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John Matthews
Joined: 04 Dec 2004
Posts: 1986
Location: United States, Michigan, Williamsburg
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 Posted: Thu Nov 01, 2012 7:29 am Post subject: |
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| Chris Hurst wrote: | | Benn Herr wrote: | The problem is three parts.
First, the power band is "wrong" for karting. It has a reasonable bottom end, good mid-range, strong peak power, and nothing past that.
I suppose you could all agree to run a conservative gear and don’t mess with the tuning. But that’s not really racing is it? |
Here is Renntech's dyno sheet of the XR50
30 ft lbs from 4000-8000rpm is "reasonable" ? Personally I think that is a little more than reasonable.
I was also unaware there was some portion of an engines power band beyond bottom end, mid range and top end. What did you mean by that comment?
As far as "not really racing" is concerned:
Have you heard of spec racing? This could be a great spec class where you put an emphasis on driver and chassis tuning rather than motor tuning. I see what you're saying, but lets be honest...if someone is a 7/10 driver but can make more power than a guy who is a 10/10 driver the chances he can make up ground are much higher than if everything is equal.
Is real racing about winning because of your machine or because you're a better driver? The flip side of your comment in my mind is actually less of real racing as it puts more emphasis on the machine than the driver.
Personally I think the most entertaining races to watch are spec races because you see the psychology of race tactics unfold. Watching the Australian V8 touring cars last weekend or even Spec Miata IMO is much more entertaining than almost any F1 race I can recall in the last 5 years.
If the XR50 was a spec series it would have my vote 100% |
It seems that everyone here has a valid point. I know Benn and if there ever was a series that the Aixro would be good for in the US he's running it. They run with a group called "Pro Autosport", or at least they used to and share track days with lots of sports car folks.
What this argument comes down to though is perceived value for money spent. The XR50 might run for 100 hours, it might run for 300 hours. But, once I buy the motor ~6k and a chassis that's worthy, another 6k I've got 12 large wrapped up in a kart that my friends will make fun of because they think I'm putting around with 8-year-olds. That same 12 grand will buy me a pretty decent Spec Miata, with a trailer, spares, etc. and it'll run just as many hours. Plus I can run it at track days, or if I'm a hotshot I can take it regional or national racing. Sure it won't have the same power to weight but I'll have something I really like in a race car, a rollcage....
If I really want the kart experience I can get a Spec Honda for a lot less money and have a ton of places to race. I've never been to Europe but I haven't heard of them running Spec Honda, or Spec Miata for that matter. I'll bet it makes a big difference when you're in a country where you can get used cars and build them into competitive racers for cheap. Also, we've got a lot of track day folks here that don't really care about sanctioned classes as long as they can drive what they want to.
So, in places like Florida where you can drive year round and there are a lot of kart racers a spec XR50 class might make sense but in most of the US folks that have 12 grand to play with are probably much more likely to go SCCA, or NASA, or mini sprints for that matter. They are cool engines but I still stay with my statement that it's more power than almost any karter needs and more money than they want to spend. The guy with that much money can probably drop 2 or 3 times as much and get one of the formula cars advertised on this forum and be strapped in when he hits the wall at 100mph 
JMHO,
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John Matthews
Heartbeat Power, LLC. |
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Brian Degulis
Joined: 13 Aug 2012
Posts: 413
Location: United States, Florida,
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| Posted: Thu Nov 01, 2012 7:49 am Post subject: |
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To be fair you can't really compare a rotary to a spec Honda. The rotary is a purpose built kart engine. The only fair comparasion would be to an ICC shifter which is very close in cost. In the real world it's diffrent becuase I can buy a used ICC shifter for 1/2 or less of the cost of new. Used rotary's are rare in the US but if you go on the European market they're common and you can get a decent kart with a rotary for around 6K.
The question really is why aren't they common here. For me the performance speaks for itself and the idea that a high RPM rotary engine would require less rebuild/repair than a high RPM recpricating engine makes sense. The only reasons I see is that it may just be to difficult to get enough of them out there without a class. Of course the obvious problem with that is how do you build a class if they aren't selling? Rotax I think did that by creating a class the diffrence is that Rotax can and often is mixed in with TAG. The rotary is so diffrent it's hard to mix them in with anything else.
Brian |
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Justin Martin
Joined: 27 Mar 2012
Posts: 328
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| Posted: Thu Nov 01, 2012 7:54 am Post subject: |
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| I have driven both a spec miata and an RX7 that hangs with hopped up vets and such, on thunderhill, buttonwillow, Laguna Seca, and sears point (infineon). I personally prefer karting. Not only is it cheaper, but the excitement, speed, acceleration, and lateral G's can't be matched racing full size cars. |
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Jim McMahon
Joined: 07 Apr 2007
Posts: 2658
Location: United States, St. Paul,
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| Posted: Thu Nov 01, 2012 8:01 am Post subject: |
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| Brian Degulis wrote: | | To be fair you can't really compare a rotary to a spec Honda. |
Its think its very fair depending on the point one wants to make. My point is that testing and plodding about is one thing. Competitive "looking for that nth degree of performance" is entirely another. This is true regardless of the origins of the motor design. You can run an oldschool FA motor at 18,000 RPM for a 10hrs plus time still turn faster laps than TaG. But get a grid of FA's together and you are looking at 40mins tops on a piston. Racing is different.
I think the reasons it hasn't taken off have been pretty well highlighted. There are a lot of unknowns and some misconceptions surrounding it.
Its not well known, not popularly raced, potentially has more power than people can handle. The initial purchase price is too high for people given these unknowns.
In terms of why do people buy shifter vs Aixro, well some people just like to bang gears.
Again, I love the package, but I'm also realistic about its potential given its current state. It needs some real push behind it to succeed.
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Greg Wright
Joined: 25 Oct 2001
Posts: 2495
Location: United States, Indiana, Clermont
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| Posted: Thu Nov 01, 2012 8:10 am Post subject: |
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| Brian Degulis wrote: | | The rotary is a purpose built kart engine. |
Are you sure about this? The first time that I heard about this engine it was on an ultra-light aircraft and set some record flying near or around Mt. Everest.
This was a couple of years of more before I first saw one mounted on a kart and that was at NCMP in 2004 or 2005. Great performance no doubt but at that time they were having a heck of a time keeping a silencer intact, the crazy high EGT on the rotary was burning through silencers on a regular basis, explosively I might add
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Brian Degulis
Joined: 13 Aug 2012
Posts: 413
Location: United States, Florida,
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| Posted: Thu Nov 01, 2012 10:09 am Post subject: |
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Jim when I said not a fair comparision I ment strictly from a price point. Performance, how it drives is all subjective and diffrent for everyone.
I don't know for sure if it is purpose built I'm basing that on the manufactures info.
I like banging gears to in fact I'm at my shop right now blasting up and down my quiet street testing out a KX125 shifter we just put together. That hasn't been mentioned so far and that might be a big part of it. Being able to change gears adds to the experience.
Brian |
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Jim McMahon
Joined: 07 Apr 2007
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Location: United States, St. Paul,
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Brian Degulis
Joined: 13 Aug 2012
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Location: United States, Florida,
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Michael Boone
Joined: 31 Jan 2007
Posts: 1184
Location: France, Not USA state, Nantes
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Johnny Brooks
Joined: 04 Aug 2005
Posts: 310
Location: United States, Virginia, Arlington
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| Posted: Thu Nov 01, 2012 1:28 pm Post subject: |
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| Greg Wright wrote: | | Brian Degulis wrote: | | The rotary is a purpose built kart engine. |
Are you sure about this? The first time that I heard about this engine it was on an ultra-light aircraft and set some record flying near or around Mt. Everest.
This was a couple of years of more before I first saw one mounted on a kart and that was at NCMP in 2004 or 2005. Great performance no doubt but at that time they were having a heck of a time keeping a silencer intact, the crazy high EGT on the rotary was burning through silencers on a regular basis, explosively I might add |
The Aixro was a kart motor before it was used in a highly modified form to power Bear Grylls over Mt. Everest...supercharged & EFI to compensate for thin air....ironically it was the ECU that failed from the cold and kept his partner from reaching the summit, Bear's motor quit just after he topped the peak....they both glided unpowered back down successfully.
Parajet (company that supplied the parasails) currently sells a rotary engine they call 'Rotron'....it uses an Aixro 'lightweight core engine'.
http://www.woelfle-engineering.com/Produkte/produkte_en.html
http://www.flycyclone.com/performance/
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Gary M Smith
Joined: 24 Apr 2007
Posts: 147
Location: United States, California, Fullerton
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| Posted: Fri Nov 02, 2012 8:03 pm Post subject: |
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I think Jim McMahon said it best, some people like to bang gears.
I moved from 125cc Tag to 80cc shifter, not to go faster, but to bang gears. I am only a tad faster than my Tag times and having blast doing so.
I have driven an Aixro, it was powerful, which led to overdriving into the corners, the owner wasn't too happy  neither were the tires.
Gary
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Chris Hurst
Joined: 27 Jul 2007
Posts: 569
Location: United States, California,
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| Posted: Fri Nov 02, 2012 8:23 pm Post subject: |
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| Gary M Smith wrote: | I think Jim McMahon said it best, some people like to bang gears.
I moved from 125cc Tag to 80cc shifter, not to go faster, but to bang gears. I am only a tad faster than my Tag times and having blast doing so.
I have driven an Aixro, it was powerful, which led to overdriving into the corners, the owner wasn't too happy neither were the tires.
Gary |
Would you say you were conscious of the fact you were over driving in corners during the drive? How would you describe the way the kart lays down the power compared to a TaG or a shifter kart?
Just curious about the comment someone made earlier saying the power band is not suited to karting. After looking at Renntech's dyno I'm curious to get the drivers perspective of someone who has driven one as well as TaG and an 80cc shifter.
What is your opinion of the performance? |
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Benn Herr
Joined: 18 Jul 2001
Posts: 1579
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| Posted: Fri Nov 02, 2012 10:17 pm Post subject: |
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Chris, The power band is poorly suited to karting because it runs you too close to a condition where the engine can hurt itself, not any problem with how it lays down power. It makes great power with a very flat torque curve from 3750 to 10,000 rpm - impressive! A curve like that would work really well with a gearbox. Wind it up, grab a gear, and do it again. The trouble starts when you run it as a one gear machine. With the power the rotary has it will easily run 80 mph on any decent sized Sprint track. Geared to do 80 at 10,000 means it will be going 30 mph at 3750. That's okay if none of your corners are below 30. Many Sprint tracks have a corner or two that are in the 20 to 25 mph range and you will definitely be off the power band. Not a big deal, you're all in the same boat. Until somebody adds a tooth to the rear axle to get a better drive off the slow corners. A little higher rpm and a little quicker into the rebuild/repair zone. And once one person does it, everybody has to do it. The guys over in the UK are asking questions about that exact situation.
Successful kart engines all have a margin of over rev before they get into a problem zone. The Briggs flathead, KT-100, JICA, FA, ICC/KZ all offer a margin of over-rev that the rotary does not. That's a big part of why they were/are popular. It's not a problem if you are willing to live within the limitations the engine has but it is perhaps more of a "Gentleman's" class than a serious racing engine.
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Benn Herr
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Justin Martin
Joined: 27 Mar 2012
Posts: 328
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| Posted: Fri Nov 02, 2012 10:51 pm Post subject: |
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Benn, can you tell me how you came up with the 80 and 30mph statement?
How do you know the gearing for 80 would drop off the powerband at 30? |
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Benn Herr
Joined: 18 Jul 2001
Posts: 1579
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| Posted: Sat Nov 03, 2012 12:21 am Post subject: |
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| Justin Martin wrote: | Benn, can you tell me how you came up with the 80 and 30mph statement?
How do you know the gearing for 80 would drop off the powerband at 30? |
10,000 divided by 80 equals 125 rpm per mile per hour.
3750 divided by 125 equals 30 mph.
3500 rpm shows about 5 hp and 8 lb torque, about as much as a stock clone engine.
20 mph would equal 2500 rpm or probably around where the standard clutch slips to. If you tried to get a clutch to work up around 4000 rpm where the torque really comes it would be really fast - as long as you can keep the clutch from turning all blue and melty!
So the rotary would have a similar problem the TAG engines do. If you spin it, you're going to be pretty slow until you get up to some kind of speed. Then when it hits 4000, hang on.
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Benn Herr
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