Apex more stable than RX-1

[Tork]

Lower polar moment of inertia.
By moving the rider (200lbs?) 6-8 inches further forward this equals centralized mass. 3 heavy items (engine, gas tank and rider) are now between the rear 40% of the skis and the center of the skid.
it will take less side force to turn the sled in a corner

Front end geometry.
A. Shorter spindles change the roll center.
B. wider ski stance in the new front end.
C. This geomety has bump steer. I believe that under compression the ski stance grows even wider, it looks like the better part of an inch.

More weight on the skis.
Many riders have dialed in more weight on the skis by lessening the weight transfer on the control rods and pulling up on the straps.
On the Apex we are putting more actual weight on the skis without the suspension compromises.

I saw where someone here said that because the seat is taller that the 06 sleds were more tippy. They felt very stable to me, and I thought it might be fair to look at the reasons why.

Lets look at some recent history on the RX 1. Many will agree that the 03 RX 1 was far more tippy than the 05 which has a taller seat. We know already that front spring rates and the roll bar are far more important than seat height for stability.

Provided that Yamaha gets the 06 spring rates and roll bar callibrated right , the above changes will give us very responsive and stable Apex/ Attack snowmobiles.

 

[LazyBastard]

C. This geomety has bump steer. I believe that under compression the ski stance grows even wider, it looks like the better part of an inch.

Thats not what bump steer is. On trailing arm suspensions, when the suspension compresses, the ski will actually point in a different direction with respect to the body. This will NEVER happen with A-arm suspension.


The 05 "stability" has nothing to do with the seat. It has to do with the springs, swaybar, and links. With the way I have my 03 set up, I can't imagine ANYTHING being more stable - 13mm swaybar, 130# springs preload min, 04/05 links, slide rails moved back 4" from stock. I had a psycho on a REV800 trying to keep up with me on real nasty twisties, and he couldn't. Thats given the fact that a) he's an experienced rider, b) he's nuts, c) he had advanced warning for curves by watching where *I* went.

 

[Tork]

Yes you can have bump steer on an A arm suspension. On the RX-1 the A arms are flat (level). On the vector the A arms angle down. Under compression the ski stance gets wider and that will also change toe in/out.

On the second point I agree. You have turned your sled into a slot car .
And stability has everything to do with spring rates, bar and arms and it has little to do with seat height. So we agree on pretty much everything except the bump steer.

The reason I wanted to start this thread is that someone a week or so ago said that the 06 sleds are more tippy because the seat is taller. He had not ridden them, just made a flat out statement as if it was an undisputeable fact. I thought that it was very unfair to the members of this forum to let it stand. I was lucky enough to ride the 06 sleds. In the trail section the corners were very icy. But I got to do some zig zags in the long ditch sections and some high speed sweeping turns on the lake and I thought it was remarkable how stable the Apex was.
Members count on getting accurate info. They are making expensive decisions based on what they read here. I did not want to let a guys statement (that was based on short sighted logic) screw anybody up.
Plus the 06 fanfare has died down a bit. I thought I'd fill in some gaps in the info that has been presented so far.

 

[LazyBastard]

The ski stance and a-arm angle has nothing to do with toe in/out. Thats the whole point of a-arms.

The tie rod is the same length as the a-arm. It is also parallel to the a-arm and positioned at the same height as the a-arm. As the suspension compresses, the tie rod precisely follows the arc made by the a-arm, thus maintaining toe in/out precisely regardless of the change of angle.

The reason why trailing arm suspensions DO have bump-steer is because the tie-rod does NOT follow the arc of the whaddayacallits precisely on account of the rear mount and the change of angle of the spindle as the suspension compresses.

From the yamaha website:

... maintains spindle castor at 23 degrees throughout the travel stroke. This gives the sled consistent and predictable steering regardless of the suspension’s activity.

 

[Red2003]

Well, you're both right. The RX1 has NO bumpsteer. But, it isn't soley because its an A-arm suspension. In race cars with A-arm suspensions we tune with bump steer all the time. Its often a handling advantage to bump toe in or toe out into the suspension to compensate for the different radiuses that the two front wheels (or skis) are being forced through as you turn sharper and sharper into a corner. The RX has none because of the placement of the tie rods inline and of equal length to the A arms. Also the bellcrank keeps everything parallel all through the suspension throw. But, modifying any of these components WILL yield bump steer in any type of suspension, which isn't necessarily a bad thing.

 

[QCRider]

Tork, while Yamaha may have made suspension upgrades on the front of the sled to minimize the effect of the higher center of gravity, the fact remains that an '03 RX-1 with the same suspension setup will corner flatter than an '04, '05, OR '06 because each year the rider's weight moved higher and higher.

It's a constant trade-off between the ability to mash moguls and the ability to handle high speed turns. You cannot be the best at both at the same time. Have you ever seen the oval racer sleds? Do they look ANYTHING like a Sno Cross sled? There are good reasons for both designs, and the truth is that a trail sled is a combination of both. Yamaha has done what everyone esle is doing, they have biased the design of the sleds more toward the ability to mogul mash, and this is based mostly on the popularity of Sno Cross. Years ago, oval racing was much bigger than Sno Cross, now the tide has changed.

 

[POWERHAULIC]

Wow thats like twice in one week LB was Wrong. Red2003 hit it on the head, A-arm suspension can have bump steer and sometimes it can be a good thing.

 

[DaveyBoy]

opinions vary

 

[Red2003]

:~)

 

[Tork]

The ski stance and a-arm angle has nothing to do with toe in/out. Thats the whole point of a-arms.

This statement is not correct.

The tie rod is the same length as the a-arm. It is also parallel to the a-arm and positioned at the same height as the a-arm. As the suspension compresses, the tie rod precisely follows the arc made by the a-arm, thus maintaining toe in/out precisely regardless of the change of angle.

This statement is perfectly correct if in fact the tie rod length and angle are the same as the A arms. I've had a Vector now for 6 months and to me it looks like it isn't. I've had the brochure now for a month and have read it cover to cover at least 20 times. It was a bold and perhaps incorrect statement I made going against the brochure. But at dinner after the presentation and ride, MrSled actually brought up what I had been thinking.
I am pretty sure he thought it had bump steer also.

The reason why trailing arm suspensions DO have bump-steer is because the tie-rod does NOT follow the arc of the whaddayacallits precisely on account of the rear mount and the change of angle of the spindle as the suspension compresses.

This one is going to take somebody smarter than me to argue. Yes the spindle angle does change on a trailing arm suspension. Based on what I know (and that is far from everything) I have not heard 'bump steer' to be connected to spindle angle. Going back to an earlier point, I think that if the tie rods were the same length and angle as the radius rods, you would have little or no bump steer

From the yamaha website:

... maintains spindle castor at 23 degrees throughout the travel stroke. This gives the sled consistent and predictable steering regardless of the suspension’s activity.

Yes I have read that too. But to my way of thinking it does not translate directly to bump steer. I could be wrong and I am more than willing to be corrected because that is how I learn new things.

Red thank you for bringing your very well stated comments to the forum.
My opinion was based very much on what I know about race cars. Bump steer is in fact a suspension tuners helpful trick. Your comments brought great clarity to the discussion.

 

[Tork]

Tork, while Yamaha may have made suspension upgrades on the front of the sled to minimize the effect of the higher center of gravity, the fact remains that an '03 RX-1 with the same suspension setup will corner flatter than an '04, '05, OR '06 because each year the rider's weight moved higher and higher.

It's a constant trade-off between the ability to mash moguls and the ability to handle high speed turns. You cannot be the best at both at the same time. Have you ever seen the oval racer sleds? Do they look ANYTHING like a Sno Cross sled? There are good reasons for both designs, and the truth is that a trail sled is a combination of both. Yamaha has done what everyone esle is doing, they have biased the design of the sleds more toward the ability to mogul mash, and this is based mostly on the popularity of Sno Cross. Years ago, oval racing was much bigger than Sno Cross, now the tide has changed.

Great point QC. Yes as sleds get taller, if the side force exceeds the center of gravity the sled will roll over easier eveything else being equal. But other things on these sleds were changed as well.
My point is stability up to that roll over point. Things like a wider ski stance, lower roll center of the new front end geometry and centralized mass requiring less side force to turn the sled will likely make the sled more stable. Actually, I cant prove it because I am not an engineer but the changes could be enough to counter the higher CG at the roll over point.

 

[impalapower]

Tork, while Yamaha may have made suspension upgrades on the front of the sled to minimize the effect of the higher center of gravity, the fact remains that an '03 RX-1 with the same suspension setup will corner flatter than an '04, '05, OR '06 because each year the rider's weight moved higher and higher.

Man, at least my '03 has one characteristic that outperforms the newer sleds.

 

[impalapower]

Wow thats like twice in one week LB was Wrong. Red2003 hit it on the head, A-arm suspension can have bump steer and sometimes it can be a good thing.

Hey, easy on the Lazy. Not everyone is perfect. 8)

 

[LazyBastard]

I wasn't at all wrong.

I didn't say that you couldn't make an a-arm suspension give bump-steer. I said that it was selected because it could (NOT-bump-steer).

THE POINT OF IT is to get rid of bump steer. THE POINT OF USING IT. THE REASON WHY IT IS USED.


SHOW me where I said that it HAD to no-bump-steer.

 

[SnowdemonsRX1]

Here we go again