Preparing a road legal FXR150 for Bucket racing

[Scotty595]

yup, you'll notice a top speed increase too.

Havent tried the top speed test yet. Although it could only haul my fat ass to 120 with the standard carb on so shouldnt be too hard to beat. (cant really check atm as speedo not working)

[Scotty595]

Just to find one.........

Pm'd ......

[Scotty595]

Hi Ray
All I have done with my FXR is add an XR400 carb (probably a bit big - it bogs at low revs) and slick tyres on the standard wheels (with tubes). I also added CRF250R dirt-bike handlebars (required new throttle cable and brake line) after bending the FXR bars. The rest is totally standard. I still have the speedo, electric start and standard exhaust. My biggest problem is the exhaust as it scrapes on the track even after moving to the inside of the mount and adding spacers. So, you don't need to do much to go racing. The most important change was the tryes. My exhaust has to go! I have a dirt-bike pipe ready to cut but I need a downpipe. Anyone any suggestions on a downpipe?
Regards
Damien

I have seen that exhaust scraping lol! Infact i have seen quite a few exhausts scraping, hence why i want to get mine moved.

From what i have heard the standard FXR pipeis pretty much spot on, just good to get it shortened and exhaust mounted under the engine.

[F5 Dave]

the only thign total loss is good for is, it means you can remove the heavy fly-wheel magent and starter gears...which will boost up your horse power...i did it to mine, makes it rev like nothign else, hard to keep the front down!!

So removing weight from your flywheel etc will boost up the horsepower yeah? Probably 2 or 3 hp yeah?

so where does that come from? It's been a while since I did physics at skool. please explain.

[thealmightytaco]

So removing weight from your flywheel etc will boost up the horsepower yeah? Probably 2 or 3 hp yeah?

so where does that come from? It's been a while since I did physics at skool. please explain.

Rotational inertia, and the fact you're throwing weight away.

[F5 Dave]

ooh a bite..

So the act of the throwing is Kinetic energy by accelerating the parts toward the bin. I have done work lobbing these bits into the skip. . . . Not sure how that makes the bike faster. The starter gears largely aren't turning more than a small idler gear until clutched in & I can't see much loss of hp in that. So this rotational inertia thing creates the hp does it? How much HP would we expect?

[woodyracer]

So removing weight from your flywheel etc will boost up the horsepower yeah? Probably 2 or 3 hp yeah?

so where does that come from? It's been a while since I did physics at skool. please explain.

it boosts the revs up, as the engine dosnt have haevy weights on it.

Got a carrot up your ass or somthing.......geez

[saxet]

What Dave is trying to say is removing the flywheel cannot increase power, seriously how can it? It has no effect on the engines state of tune.
What it does do is remove some of the mass the engine has to accelerate, letting it spin up faster. It also tends to remove the engines torque effect and reduce engine braking.

[Kendog]

So the act of the throwing is Kinetic energy by accelerating the parts toward the bin. I have done work lobbing these bits into the skip. . . . Not sure how that makes the bike faster.

Some FXR's were throwing bits off their bikes during the racing in the weekend, not sure we got a pre and post measure of HP or lap times to validate performance improvements.

[F5 Dave]

Arse sans carrot. Check.

So first it boosted up the horsepower, now it boosts up the revs? So now it will rev higher? is that good? Just engage brain before keyfingers when giving 'advice'. What you mean, as Tex pointed out (although for torque effect I would have said flywheel effect to avoid confusion), is it will accelerate the lighter rotating objects faster. That is not an approximation of either of your statements.

[woodyracer]

Arse sans carrot. Check.

So first it boosted up the horsepower, now it boosts up the revs? So now it will rev higher? is that good? Just engage brain before keyfingers when giving 'advice'. What you mean, as Tex pointed out (although for torque effect I would have said flywheel effect to avoid confusion), is it will accelerate the lighter rotating objects faster. That is not an approximation of either of your statements.

Well that was what i was told by the mechanic that did it......,And you coulnd notice a huge difference in power.....
I am engaging my brain.....but your just ingaging your attitude........Sorry to distract you from your butt-scratching exercises.

-max

[thealmightytaco]

The flywheel's attendance or lack of has no effect on horsepower created by the engine, but the fact that precious horsepower is no longer needed to get a heavy wheel spinning (energy sapped by rotational inertia) means there is more available to instead rotate the wheel and propel your arse sans carrot forward.

Rotational inertia doesn't decrease or increase power, but it will steal some, thereby decreasing it at the wheel perse'. Parry dodge spin.

[woodyracer]

The flywheel's attendance or lack of has no effect on horsepower created by the engine, but the fact that precious horsepower is no longer needed to get a heavy wheel spinning (energy sapped by rotational inertia) means there is more available to instead rotate the wheel and propel your arse sans carrot forward.

Rotational inertia doesn't decrease or increase power, but it will steal some, thereby decreasing it at the wheel perse'. Parry dodge spin.

oh, i though you loose torque not horse power.

[thealmightytaco]

oh, i though you loose torque not horse power.

Get both probably, horsepower is essentially rate at which torque is produced (power = work over time), and with the flywheel no longer absorbing precious torque to wind it up, there's more torque, and over several thousand revs, there's more power.

[saxet]

I should've said flywheel effect at low revs where a road bike can stall at manouvering speeds. If you're moving at almost an idle, in particular, a wheighty flywheel gives inertia to the engine to decrease the chances of stalling. Once the flywheel is lightened the engine has less mass to spin up when you accelerate it.
Once the engine has accelerated it makes no difference to the engines outright power.
I would be curious to see what it does to the torque curve on a dyno, I suspect it could be similar to gearing an engine down.
E.g. it allows the bike to accelerate faster but does'nt give it more outright power.