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Hawk 7 fast after it warms up - any idea why?


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#1 MarkH

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Posted 01 February 2017 - 07:11 AM

First, lets not turn this into yet another $13 Chinese motor thread.

 

I am just curious if anyone else is seeing these motors pretty far off the pace for the first 10-20 laps and then they come up to expected performance. I do not understand what is going on to cause this issue but I have had five of the last seven motors behave this way. One of those had soft brushes and the rest had hard brushes.

 

To be fair, I build cars for another racer and just pull a motor out off the peg board and put one in. Fortunately for him he has been getting the "cherries." They are fast at power on and seem to be lasting a long time. The last one I put in for him was just next on the board and it also had hard brushes and it is very fast at power through the whole of our long races.

 

The last two I put in my cars have been thrown away due to losing too much at the start of the race.

 

Your thoughts...


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#2 dalek

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Posted 01 February 2017 - 09:46 AM

Although it doesn't seem to make any sense, it is possibly hardware related.
 
I had three Hawk 7s that all took about a minute or more to get up to full power, each lap I could hear them revving higher down the straight, and of course, lap times were dropping.
 
I was putting them into a JK Indy car (screw-in mounting), and had put Sonic 10t lightweight pinions on them, mated to a 37t Cahoza spur. I haven't used those pinions before or since.
 
I didn't use any of the motors, but just marked them "slow cold" and put them back in my box.
 
Sometime later, I put one of them in a GT1 RTR car and it didn't have the "slow cold" problem.  
 
I don't positively remember the fate of the other two motors, but I think the outcome was the same, or at least similar (only three or four laps to get up to full power).

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#3 tonyp

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Posted 01 February 2017 - 12:01 PM

When running motor when it heats up the spring arms lose tension and RPM increases. I had a couple of motors that would need to warm up before they ran well.

 

What I did was break in the brushes a lot more which will decrease the tension and it made these motors fast right off the bat.


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#4 slotcarone

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Posted 01 February 2017 - 12:09 PM

A very common problem with FK motors in general. I have not found a solution except what Tony said.

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#5 Taylor Davis

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Posted 01 February 2017 - 12:45 PM

I had a Retro do that for about the first 30 seconds of every heat last month, with three heats to go I had my pit man douse the comm with lighter fluid while it was running between heats and it went away.

 

Maybe just a dirty comm. Or brushes?


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#6 DOCinCocoa

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Posted 01 February 2017 - 04:01 PM

This business of running slow at the beginning of the heat for maybe 30 seconds (it's not real bad) is why I do not cool down the Hawk Retro between heats any more. All of these motors have been the older version of the Hawk Retro.

 

I have not seen this with the 7R version, but I do not have a lot of experience with the newest version. Of the five 7R motors that I have run, they have been very consistent in performance and have not exhibited the slow/fast phenomenon.

 

Are you cooling the motor down between heats?


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#7 Taylor Davis

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Posted 01 February 2017 - 04:11 PM

No, it was the older version, I think, but I'll have to check. I have only purchased one 7R 7R and it's been okay/middle of the road for me with the extremely limited experience with them.



#8 MarkH

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Posted 01 February 2017 - 05:04 PM

The spring tension sounds right to me. Thanks, Tony.

 

I am not using a cooling fan between lanes but the motor does not take as long to go fast after running a lane. This adds more evidence to the spring tension changing.

 

Maybe I can dig out the last one I tossed in the can and see if more break-in will work. These are motors with 7777 acid etched in so I think they are from a current batch.

 

Funny thing is when they are running as expected they are very fast.


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#9 MarkH

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Posted 01 February 2017 - 08:18 PM

I took a good look at the brushes. They are already worn down to around 25% left. This was a brand new motor with brushes that took a bit longer than normal to break in. Not sure why they would have worn down.

However, it took 23 laps for the car to finally get up to speed and could run easy 3.65 on my track. I cooled off the motor and put it back on the track, 4.15-4.20. Sixteen laps later it heated up and was again running in the mid 3.6s.
 
So I took a pick and raised and lower each spring in an attempt to reduce the tension. No change in performance.

I then heated the wire post up with a soldering iron at 420c and let them cook for 3 seconds each. The thought being extra heat might release some tension. Nope, same story.
 
So, I grabbed another motor, broke it in, and mounted it in the same car with the same gears and tires. First laps were in the low 3.7s. After 5 laps easy 3.63-3.64. Let it cool off after the 20 lap run. Put it back on the track and had instant 3.65s and few in the 3.5s without trying very hard. So maybe I finally have a winner.

I know at one point a couple years ago a batch of 10 or so H7 I bought were all bullets except one and it was very good. I guess it is just the law of averages.
 
Thanks for all who gave advise.
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#10 Samiam

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Posted 01 February 2017 - 09:23 PM

I have seen racers running their cars on power supplies between heats in an attempt to keep the motor from cooling off. Is that effective?


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#11 Taylor Davis

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Posted 01 February 2017 - 10:56 PM

I do this in wing car racing to clean the comm on low voltage due to the high amp draw during race power, don't know if it would effect such low drawing motors as much though...

#12 slotcarone

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Posted 01 February 2017 - 11:11 PM

Happens with the 7R motors, too! :)


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#13 Jay Guard

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Posted 01 February 2017 - 11:24 PM

I'm not really convinced that the brush holders/springs are really the cause of the slow/fast phenomena. Generally speaking the characteristics of spring quality brass is not affected much at all by the kind of heat that these motors see. Unlike most steels, brass is work hardened and not heat treated to gain its spring (i.e. hardness) qualities.

 

I tend to like the theory that when the motor heats up it is essentially softening the glue around the comm and allowing the timing to shift slightly. I've got no hard data to back up the "glue melt" theory but to me it seems a bit more likely than the spring tension theory.


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#14 MSwiss

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Posted 01 February 2017 - 11:58 PM

I've had one Retro Hawk that ran that way.

My money is on something to do with the comm connection.

Next time I email my old boss, I'll ask him if he has any insight.

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#15 NSwanberg

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Posted 02 February 2017 - 03:51 AM

When running motor when it heats up the spring arms lose tension and RPM increases. I had a couple of motors that would need to warm up before they ran well.

 

What I did was break in the brushes a lot more which will decrease the tension and it made these motors fast right off the bat.

 

How do you know the spring tension was not getting higher?

 

I have often thought this problem was related deformation of the brush tabs/spring occurring while soldering wires to the tabs that essentially serve as the brush springs as well. 

 

Is there enough experience out there to say it does not seem to happen to racers that use clips to the tabs verse soldering the lead wires to the tab?

 

AJ Hoyt seems to prefer the clips. I don't see his motors suffering this problem as often but I still think it has happened to his motors.

 

I prefer soldering and I try to do it quick!


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#16 MSwiss

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Posted 08 February 2017 - 07:59 PM

Next time I email my old boss, I'll ask him if he has any insight.

 

His reply was "odd."

 

Possibly something to do with the formulation of the brushes.


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#17 MarkH

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Posted 08 February 2017 - 08:17 PM

Perhaps the thought is some materials used to make brushes will conduct better at higher heat. If there is more of that in the mix that makes some sense.


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#18 zipper

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Posted 09 February 2017 - 07:05 AM

Carbon resistance has a negative thermal coefficient.


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#19 MarkH

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Posted 09 February 2017 - 07:18 AM

So negative thermal coefficient would support hotter brushes conducting better or worse?

 

Sometimes engineering terms mean the opposite of what they appear to say.


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#20 gotboostedvr6

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Posted 09 February 2017 - 07:56 AM

I can't think of any material that will reduce resistance as its temperature is increased from room temp. At least nothing that would be used in a brush.


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#21 Brian Cochrane

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Posted 09 February 2017 - 01:11 PM

For educational purpose only, take the problem motor apart and take another motor apart and swap endbells and retest it. And then swap brushes and retest it. See if you can find the problem that way.

 

I think if it's not the brushes or the brush arms/springs, It's probably is the wire connection at the comm. It is not making full contact and when the motor gets some heat in it the contact increases which makes the motor "wake up."


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#22 MSwiss

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Posted 09 February 2017 - 01:59 PM

Even though Stu guessed brushes (note, a guess without much conviction) I still say comm connection, based at how abruptly the motor speeds up.

If the OP has two that he's given up on, if he can send one to JK and one to me, maybe we can get to the bottom of it.

I would take the arm out of the motor and check for resistance, look for anything odd, and hit it with a heat gun to see if any of the resistance readings go down.
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Mike Swiss
 
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#23 gotboostedvr6

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Posted 09 February 2017 - 02:00 PM

I agree, it sounds like a poor magnet wire to comm tab connection. Once it gets hot the metal expands and makes a stronger connection.

Maybe you can sneak a soldering iron in there...


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#24 MarkH

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Posted 09 February 2017 - 04:54 PM

I know they were tossed in the trash. Pretty sure the can has not been dumped and would be happy to send you guys these for expert evaluation.


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#25 MarkH

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Posted 10 February 2017 - 11:29 PM

OK, I found the two motors in the trash. I can send them your way if you still want to investigate.


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Mark Horne

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There are only two things in life that make me feel alive. Racing is one of them.






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