# Fray Arms



## bolts69 (Dec 24, 2009)

Do you really need 16 ohms on all 3 to have a screaming car?


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## alpink (Aug 22, 2010)

resistance balance as well as static or dynamic weight balance are incredibly important.
the ideal arm would be dead on 16OHMs on every pole, but those are more difficult to find everyday.
all rotating parts being properly balanced, shafts straight and friction minimized are the absolute goas of any racing machine.
brush and pick up shoe tension are apparently a matter of personal preference but there are many who hold to one philosophy.
I think if you go to any sanctioned event for these types of cars and talk to the top racers when they are not busy, they are more than happy to tell you the "secrets" to building 'screaming' cars.

I am NOT one of those!

LOL


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## smokinHOs (May 30, 2006)

Bolts.. I'm relatively new to the "Fray" tjet racing scene but I can tell you that drivers prefer different motors depending on the track design and driving style. I have found that cars have different characteristics like brake or coast, torque, or top end. I have cars that don't dyno well but are excellent on the track and other's that dyno much higher but I can't keep the rear end planted. All part of the "fun" of spending hours and hundreds of dollars to make a car run like a tyco you just took out of the package. But - as others before me I have been pulled to the fray side of things because the racing is a blast and it seems that the racing "design" has lived on despite a hobby that seems to barely hang on at times.

Anyway- back to ohms. The vast majority of my arms ohm between 16.0 and 16.8. Most are within 0.1 difference. I agree with Alpink that there is so much that goes into these cars that I would just start with a nicely balanced closely ohm'ing arm. Anything from 16-17. You can always change the arm in the future. Just remember that good gears, proper brush tension, shoe travel, magnet strength and orientation (etc) all go into these not so swiss army knife slotcars. 

Enjoy the journey, it never ends.. BUT neither does the fun.

-Marc


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## Rich Dumas (Sep 3, 2008)

Ohms by itself does not tell you how well an armature will perform, it is probably more important for each pole to measure nearly the same as it is to get right down to 16 ohms. A lot of the late production arms are a real mess. I have seen quite a few where the stacks or the commutator was not centered. If the stacks are then ground the arm will be out of balance for sure and it will take a gob of epoxy to get the arm balanced. The hitch there is that each pole will have a different mass of iron, the pole with more iron will become a stronger magnet when it gets energized, so magnetically the arm will be out of balance. If the commutator is not centered the timing will not be the same for all three poles. Worse yet I have seen commutators where the segments were not the same. The best builders start with arms that are nearly perfect, some builders are not so fussy, so their arms are likely to me mediocre at best.


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## Boosted-Z71 (Nov 26, 2007)

I agree with Rich, that ohm number is way less important than an equal electronic balance, Another thing to mention is the flatness of the com plate and the perpendicularity to the shaft and stack. I dont like to grind stacks due to the above mentioned reasons. The other thing to easily overlook is the electrical connections, they must be surgically clean, and any friction or magnetic drag is your enemy. 

One other thing if your dyno results don't mimic your track results, your dyno is spitting out poo, I ran into this a long time ago and built my own dyno with an adjustable load setting to duplicate what the car is seeing on the track. If its a dog on the dyno, it should be a dog on the track & likewise for a screamer. 

I have built a few fast cars, I dont get to race as much as I would like but I have benchmark cars that I bought from some of the top builders and they are my measure to compete against, on the dyno and track, a dyno will tell you almost nothing on handling unless you have bent parts. 

I have several Fray style builds that are faster than there judging brothers and built my share of dogs, but with my notes on what component parts should blueprint and measure too, I can build an out of the box Fray style car in about 7 hours and predict the performance pretty dang close. Trust me I have a ton of data and patience in tuning is crucial. 

There is nothing better than waxing an inline with a t-jet or AFX maggie

And Al you have told me a couple of things that I let soak in 

Boosted


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## slotking (May 27, 2008)

forget ohms, I have had fast 16 ohms arms but more often than not my faster arms are closer to 18.
I do not even check or even ohms any more.

I simply test out arms in the chassis using a tachometer setup i made, That way i do not have to take gears off all the time. 
I then put all the high rpm arms in a group and retest for how fast the get up to speed.

even this ths not the tell all answer in that 2 fast arms may not show the needed torque.

There is no substitution for testing


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## Rich Dumas (Sep 3, 2008)

I agree that at the end of the day all that matters is what an armature will do on the track. Still it is nice to be able to at least weed out the real stinkers. Some dynos are just simple tachometers, in order to get a better idea of what an armature will do you need one that can apply a load. One way to do that is the have the car's wheels spin a second motor. If you short the terminals of the driven motor that will apply a load to the drive motor. If you have a potentiometer across the driven motor you can vary the load.
Some years ago I saw an interesting post on a long defunct slot BB. A person drove one motor with a second motor and looked at the output of the driven motor using an oscilloscope. For each rotation three pulses would be generated and those should be equal in amplitude, if not there would have to be something wrong with the armature.


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## alpink (Aug 22, 2010)

Boosted-71, LOL all day.
I doubt I can tell you anything about building fast HO cars.
especially pancake motors.

folks, Boosted-71 sent a car to Jim Sgrignioli whom hosted a contest for the fastest drag car within certain parameters.
I was there for the testing.
Boosted-71's car was in a class by itself.
now, Jim did a tune up on many of the cars after their initial runs and improved them all.
still no competition for the Boosted-71 entry.
and, I am pretty sure that Jim didn't attempt to "tune up" that car.

so, all the information you gotten here from the real roundy and straight line racers is valid and completely applicable.
time and patience .... trial and error .... and make notes af what worked and, just as important, what didn't work.

I'm sure there are some more comments on this subject and I am anxious to read them all.

:laugh:


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## Rich Dumas (Sep 3, 2008)

If you race your cars you have to check the ohms to be sure the armature is legal. If your car is underperforming it is sometimes difficult to tell if that is because of a dud armature. If you do have a fast car you could put a new armature into that, which would eliminate other variables like gear mesh, motor brush pressure and pickup shoe adjustment.


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## Tjet Fun (11 mo ago)

New guy here....
I am getting back in this after years away. I know Im posting in an old thread but would like to offer my two cents.
What Ive learned about testing searching out arms is....
Free rev zing um off a transformer regardless of ohms. After I give um a minute to seat the brushes on a 9v battery, I then give um a 2sec zing on an old track transformer. You'll know when you've found a special one!!! 
I have been on a quest to build(for home drag racing) a pure stock Tjet sticking to most clubs 16ohm rule. (A personal goal more/less as there are no clubs near me and its been close to 20yrs since Ive been down an actual 1:1 strip) and I need an outlet LOL 
So ive been scouring ebay and other places for stock unmolested tjet arms. I have arms ranging all sorts of ohms. All things equal lower ohms should mean a stronger arm..... But for example.... My VERY best arm measures 16.1/16.1/16.0 it screams like a dremel on high. I have another that ohms the same, that feels like a dentist drill when you zing it and it revs like a turd. I have one that measures 15.8/15.8/15.8 thats so/so a bit better than average but blah. Ohms are only part of the equasion. My 2nd best arm for the record measures 16.8/16.8/16.7
A trick I learned(maybe reading here??) and I now 100% believe in, without fancy expensive tools, is to spin the arms like a top on your table/bench. My two best will spin and spin and spin(I have to chase um around the table as to not drop um) my middle ground ones spin a bit then topple and my duds you cant get spinning at all. Its in the balance(and comm flatness too).
And also the chassis/topplate and arm need to work well together...my very best arm that SCREAMS, screams in its chassis....I zinged it in another matched topplate/chassis combo using the same pair of magnets and it clearly was missing a hair of horsepower/torque. My guess is slight variances in the chassis molding, making the magnets spacing/airgap slightly differant.
Anyways get your best few chassis, match up topplates for um and get your best few arms and keep swapping stuff around and taking notes(or piles of parts and mental notes like me LOL)


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