# Performance of Brushed Stock Motors



## dhiracing (Jul 19, 2005)

In general ...

What is the effect on runtime if :
- Using a stiffer motor spring on positive when compared to negative;
- Using a stiffer motor spring on negative when compared to positive;

Besides runtime, what are benefits are derived from above combinations?

Thanks,
Mark


----------



## rctazmanmc (Oct 8, 2001)

I think in general that lighter springs will equate to longer run times due to less friction/heat on the com, but produces less torque and higher rpms.

Heavy springs will generate more friction/heat but produces more torque and less rpm.

What type racing are you doing? I run lighter springs for 1/12 stock, but heaviers for 1/10 tc and 1/10 stock truck.

Hope this helps a little.

Also lots of information on this and other websites for this. I think hank has some of Big Jims motor information on this site.

mc


----------



## dhiracing (Jul 19, 2005)

I am trying to fine tune a 21 turn corally sealed stock motor for 10 mins racing with 3000 batteries, always using a 1:10 touring car. These are the current rules for the beginners class in Malta.

I read a lot of info and I might say that some details are really confusing. There are people who say lighter springs give more revs, whilst others exactly the opposite. The more I read, the more I find contradictory matters ...

Where can I read more? hehe ;-)


----------



## tfrahm (Nov 14, 1998)

The best answer to your question is a long one, but I'll try to give you a condensed version...

The earlier post is generally correct..

Where I see people make a mistake is in going too far or not considering their racing conditions... "Lighter" or "stiffer" are relative terms, and not absolutes... 4-cell 12th scale can use very light springs because of the lower voltage, lower amp draw, and the need for 8 minute runtime efficiency. 4-cell Oval needs "stiffer" springs because the amp draw is much higher (only 4 minute races) and power overrides the need for efficiency. 6-cell racing produces higher RPM than 4-cell, so there is a greater demand for "stiffer" springs to minimize brush bounce (and the harm it does to the motor).. Etc., etc....

If there were no friction between the brushes and the comm, stiff brushes would always be better, as they would eliminate brush bounce, but there IS friction... The brushes are passing the current to the armature, but they are also like brake shoes, creating drag and friction in the process... You have to balance all the factors...

A super smooth racing surface allows lighter springs because there is less vibration, etc... Offroad requires stiffer springs because the vehicle really bounces around, and you MUST hold those brushes on the comm...

For your application, the normal spring setup using EPIC style springs would be Red +, Green - ... Both are in the general "medium" category , with the Red being "medium-heavy" and the green being "light-medium" (based on "fiddlestick" measurements). You need to stay as soft enough to get 10 minute runtimes, but that same long runtime and 6-cells means you cannot go too soft... 

Sorry I can't be more specific -- I've never worked on a 21 turn Corally motor, so...?


----------



## me21 (Nov 7, 2003)

I have to disagree, with the last posts, Lighter Springs will give more torque, While Stiffer Springs will give you more RPM's, you biggest factor in making runtime will be in brushes, more then springs, I would run 2 Red Springs, and like a R brush, You DON'T want any kind of serrations because they will increase your amp draw, almost any full face non serrated brush will work.


----------



## tfrahm (Nov 14, 1998)

Lighter springs, in the real world, work only as long as the comm is "fresh" and clean. Years of working on 100's of motors and interpreting the results on a TD45 document this fact. Reduced brush drag increases efficiency, allowing the motor to show more torque on the dyno. However, IN A CAR ON THE TRACK, the added vibration and the contamination of the comm surface (dust, dirt, tire dust, carpet fibers, etc., etc.) quickly kill that effect. Very rapidly, that illusion of torque vanishes and motor performance fades.

Similarly, on the TD45, when measurements are taken at various amp draw values, overly heavy brush springs will make the motor draw more amps. The dyno translates this into an impression of reduced torque and high RPM. However, IN THE CAR, ON THE TRACK, the stiff springs ensure that current flow from the brushes to the comm stays consistent in spite of vibration, overcomes comm wear and can reduce the impact of comm surface contamination. Remember that power in Watts = current squared * resistance... The resistance for a given armature winding is effectively a constant (ignoring hysterisis effects from the switching on/off of the electromagnet the winding represents) -- thus maintaining good current flow ensures more power...


----------



## erock1331 (Apr 1, 2002)

I see both sides having a valid point on the big debate of what heavy vs light springs do for a motor. So I will just stay out of that discussion...

I say give the motor what it wants. I build with Green springs, then lightly press on the back of the brush during break-in. If It picks up only about .02-.03 amp draw then I leave the spring. If it picks up about 2+ amps then that tells me its sprung too lightly, not making good contact or bouncing. So then I add a red spring to that side.

Just another tid bit of info I found.
On our asphalt track this past summer, temps ranging from 70 to 90 degrees outside, I ran red springs on epic motors. Seemed to be really be working, but towards the end of the season I went with 2 green springs and the motor finished a little stronger and did not get as hot. Overall a faster run.

Keep this in mind:
Your car has to be on or around a track record pace, if not I highly doubt you are ever going to feel a spring change. 
I think my car has only been 98% on a couple times in the last 5 years.
Conclusion: If you can feel a spring change you are a better man than I, lol


----------



## erock1331 (Apr 1, 2002)

additonal reading on the subject.

http://www.hobbytalk.com/bbs1/showthread.php?t=124274


----------



## patcollins (Sep 29, 2001)

The only place I could ever tell the difference from a spring change was on the lathe after the race. The lighter springs definately required more cuts on the com to get rid of the "half moons" at the edge of the com slots from the brush bounce.


----------



## tfrahm (Nov 14, 1998)

erock -- You raise one key point... The TRACK is where it matters... A couple of years ago, I had a stock motor in my sedan on the local parking lot races that was strong on the dyno and strong for about 3-4 minutes, then "soft" the last minute of the race... I went up one step stronger on the springs, and it actually "looked" slightly less powerful on the TD45 (but not by much)... On the track, the stronger springs seemed to eliminate some heat buildup from brush arcing -- the motor was strong for the full 5 minutes, and I gained almost a full lap on the track...

"Give the motor what it wants"... 

I like the sound of that -- can it be that simple...?


----------

