top speed reduced..

[Newfie Apex]

On our last run of the year I had the opportunity to make a few top end speed runs....Before I did all the mods to my '06 Apex rtx totally stock I did 106.8 mph on a gps...Since then I added the Phazer mountain skis, extended the sled to a 136 and added a 1.5" track. Dropped a tooth on the top gear, and used an Ulmer stage 2 kit.(springs weights and helix).... top speed with setup was a dismal 90.5 mph... ...My rpms on takeoff are hitting 11ish and i am going to say stay around 10800ish....That is over 16MPH lost ...How.. why?

 

[kinger]

Everything you did would reduce the top speed. Just going to a 1.5 track in a 121 I would say you would lose 6-8mph, dropping a tooth will reduce top speed theoretical max by 5mph, wide skis add air resistance and drop speed, going to a heavier track with more rotating mass would drop that.

I say each one individually add up to more then 16mph and the clutch kit probably got you a few mph back.

I'm going 13.5" wide by 151 this summer and cant wait to see the effect on my top speed.

 

[APEX 06]

Everything you did would reduce the top speed. Just going to a 1.5 track in a 121 I would say you would lose 6-8mph, dropping a tooth will reduce top speed theoretical max by 5mph, wide skis add air resistance and drop speed, going to a heavier track with more rotating mass would drop that.

I say each one individually add up to more then 16mph and the clutch kit probably got you a few mph back.

I'm going 13.5" wide by 151 this summer and cant wait to see the effect on my top speed.

X2

 

[Dano]

1.5 track is the main cause then I would blame the clutch kit. Probably a good combo for get up and go, but clutch kits often cost on top end as well. I would say a 23 top gear wouldn't effect top speed as I seen better top speeds by gearing down a tooth. Still good for 125ish mph with 23/38 gearing so you'll need more umff in the motor for pulling your gearing to 1 to 1 ratio at 10,800ish rpm.

Dan

 

[Bucky the d.o.g.]

I put a 1.5 ripsaw on my 07 121 and it lost quite a bit on top end(with no other mods) however was able to beat guys out of the hole that I could not before.

 

[steiner]

The 136 will cost you 10 mph alone

 

[grader]

it was all the things noted above, except the comment on the track lengh. my rx ran 116mph as a 121 and at 136, and i seen 115mph this year as a 144. its hard to pinpoint what caused the most loss, but i would bet on the 1.5 inch track as the main one, followed by the clutch kit. a 1.5 inch track pumps a lot of air into the front of a tunnel where it has nowhere to go, and that kills speed.

 

[raginyamaha]

i went from 121 1.25 to 136 1.5 on my nytro and gained a couple mph....... also did undertunnel exhaust and cr10 plugs. Same gearing and clutching....

 

[bottlerocket]

i went from 121 1.25 to 136 1.5 on my nytro and gained a couple mph....... also did undertunnel exhaust and cr10 plugs. Same gearing and clutching....

Thats odd~you went longer and bigger and gained mph?

 

[steiner]

My Viper lost almost 10mph going from 121-136. Same clutching, gearing and lug height on the track.
More rotating mass in the drive line should slow the sled down unless you have soooooo much power that now you are getting traction to get that mph gain.

 

[raginyamaha]

i went from 121 1.25 to 136 1.5 on my nytro and gained a couple mph....... also did undertunnel exhaust and cr10 plugs. Same gearing and clutching....

Thats odd~you went longer and bigger and gained mph?

only saw 98 on the dreamometer before and saw 101 this winter with the swap....

 

[ShootToThrill]

Conditions play a huge role, different day, different speeds. But the 1.5 was the biggest factor IMO.

 

[grader]

think about it this way, the longer the track, the lower the rpm of that track at any given speed. it has its limits of course, but i lost nothing from 121 to 136, and next to nothing from 136 to 144. 2 strokes are another story as my buds with ski doos cant pull the same speeds when going even to a 136. must be a torque thing.

 

[Ratpack]

Every 1/4" on the lug =5 mph.

 

[arteeex]

think about it this way, the longer the track, the lower the rpm of that track at any given speed. it has its limits of course, but i lost nothing from 121 to 136, and next to nothing from 136 to 144. 2 strokes are another story as my buds with ski doos cant pull the same speeds when going even to a 136. must be a torque thing.

This isn't true with a track drive. The angular displacement of the drivers is transferred to the track therefore the track length doesn't matter. For example, ten revolutions of the drivers will move any track length the same distance forward. Without slippage, a 121" track is moving at the same velocity as a 163 track at a given speed; the shorter track will make more revolutions but it isn't moving any faster. Regardless of the track length, there's still the same number of inches in a mile that the track has to cover.

The inertia of the longer track will effect the top-end speed because it takes more energy to accelerate the additional mass.