# 'Domed' T-Jet brushes...



## lenny (Feb 29, 2000)

OK. Stupid question time. Why are the old Aurora and newer JL/AW T-Jet brushes 'domed'?

Dan


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## alpink (Aug 22, 2010)

the V in the bottom is to prevent spinning and when the "dome" finally reaches the "sweet spot" the car is wicked fast.


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## lenny (Feb 29, 2000)

alpink said:


> the V in the bottom is to prevent spinning and when the "dome" finally reaches the "sweet spot" the car is wicked fast.


I know why the 'V' is in the bottom... But why a 'dome' on top? why not flat allowing more brush to hit the comm...


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## pshoe64 (Jun 10, 2008)

Less friction maybe. That would be my guess.

-Paul


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## alpink (Aug 22, 2010)

well, it is actually flat on the top and tapered from a narrow surface that meets the commutator. multi purpose. as shoe said, less friction and allowing it to get to that "sweet spot" where the size of the contact patch doesn't create more friction than connectivity quicker. yes, later versions, Magna-Traction went back to flat brushes that had coil springs beneath them and, no, I don't know why.


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## slotcarman12078 (Oct 3, 2008)

I can think of a couple of answers, none of which may be even the slightest bit right...

Hypothesis 1. Having the dome reduces the amount of drag on the comm plate when the brushes are fresh. As they wear, the brush tension is gradually reduced, and the larger surface makes up for it.

Hypothesis 2. It makes the brushes wear out faster, in an attempt to sell pit kits.

Hypothesis 3. Putting the notch in the bottom of the brush makes it sit lower, so the dome raises the height of the brush to accommodate the difference, netting about the same wear rate as flat brushes.


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## lenny (Feb 29, 2000)

if the reason is that the brush has less tendency to catch the edge of the comm as it comes around, thus reducing friction and hop, is it domed 'properly'? Wouldn't part of the flat top still catch the edge of the comm?

The dome is pretty 'severe' and eliminates quite a bit of brush material on top. Why not just slightly bevel the edge along the perimeter?

Or maybe better yet, why not a type of 'mushroom' top where the top is wider than the body of the brush, allowing it to use more of the comm?

Just thinking about brushes since I haven't sourced that yet and really looked at the dome on the Aurora and AW brushes and it didn't make all that much sense...


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## Ralphthe3rd (Feb 24, 2011)

In my opinion, it doesn't make much difference, only the brush material/content makes a difference.
IE- All the aftermarket T-Jet brushes are the old flat(top and bottom) type, and trusted names like JB's, Wizzard and now Slotteck, use the original Aurora (pre Tuff Ones)style, albeit with either high Silver content, or high Copper content.


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## Ralphthe3rd (Feb 24, 2011)

PS to Dan/Lenny, Aurora ONLY used the domed brushes on the Tuff Ones T-Jets and the first A/FX non MT chassis. All original T-jets - including the Wild Ones used the Flat Style brushes- although the Wild Ones Chassis used extremely HIGH Silver Content Brushes.


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## joeslotcar (Mar 12, 2003)

*Brushes*

More brush surface touching the comm plate does not necessarily mean it will work better. Most important is the that the brushes have proper tension against the comm. Too little tension and the brush will deposit material onto the com and form black streaks. Too much tension can abrade the surface and will make grooves in the comm. What you want to see is faint charcoal streaks on the comm. Also, brush composition is a factor is performance.
Here is an article that covers brushes and comm wear:
http://ecmweb.com/content/maintaining-dc-motors

-Joe


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## partspig (Mar 12, 2003)

Speaking from experience, ........ using a larger brush in most HP inline motors, makes them run faster and cooler. I would not apply the information in that article to the flat coms on a pancake style motor as they are a different animal entirely. Two things you can do to increase the performance on a pancake motor are to stop the brushes from spinning and keep the contact patch as large as possible. The reason for the domed brushes is a mystery to me, only the Aurora engineers know for sure. They may have been used to lessen friction on the com, stop the brush from spinning, and keep the contact patch more constant. Who knows........ JMHOFO!!! pig


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## Bill Hall (Jan 6, 2007)

*Be kind to your Arms*

It was someones great hop up idea until they changed their mind. 

My observation is that domed brushes like to cant in the guides. Canting leads to binding. Binding leads to death. The working geometry of the original lever spring only aggravates the condition. Tweaking the comm tension can further alter the brushes thrust angle. The flat top brush is the optimum compromise as the added material contacting the comm tends to right the brush in it's guide throughout it's useable service length. Notably the factory chose to drop the hopped up domed brushes and straighten the thrust angle of the brushes to vertical in the Super ll and the Magna. The original guide and spring arrangement just couldnt handle it anymore. Lever spring fatigue is a common conditon in AFX and modified T-jet. We then saw the brushes change back to a round flat top with centered coil springs and the re-inforced holders. 

Never cared for the way domed brushes wore the comm segments. Sometimes difficult to clean up after the fact. I've got AFX cars that still have domed units in service, but I'm happy to convert them to flat-tops when the time comes; as I consider it to be an upgrade. I do not use domed brushes in any T-jet applications. 

I also consider that wider contact patch to be a larger conductor and for what it adds in friction it disperses in tension across a broader area of the comm segment. 

Pound for pound, around here, where it's always just road racing; the cylindrical style flat-topper brushes allow longer service intervals, ease comm segment wear, and eliminate comm spring fatigue by running cooler. For me, it's about longevity, reliability and piece of mind.


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## joeslotcar (Mar 12, 2003)

partspig said:


> I would not apply the information in that article to the flat coms on a pancake style motor as they are a different animal entirely. pig


Nope. The main point of the article is about having proper brush tension and correct compound to have maximum performance. That applies on both round or flat commutators. 

Getting back to the original question, which is, why were the brushes domed? 
I think Slotcarman12078 is closest with his explanation, especially hypotheses 1 and 3.


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## tjd241 (Jan 25, 2004)

*As a layman...*

I don't know about specific metals and their unique compounds and properties.... As a (so far) Aurora tjet-only guy I can only judge by performance and what has worked better for me. One of the very first things I do when I prep a chassis is to dump out the stock brushes and drop in a pair of ThunderBrushes. I've tried them all. There has never been any other that worked better for me. There is no substitute for consistent quality and an immediate noticeable improvement in performance. IMHO that's the benchmark you should work towards. Sometimes there's a good reason to be a one trick pony. :dude:


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## smalltime (Jun 3, 2006)

joeslotcar said:


> Nope. The main point of the article is about having proper brush tension and correct compound to have maximum performance. That applies on both round or flat commutators.
> 
> Getting back to the original question, which is, why were the brushes domed?
> I think Slotcarman12078 is closest with his explanation, especially hypotheses 1 and 3.


It started with the original AFX chassis. But those brushes were a silver alloy of some sort. They were VERY good when adjusted right, and clean. But they gummed the comm quickly. I used them for oval stuff, but never for road course apps.

As for the JL/Autoworld brushes, I think they simply copied the AFX offering. They aren't bad if you files them to remove the facets. Tyhen sand, then adjust brush springs, then polish, then seat...........Well you get the idea.


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