The pivot point and axle centerline

Glenn S · Post by **Glenn S** » Sun Apr 06, 2003 4:50 am

Peter,
Both your trucks and Seismics have the axle centerline not exactly on the pivot point.

To me I see that as the axle centerline arcing above the pivot point. I see it as shifting back and forth a bit in this arc over the pivot point.

So in a sense it is not the “perfect pivot” where the axle centerline would be exactly on the pivot point.

Does newer R3 have the axle centerline further away from the pivot point than R2? Did this relation change at all between the two versions?

And would it not be best to have the axle centerline right on the pivot point?

With Seismic I see that they cannot have the axle centerline on the pivot point because they are using an axle that goes completely through the truck hanger.

And your design has the grommet plate and housing that would stick up considerably if you brought the axle centerline inline with the pivot point.

I guess you could remedy this a bit if you could have the grommet access on the side of the truck, to reduce the height of having it on the top.

But still it just seems best to me if a design had the axle centerline on the pivot point itself.

I’d like to hear what you think of this.

Thanks,
Glenn

[ This Message was edited by: glenn on 2003-04-06 13:03 ]

Terry Kirby · Post by **Terry Kirby** » Sun Apr 06, 2003 6:34 pm

Glenn, how would this effect turning in a race situation and why would it seem best to have the axle centerline on the pivot point itself? Would it effect traction,steering or both? I guess my question is what specific problem are you having in terms of performance that needs to be remedied by this design change? Thanks, TK

Glenn S · Post by **Glenn S** » Sun Apr 06, 2003 7:27 pm

No it just seems to me that it is a compromise because if they did put the axle centerline exactly inline with the pivot point that it would further reduce the hanger/road clearance.

That if they could, they would want to design the truck that the axle centerline would be in alignment with the pivot point. That not doing so is really just a compromise because of ground clearance and other design factors. And Seismic just can’t do this because of the way the axle goes completely through the hanger.

I'd like to hear Peter comments on this and even Dan from Seismic too. I'm just curious. This is a good question.

I like my PVD's but someone recently brought this to my attention, that the axle centerline is arcing, and in a sense shifting, if it is off the pivot point.

My gut feeling is that if they could they would put the axle centerline directly on the pivot point. That doing this would be the most efficient design.

Peter Verdone · Post by **Peter Verdone** » Sun Apr 06, 2003 7:30 pm

there is no such thing as a "perfect pivot point." different situations call for different geometries. changing the distance that the axle is from the pivot axis raises or lowers the roll center given a certian steering angle and offset distance. my offset distance was chosen to maximize traction while maintaining the desired 'feel' for how the board handles (roll center).

Peter Verdone · Post by **Peter Verdone** » Sun Apr 06, 2003 7:33 pm

some randal trucks have the geometry that you describe to be the "perfect pivot." i think that if you take a ride on some of those trucks you can see if it really makes a difference.

Glenn S · Post by **Glenn S** » Sun Apr 06, 2003 7:57 pm

Maybe my wording of Perfect is not the best way to describe what is just an exactly aligned pivot. Using the word Perfect implies "best", which I am not one to say that it is. I am just seeking insight into this theory of why or why not have them aligned exactly.

Are you saying that you are offseting the axle centerline from the pivot to gain traction? That if the axle centerline was directly on the pivot point there would be reduced traction?

If you don't mind I'll post a picture for other to see the relation of axle centerline to pivot point:
<img src=http://members.cox.net/oceanslider/R3-3-5-inch.jpg>
The offset in the picture above, of the axle in relation to the ends of where the pivot is supported has no relation to what I am questioning. It is bringing the axle down so that it is in on the pivot point itself. As if the axle centerline went straight through the pivot. I know that Peter understands what I am questioning. I'm just saying this for anyone that might not. There are a few different offsets that could be construed here.

[ This Message was edited by: glenn on 2003-04-08 12:54 ]

Terry Kirby · Post by **Terry Kirby** » Sun Apr 06, 2003 8:04 pm

please guys ,help me out here. I am not an engineer but I do race these trucks. In simple terms, why would I want those things to be lined up. WHY would they turn better? HOW would they turn better? WHY and HOW would they grip better? I'm confused, TK

Glenn S · Post by **Glenn S** » Sun Apr 06, 2003 8:59 pm

TK,
To some small degree you could think of this as:

If the wheels were stationary on the ground and you lean on the deck to one side. The deck is in essence doing an upside-down pendulum. The further away the axle centerline is from the pivot point the larger the swing is of this pendulum. Seems as if this would shift how the weight is distributed to the truck.

If the axle centerline where on the pivot point, there would be no pendulum effect whatsoever. The weight would always be distributed to the center of the axle evenly. But this is just my guess.

Now given, I know that the axle centerline is not far off the pivot point. I just want to understand the why or why not. Is the reason that it is not a compromise or actual theory that it is better this way? I'm just wondering how this affects performance.

[ This Message was edited by: Glenn on 2003-04-08 12:59 ]

Terry Kirby · Post by **Terry Kirby** » Sun Apr 06, 2003 9:12 pm

Hmmm, I'm still not sure if I get it. I have always had a hard time with theory, I can't read blueprints and I can't do geometry either. Here's how I figure out what works and what does not work for me. For my profession which is training dogs there are lots of theorys that sound great untill you test them on the Airdale who wants to eat his owner. Then you find out what REALLY works and what sounds like it SHOULD work. For slalom racing and slalom gear here's what I do.. I set a course, set up my timer, get 2 identical boards except for the part I am testing (you can only change on perimeter at a time) . Now comes the fun part. I run the course dozens of times with both set ups, I look at my notes and I race what the timer tells me what is fastest. John once told me fulltracs on an old fullnose was fastest. The timer told me he was full of shit. I told him High camber was faster, The timer told me I was full of shit. I bet Kenny he could never get his 36" PP through the course I just set. Him and the timer told me I was full of shit. I say screw theory, the timer never lies!

Peter Verdone · Post by **Peter Verdone** » Sun Apr 06, 2003 9:52 pm

i agree with tk entirely about the timer saying it all.

about the offset. think in terms of roll center not pendulum. the swing of the axle while offset is good for 'making' speed.

Glenn S · Post by **Glenn S** » Sun Apr 06, 2003 11:28 pm

Yes, the proof would be in the pudding. So it that sense it is possible that it might be better to have the axle centerline on the pivot. We would not know unless we had two nearly identical trucks, one as it is now, and another with only the axle centerline moved to be on the pivot point.

I'm not saying that PVD's are bad in any way. I am just wondering what the reasoning might be behind the choosing to have the axle centerline on the pivot or offset from the pivot some like it is now. My initial thought was that it was a bit of a compromise because doing so would make for less ground clearance.

I’m sure Peter does not mind the question. As it is obvious that he in the quest for the best truck possible. And if you’re in the quest for the best truck possible then all questions are welcomed.

I like my PVD R2's. And I do get the feeling that the affect either way would be ever so slight anyway, as if to be hard to tell the difference. I was just curious.

Peter, thanks for taking the time to answer my question.

[ This Message was edited by: Glenn on 2003-04-06 17:32 ]

Peter Verdone · Post by **Peter Verdone** » Sun Apr 06, 2003 11:45 pm

i have answered the question. i really cannot spend the $300 extra programing and setup to build a single truck that probably is not what a certian market is looking for. how many people are running randals on slalom?

"changing the distance that the axle is from the pivot axis raises or lowers the roll center given a certian steering angle and offset distance. my offset distance was chosen to maximize traction while maintaining the desired 'feel' for how the board handles (roll center)."

Glenn S · Post by **Glenn S** » Mon Apr 07, 2003 12:40 am

Your talking about the Downhill version of Randal's yes? But Randals are not on a solid pivot that does not move about, so it would be hard to compare. Not to mention any slop that might be in the pivot cup. The hanger on Randals can shift about on the bushing just like any conventional truck can. I did see a few of the open class guys using Randal's for slalom at Avila last year. Also because Randal's put the axle centerline on the pivot and their overall design they are very tall trucks. And you'd have to modify the hanger to get it smaller and more like the prefered 101mm to 106mm hangers.

And Downhil Randal's are very popular with the high speed crowd are they not?

[ This Message was edited by: Glenn on 2003-04-08 13:02 ]

John Gilmour · Post by **John Gilmour** » Fri Apr 11, 2003 7:50 pm

IMHO here is what I think is going on- From a biomechanical point of view.

We weigh a lot relative to the board we are influencing.

We are balancing our body mass on 2 pivots.

A high roll center - at least as it was explained to me is the same thing as your upside-down pendulum.

So it would figure that the most stable system has us balancing on or extremely close to the axis.

Turn initiation and application of the normal force into the ground is another issue...and in snowboarding and in skiing often we add risers under our bindings to increase the height of the "roll center" to aggressively lever into the snow once a turn has been initiated. You'll see lots of top racers using lift under their bindings in skiing- some are marketed under the name derby plates.

I've coached/taught several motion sports.
2 kids I coached snowboarding won 1st and 4th in the Nationals. I taught inline skating performance drills and at one time every nearly top Inline Speed Skater in New England learned this drill I developed ). I'd like to think I helped TK, Ur13, Tway, Jon Rutherford and others to skate better.

Well here goes.

In regards to traction. The more centered your body mass is over the contact patch .... in regards to "the Normal Force" the better traction recovery you will have when entering a skid.

I've told Inline skaters if they are trying to make a hard turn in dicey situations NOT to lean into the turn- but to stay upright/ shoulders level/ flexed downwards but not leaning into the turn (weight directly over the wheels but ankles and kness doing nearly all the angulation). Doing so will prevent your skates from completely sliding away from you in a event of lost traction.

In snowboarding I teach people to move their arms away from the slope in a hard carve to maximize traction through the normal force and not to exert more lateral force against the snow (hence breaking traction) in a turn- PARTICULARLY at the APEX of the turn. Mark Fawcett keeps his shoulders completely level in a turn.

ICE Speed skaters are a little different because their skates will offer no traction unless really tilted- but even then they crouch in the turn to minimize their lateral displacement in a turn. (you will exert the most force laterally leaning into the turn in speed ice skating- your blades will grip the best this way- because they don't grip at all when flat, and you'll still have the normal force working for you + the surface is uniform so you are unlikely to lose traction......but if you do....you are falling down for sure. The highest speeds hit on ice speed skating are in a crossover (not straight ahead) and since the normal force is always nearly constant (their navels don't move up and down much) the only way to get more force into the ice to generate speed is to increase the lateral force by leaning into the turn- however this is a "make it or break it move" since recovery is very difficult. Lateral force converted into forward speed is what makes you fast on the ice. Lateral force means speed to the ice skater.

Slalom skateboarding isn't like ice speed skating since the pavement is rarely uniform and skidding to break and then quickly recovering are can be a part of the course technique and strategy.

So if the axle centerpoint is at the center of the pivot point you will get the most traction............ ONLY IF the place to maximize traction IS ... EXACTLY between the wheels. But if the place where traction is maximized is over the inside wheel- then this would not necessarily be the case. A higher roll center truck would help transfer your mass and normal force over to this position.

In Inline skating I teach students when entering a dicey fast turn to pressurize the inner skate more and angulate the inner skate more because if traction is lost on teh inner wheel you ahve the outside skate to catch you. in skateboarding if you are concentrateing too much force on the outer wheels you are likely to "overwhelm" the urethane (ie. it deforms too much and loses traction)on the outer wheel. Better to spread the force over both wheels. If you are definitely going to skidding almost all of the time- as opposed to not skidding - then an axle at the centerpoint is likely to be easier to moderate and control.

Having a roll center that is slightly higher might help in generating speed though. You would have more leverage on the system and more "spring" and resonant energy effect on the grommets. If this is the case then a "compromise" is found...and likely the "happiest compromise" is the one that gives you the lowest time.

The reality is that we are talking very small amounts of length and when compared the the over all height of the center of mass of the "skater + system" it is a very small indeed.

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