[Allracing]

Hello all

1st time poster from overseas with a sick idea / pipedream.

I am planning a race bike build to compete in the Nordic "Supermono Cup". We are talking about a road racing class equivalent to the AHRMA "Sound of Singles" in the US, only difference being that displacement is capped. Basically everything is allowed as long as the engine is a 4 stroke naturally aspirated no more than 800cc (~49") engine that runs on pump gas.

I am planning a steel trellis chassis with high end suspension and mag/CF wheels.

Big question is what engine to use for this build.

Competitive bikes in this class fall into 2 categories:
- 450cc MX engines with 125GP like chassis (read: very small bikes, extremely lightweight. Looking at down to 220lbs wet weights)
- larger engines in 250GP like chassis. Geometry closer to 600cc crotch rockets. KTM 640, Honda XT, Suzuki DR, BMW/ROTAX 650 engines. Wet weights in the 290lbs area

The ROTAX 650 is by far the most popular engine in the class.

Soooo...how bout basing the build on a big twin

What I am thinking is going for a dummy piston much like what Ducati did when they made their infamous Supermono:

https://picasaweb.google.com/lh/photo/k4TemiVRgTExsRF6szG6QNMTjNZETYmyPJy0liipFm0?feat=directlink

That way, one could obtain perfect primary balance. I don't think it would be unfair to say that the Ducati twins do not have vibration issues at all due to their perfect primary balance.

Now I realize that a big twin is not the perfect engine choice for this application by far but I do see some pro's:

- No problem maxing out dispalcement within class rules. Mentioned MX and Enduro engines are limited when it comes to how big you can go on displacement.

- the detached tranny of a big twin would yield many option for drivetrain configuration. Think Husaberg engines:

https://picasaweb.google.com/lh/photo/FMRNQDjULyYp3VuRWacqmNMTjNZETYmyPJy0liipFm0?feat=directlink

- The huge aftermarket offering reasonably priced performance parts for Harleys are very tempting

- The cool factor would totally go through the roof

Con's/Questions:
(this is where I was hoping you guys would chime in)
- Primary drive. The build would depend on NOT having to use the primary cases. I'm thinking both geometry and weight.
Would it be at all doable/feasable to run a big twin without the primary cases?

- Weight.
even without the primary cases, would the engine/tranny be light enough? Would anyone have an idea about what the following components weigh: tranny - cylinder - head - bottom end?

- RWHP. I would need to shoot at 70+ RWHP to be competitive. I am guessing that in a single configuration the head would have better conditions than usual given that it doesn't have to share the intake with a 2nd head. I would be looking at borexstroke along the lines of 4" x 3 3/4" , which would yield close to 800cc. Don't get me wrong - I realize that we are talking about an all out race engine probably sporting exotic components.
What is you opinion as to whether 70+ RWHP is possible?

Lastly I realize that Sound of Singles roadracing is hardly your average topic at this forum. Please let me now if I am in the wrong place.

Best Regards,

JT

 

[dsvracer]

This project seems like a whole lot of work, but as you said the fun factor will be awesome. The idea of running a dummy piston is interesting but wouldn't it be easier to just cap off the cylinder not being used ?? It would also be considerably lighter not having that extra cylinder.
Yes you can run the bike without a primary case, we have belt drive systems that do not require the inner or outer primary.
A complete v-twin motor weighs about 125 lbs, trans 30 lbs, a cylinders weighs 15 lbs and the head weighs 14 lbs. These are just rough weight, everything can be made lighter if needed.
Getting 70 hp to the rear wheel will be a challenge but you have to remember harleys make more torque than the average metric bikes do. If I can help just ask. [email protected]

 

[Hollywood63]

Buell Blast with a big bore kit 600cc ?? Then again that Rotax 650 is going to be a tough one to beat

 

[Jeffytune]

If I were to do this project with a Harley engine A XR750 would be my choice.

 

[qtrracer]

Of course its possible, remember the v-4,a chev v/8 that had the front 4 cylinders removed? Pretty darn competitive in sand drag competition but the v-6's and v-8's ultimately won out. But if you are looking to win, the TC based single will be a fool's errand.

 

[Allracing]

This project seems like a whole lot of work, but as you said the fun factor will be awesome. The idea of running a dummy piston is interesting but wouldn't it be easier to just cap off the cylinder not being used ?? It would also be considerably lighter not having that extra cylinder.
Yes you can run the bike without a primary case, we have belt drive systems that do not require the inner or outer primary.
A complete v-twin motor weighs about 125 lbs, trans 30 lbs, a cylinders weighs 15 lbs and the head weighs 14 lbs. These are just rough weight, everything can be made lighter if needed.
Getting 70 hp to the rear wheel will be a challenge but you have to remember harleys make more torque than the average metric bikes do. If I can help just ask. [email protected]

Thanks for your reply! I quickly checked out your website but couldn't find the caseless beltdrive system. I'd love some info on it though When it comes to the unused cylinder the plan is indeed to cap the cases off. However I plan to keep the rod and "hinge" it to the cases (@ 90deg to the other rod!!), much like Ducati did with their single. Given good/perfect primary balance, it could make for an exceptionally smooth engine at least for a single. The non offset cylinder config of the HD engines also eliminates the coupled forces that all other engine configs would suffer from:woohoo:

JT

 

[Allracing]

If I were to do this project with a Harley engine A XR750 would be my choice.

So would I but the class requires a single cylinder engine!

 

[Allracing]

Of course its possible, remember the v-4,a chev v/8 that had the front 4 cylinders removed? Pretty darn competitive in sand drag competition but the v-6's and v-8's ultimately won out. But if you are looking to win, the TC based single will be a fool's errand.

I guess that deep down inside I suspect that you are correct. However I cling to the fact that HD engines have a broad flat torque curve and also that I would be running a larger displacement engine than most of the competition. And somehow hope that competitiveness could be achieved. Even if the bike would get smoked on the straights, at least the fastest Rotax engined bikes, maybe that could be compensated for out of turns and so on.

Anyway what would the main limitation be in real life? I am thinking that the pushrod/rocker arrangement would prevent the engine from reving high enough and thereby limiting max HP? Can this be compensated? Are there light weight valve train parts out there?

Also, how high can the TC engine rev in real life? I'm gonna couple that question with what kind of reliability I would hope for: 30hrs of all out racing between rebuilds. :woohoo:

 

[Allracing]

I would love your opinion on this:

What power would a TC based single make compared to a twin with everything else being identical?

I was thinking the better intake conditions (not having to share intake with a 2nd head) would point towards it making more than 1/2 the power of an identical twin?

Anything else to take into account?

Whats your opinion?

JT

 

[dsvracer]

belt drive systems

For belt drive systems look into Primo belt or check out some of the major vendors like drag specialties or CCI they carry a full line of these products. dsv

 

[springer-]

I would love your opinion on this:

What power would a TC based single make compared to a twin with everything else being identical?

I was thinking the better intake conditions (not having to share intake with a 2nd head) would point towards it making more than 1/2 the power of an identical twin?

Anything else to take into account?

Whats your opinion?

JT

It would seem to me the twin cam engine is much too big of a platform to build a single cylinder performance engine from for your application. I would guess you would get less than half of the power due to a single piston needing to keep the flywheel turning. At least with two pistons, every rotation of the flywheel gets a power stroke. That coupled with the weight of the engine doesn't sound like a winning combination to me.

Realistically, 800cc twin cam single pushing 70hp is equal to a 1600cc actual twin pushing 140hp. 1600cc is approximately 95" and 110hp would be quite an achievement, doubtful you could get to a REAL 140hp with two cylinders, even more doubtful you would get to 70hp with one cylinder.

A very well built Twin Cam might see 7000 RPM but holding it there all day, I wouldn't recommend trying that.

I thought the same thing as Hollywood63, start with a Buell Blast engine and go from there. It is based on the HD sportster engine but already built with one cylinder. 70hp would be amazing to see out of it.

Good luck which ever way you go and keep us posted if you do try.

 

[Allracing]

Thanks to everyone replying!

From what I read the TC based single is perhaps a dead horse...

I guess now is the time to start reading up on the Buell Blast engine