reasonably firm. I'm tearing it apart as the main rails looked torqued/sprung and reusing the tongue, side pans and motor mount. it did have a threaded axle, so I've got THAT going for me too! LOL

 

I bought a Precision Slot Cars jig and have the bits laid out bare.

 

As the Great One once said, "And Awaaaay We Go!"

 

Thanks!

ScottH

[crazyphysicsteacher]

Scott, I will give you one big tip tho. When setting up the rear Axle height, make sure to use the right jigs. Using 812 jigs for F1 does not let you use a 790 tire. Not super important but one of those that can cause a head scratcher.

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[MSwiss]

It does, if you are allowed to use .040" clearance like I think you are able to, racing IRRA® rules, in Retro East.

[Shruska55]

Thanks both Chris and Mike,

 

Timely tips too! I did set the initial axle height at .812. My tech tool does slide cleanly, but since I'm early stages, and as everyone has suggested, I'll experiment. Another buddy has offered a loaner for the next round if I'm not ready when the quarantine comes off.

 

The beauty of this information is that I can apply the scratchbuild techniques to stamped chassis when tuning for loose/firm - bite bars et al. Those I have control over if not the actual build of the frame.

 

ScottH

[crazyphysicsteacher]

Mike, I ran that test at the last retro east race at the race place. We had some jk premium tires that were 790 out of the bottle. The guys who had cars built by others did not clear with the 0.040 clearance but mine that I built with the 790 jig wheels did. It is a very minor thing but can be frustrating when you are at a race and get tires that work from your buddy only to find they won't clear.

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[MSwiss]

Were their tires .795" or .790"?

 

Regardless, then their Can Am and GT Coupe cars wouldn't clear .050", with .810" tires.

 

It all depends on who made the jig wheels.

 

With the corian ones, it doesn't hurt to sand the bottom, a few thou, to give you some wiggle room.

[Dominator]

When it comes to jig wheels I use a pair of 29 crown gears and cut them down on my tire truer to the desired size.  I usually use .710 jig wheels for the rear and .708 for the front.

[crazyphysicsteacher]

Were their tires .795" or .790"?
 
Regardless, then their Can Am and GT Coupe cars wouldn't clear .050", with .810" tires.
 
It all depends on who made the jig wheels.
 
With the corian ones, it doesn't hurt to sand the bottom, a few thou, to give you some wiggle room.

790. The tires were slightly larger out of the tube but not rounds so they all got cut to 790 just to be round. I use the psc jig wheels and had no problem. One car cleared with a set or 793 tires tho. And yes, I measure every set I have to adjust.
Hey Dom, that is a good idea. Just wish I did not need to use them in my coupe cars from time to time.


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[MSwiss]

How do you cut tires down to .790, practice even a few laps, and still make tech, for a .790" minimum?

PS - I didn't think racers bought .790" tires for F1.

Just used, used ones, or ground down .820"'s.

[Shiggy]

If someone want to hear about body pin placement I can dive into that as well.

YES, please.

[JerseyJohn]

Unless a F1 chassis is jigged for .790 tires with .040 clearance it will not clear using .790 wheels on a .8125 jigged chassis. i think most believe that .790 at .040 equates to .8125 at .050 it doesn't . The minimum tire size for a 812 jigged chassis at .040 is .792 <corrected) Not .790' Using 790 tires on a 8125 jigged chassis will drop below ..040 to about .0375. 
Now if you jig for 790 your pretty much stuck with the 790 tires. if you put even 812 on it the rear clearance will be a stout .062.

[Eddie Fleming]

I do not agree John. the difference between .790 and .812 is .022 you lower the car by half that by changing tires. so the car drops by .011. so it is so close to .040 that if you are cutting it that close not passing is on you. 

[MSwiss]

Unless a F1 chassis is jigged for .790 tires with .040 clearance it will not clear using .790 wheels on a .8125 jigged chassis. i think most believe that .790 at .040 equates to .8125 at .050 it doesn't . The minimum tire size for a 812 jigged chassis at .040 is .802

J.J.
Like Eddie implied, your info is incorrect.

Minimum tire size is .792".

Dropping the clearance .010", allows you to use .020" smaller tires.

Tires are a diameter.(visualize a top and a bottom)

Clearance is a singular distance.(visual just a bittom)

My point in questioning Chris is that a seasoned racer, let alone a new one, shouldn't be cutting everything that close on minimum clearance and tire diameter.

[Jim Lange]

If 2 chassis's are identical except #1 requires a .799 tire to tech at .040 and #2 requires a .790 tire. Which chassis is typically faster? 

[JerseyJohn]

[quote name="Eddie Fleming" post="777367" timestamp="1586107427"]

I do not agree John. the difference between .790 and .812 is .022 you lower the car by half that by changing tires. so the car drops by .011. so it is so close to .040 that if you are cutting it that close not passing is on you. [/quote

Like I said .0375 FYI tires are jigged by diameter not radius.i get it I'm the outside guy.lol your killing this.

[JerseyJohn]

That's exactly what I said just a different way Jesus

[JerseyJohn]

812 minus 790 is .022 .
812 minus .022 is .790
812 at 50 = 790 @ 38 as I said before chassis jig at 8:12 will not clear .040 with a 790 Tire more like 038

[Eddie Fleming]

I don't think so John. if what you said in post 81 is the same as what I said in post 82 or what Mike said in post 83 then I don't understand English or math. Now that is possible as I am no rocker scientist.  I will agree that a car jigged for .050 with .812 tires will not pass .040 with .790, it will only be .001 low. I don't see the .375 in this at all. 

 

So where am I missing what?

 

Ok I see your 81 post is .0375 not .375 so we are very close.My bad. 

 

The point is if you allow any kind of room for error in your construction and setup you can build .050 with .812 tires and be fine with .040 and .790. 

 

Like I said if I am cutting it that close it is my fault I fail tech.

[JerseyJohn]

I incorrectly stated the number as 802 which in fact it would be 792 that was a error on my part so given 792 at40 aa790 Tire will give you thirty eight which is what I was trying to say

[JerseyJohn]

I don't think so John. if what you said in post 81 is the same as what I said in post 82 or what Mike said in

post 83 then I don't understand English or math. Now that is possible as I am no rocker scientist.  I will agree that a car jigged for .050 with .812 tires will not pass .040 with .790, it will only be .001 low. I

don't see the .375 in this at all. 
 
So where am I missing what?

My fat freaking fingers I can't type good fast it should be .00375

[MSwiss]

J.J.
You said "The minimum tire size for a 812 jigged chassis at .040 is .802".(edit - I just saw you corrected that)

It isn't.

It's .792".

So jig your car the same way, and run a .792" diameter, and not worry about getting busted for a tire too small.

Ad I said before, who techs their car in, with both tires, exactly at .790"?

Who can scuff them in and have both identical?

[Eddie Fleming]

Glad that's all over.

 

I hope. 

[JerseyJohn]

Eddy I go for 800 .lots of practice

[Dominator]

Calculating jig wheel size for desired clearance is really easy.

 

Take the minimum tire diameter you want to run. Take the clearance and multiply by 2.  Take this number and subtract it from your tire diameter and that is your jig wheel size. 

 

Example: .812 diameter tire - .050 clearance (x2 = 1.00) = .712 jig wheels.

IRRA F1: .790 diameter tire - .040 clearance (x2=.080) = .710 jig wheels.

 

Keep in mind that if the front of the car is lower than the rear you will loose a little clearance at the rear with the angle of the chassis resting on all four wheels.  This is why I use .710 for rear jig wheels on all my cars. 

 

For breaking in tires, wonder tires I start at .007" above the minimum diameter and put about 30 laps on them.  Depending on the track and class I can usually still get two races on them plus a F1 race.

 

Shiggy, I'll post about pin tube/body placement when I get back home tonight as that is going to be a long one lol.

[crazyphysicsteacher]

J.J.
You said "The minimum tire size for a 812 jigged chassis at .040 is .802".(edit - I just saw you corrected that)

It isn't.

It's .792".

So jig your car the same way, and run a .792" diameter, and not worry about getting busted for a tire too small.

Ad I said before, who techs their car in, with both tires, exactly at .790"?

Who can scuff them in and have both identical?

Guys sorry I stepped away, needed to have some margarita time.

Mike, if you run the east coast you know who I am talking about. Jerry will cut the tire to the minimum size and run enough laps to know the car. If it is big, he runs the car til it just passes tech. There is no plan to have to move up from a lower main.

JJ, we both know the green hornet does this and runs the smallest tire he can. That is part of his DNA, no changing it. He does not need to run hundreds of laps to know what his car does. Just like in canam he runs 812. We have both seen him at the tech table watching to make sure it passes and sometimes arguing a little.

Now, with jk tires out of the tube. Some are good at being slightly large so you can cut them round. There are a bunch, tho, that come out of the tube at the size they are listed at. Not only that but they are not even round and still need to have the edges rounded off. Also, those tires bought at that size are the small hub tires which in the past would be 800 out of the tube and give you a little room to trim. Plus, the only class to run those in is F1 because of the grip level needed.
Now, I will run a tire that is just at spec. Why, if I have tested the car and that tire works, no more laps needed. Into tech because I do not want to chance a problem. I also will do the same with my scale cars when I race. Why? Good car means good car. Checking a setup is only a 3-4 lap run, more and I risk wrecking or working working a motor more than it needs too.

Way too many guys get to the track and plant on a lane for way too long. I understand when the track is green but running your race car that much is just a waste of motor. I try to not spend much time on the track, a set of laps trying the car then push the car a little. If I need to put a whole heat of laps on my race car then it is either too far off or I am trying to get a motor to come around.

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