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Old 10-27-2014, 02:25 PM   #496
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Originally Posted by lpittman View Post
So if less lateral weight transfer means more grip, why are guys working so hard to create flex in their chassis?

I've been experimenting so far this year by getting my chassis to be stiff as possible (XRay T4 '14 with all screws in top deck, motor mount post and servo brace with top deck connection) and am finding I am driving much quicker than when I was trying to create flex like recommended to me.
I'm not sure I can answer the first part of your question, but regarding stiffer being faster... I think a part of it may have to do with flexible cars generating too much grip and thus scrubbing speed in corners. A car that is easy to drive (read: ton of grip) is usually not as fast as a car that's balancing on the edge of traction.

Good drivers are smooth with both throttle and steering, so they can walk that fine edge and make the cars look controlled. Try driving the car of a very fast guy, I'd be willing to bet that it'd feel like it needed more grip front and back. Of course this also depends on the class, as faster motors can make up for the speed they scrubbed in corners.
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Old 10-27-2014, 02:35 PM   #497
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Hey Guys...thanks for adding to this thread...it helps everyone!

Quote:
Originally Posted by Josh Hohnstein View Post
I'll take a shot at this one.

Putting a stiffer swaybar on one end of the car will reduce lateral grip because you end up with less longitudinal weight transfer.
Sorry Josh....

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Originally Posted by metalnut View Post
I think it's because the way sway bars work, you're actually picking the inside tire off of the ground while the outside tire is moving upward, so in addition to less lateral weight transfer (due to less body roll) you're also actually reducing the amount of vertical load on the inside tires.
That's pretty much it...with the only exception being that a stiff swaybar will actually increase the lateral weight transfer from the inside tire to the outside tire.

You are correct that the root of the answer is because the swaybars get pushed upward by the outside tire leaning, which actually lifts the other side of the swaybar (inside tire side) up a little causing the inside tire to have less downward force.

Now...since the car remains flatter there would normally be more weight on the inside tire, but that tends to be overcome by the increased upward force on the inside tire and the increased downward force on the outside tire caused by a stiff swaybar.

In my testing some amount of swaybar is a good thing, but go too far and you loose overall lateral grip. Swaybars do make the car less lazy and as such the car is quicker to transition, so if you have chicanes, then a sway bar helps with those transitions. I have also found that if I completely remove sway bars I tend to loose corner speed in large sweepers. So like most things in setup a little goes a long way, and you can over do it with sway bars.

So yes, you want to use sway bars.

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Originally Posted by lpittman View Post
So if less lateral weight transfer means more grip, why are guys working so hard to create flex in their chassis?

I've been experimenting so far this year by getting my chassis to be stiff as possible (XRay T4 '14 with all screws in top deck, motor mount post and servo brace with top deck connection) and am finding I am driving much quicker than when I was trying to create flex like recommended to me.
Flex in chassis is a strange thing in RC car racing, as in full size car racing, you do everything to make your car more rigid. But chassis flex is always there and does contribute to the balance of the car, not so much the overall lateral grip however.

Here is the content from my learnsetup.com app talking about chassis flex

Chassis Flex affect on Handling

From my testing it seems that the more flexible you make the front portion of the chassis the less steering you have in the middle of the corner off, and the more steering you have on initial turn-in. If you stiffen up the front chassis flex, this gives you more mid corner on-power steering. From my testing the car seems to have a much more abrupt and aggressive feel to the steering near the apex when you start to apply more throttle. This is a similar feeling you get when you raise the front roll center, which makes sense.

As for the rear chassis flex, the more flex I had in the rear of the chassis, the less predictable the on power steering felt. It was stable, but i felt like the car was inconsistent compared to having less flex in the rear of the chassis.

So if you want a smoother feel it would seem having more flex in the front of the chassis and less in the rear can help with that. If you want a more aggressive rotation in the middle of the corner a stiffer front chassis flex seems to help.
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Old 10-27-2014, 03:07 PM   #498
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If you were talking about bars on a full size car your theory might be true but for a small car with a relatively tiny bar it is not the case.

A bar on a TC is not much more than a progression or second stage of the spring.
There is no way that a tiny bar like we run will ever overcome the force of a lets say 14lb spring. You would have to run like a 5mm sway bar to even think about making the inside wheel move up with the outside wheel.

Try this: remove your left rear shock from the shock tower, hold the car in the air and compress the right rear suspension.
The left rear will not even move with no shock attached.
The swaybar doesn't have enough strength to lift the weight of the suspension and tire.
So if it cant even lift that small amount of weight how is it going to lift that weight AND compress the inside spring as indicated it your theory?
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Old 10-27-2014, 03:54 PM   #499
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If your suspension is working correctly ie nice and free, remove the left shock and push the right wheel up the left wheel will certainly go up.

With the shock attached you should see a small amount of movement in the left wheel.

Typically sway bars contribute about 5-10% of the overall roll stiffness in a TC.

In full scale cars due to the aim of better ride compliance sway bars contribute typically 10-20% of the overall roll stiffness.
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Old 10-27-2014, 04:01 PM   #500
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Quote:
Originally Posted by frozenpod View Post
If your suspension is working correctly ie nice and free, remove the left shock and push the right wheel up the left wheel will certainly go up.

With the shock attached you should see a small amount of movement in the left wheel.

Typically sway bars contribute about 5-10% of the overall roll stiffness in a TC.

In full scale cars due to the aim of better ride compliance sway bars contribute typically 10-20% of the overall roll stiffness.
A 1.2mm bar which is standard in the rear will not lift the weight of the suspension and tire.....if yours will I would love to see a video.
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Old 10-27-2014, 04:36 PM   #501
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Originally Posted by Josh Hohnstein View Post
A 1.2mm bar which is standard in the rear will not lift the weight of the suspension and tire.....if yours will I would love to see a video.
It doesn't have to fully lift it to reduce the vertical load the tire is producing. I think that's what we're saying... a 10-20% reduction was the above point.
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Old 10-27-2014, 05:12 PM   #502
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Default getting more uptravel

When I build the shocks on many of my on-road cars to the manual-specified length, I find little or no upward travel when I lift the car at one end. The droop screws are not extended past edge of the suspension arm, so they are not limiting travel. I don't want to switch to short-length springs as I have lots of regular length springs. The springs are almost touching the adjuster ring on the shock.

What do you think about these ways of getting 2 mm more of uptravel?

1) Unscrew the shock shaft/ballconnector extend it 2-mm
2) Put 2-mm spacers between the chassis plate and the bulkheads
3) Move the upper shock mount point upwards 2 mm
4) Put the shock springs in a vice an clamp them overnight and hope they compress a bit.(haha)
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Old 10-27-2014, 05:47 PM   #503
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Originally Posted by F1Jet View Post
When I build the shocks on many of my on-road cars to the manual-specified length, I find little or no upward travel when I lift the car at one end. The droop screws are not extended past edge of the suspension arm, so they are not limiting travel. I don't want to switch to short-length springs as I have lots of regular length springs. The springs are almost touching the adjuster ring on the shock.

What do you think about these ways of getting 2 mm more of uptravel?

1) Unscrew the shock shaft/ballconnector extend it 2-mm
2) Put 2-mm spacers between the chassis plate and the bulkheads
3) Move the upper shock mount point upwards 2 mm
4) Put the shock springs in a vice an clamp them overnight and hope they compress a bit.(haha)
5) None of the above

Seriously, I'm not sure what is going on but that doesn't sound right. If you built the shocks right and are using the correct rate springs, the car should sag quite a bit and you should have to use the preload adjusters to get the correct ride height. After that, droop screws should be used to control the amount of droop/uptravel at each end.

If your car isn't sagging down at all when you set it down (with a battery, ready to run), you most likely have springs that are way too stiff, or something is binding.
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Old 10-27-2014, 06:47 PM   #504
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Josh

regarding the sway bars not being strong enough to do what I explained in my post....

Lets assume you are right for a moment and the theory of large cars does not apply here...then how would you explain that adding thicker swaybars at one end of the car removes lateral grip at that end of the car?
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Old 10-27-2014, 09:56 PM   #505
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Hi Martin,

What are you thoughts on varying the quantity of oil in a gear diff?

IE filling the diff with 50% oil vs 100%.
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Old 10-28-2014, 07:10 AM   #506
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Quote:
Originally Posted by Martin Crisp View Post
Josh

regarding the sway bars not being strong enough to do what I explained in my post....

Lets assume you are right for a moment and the theory of large cars does not apply here...then how would you explain that adding thicker swaybars at one end of the car removes lateral grip at that end of the car?
My theory:
We have established the longitudinal weight transfer = more grip.
When you let off the throttle at corner entry and weight transfers forward you have more steering (assuming you don't overload the front end).

If you picture the weight transfer as an X instead of just front to back or side to side this might make more sense.
Lets picture a car entering a turn, a 3ft radius off power left hand turn.
When you lift off the throttle the weight will transfer to the right front tire.
With a softer bar the right front will compress pretty far giving you a ride height of 1.5mm at the right front during cornering (note estimated). So you have transferred a large amount of weight to the front resulting in a large amount of steering.

Now lets swap out the small front bar for a large one.
Same turn.
A stiffer bar will not allow the suspension to compress as far (because it acts as a progressive helper spring). For lack of a better term.
So now your car is mid turn with an estimated ride height of 3mm at the right front.
You end up with less weight transfer resulting in less steering than the smaller bar.

My theory:
Putting a stiffer swaybar on one end of the car will reduce lateral grip because you end up with less longitudinal weight transfer.
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Old 10-28-2014, 09:45 AM   #507
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Quote:
Originally Posted by Josh Hohnstein View Post
My theory:
We have established the longitudinal weight transfer = more grip.
When you let off the throttle at corner entry and weight transfers forward you have more steering (assuming you don't overload the front end).

If you picture the weight transfer as an X instead of just front to back or side to side this might make more sense.
Lets picture a car entering a turn, a 3ft radius off power left hand turn.
When you lift off the throttle the weight will transfer to the right front tire.
With a softer bar the right front will compress pretty far giving you a ride height of 1.5mm at the right front during cornering (note estimated). So you have transferred a large amount of weight to the front resulting in a large amount of steering.

Now lets swap out the small front bar for a large one.
Same turn.
A stiffer bar will not allow the suspension to compress as far (because it acts as a progressive helper spring). For lack of a better term.
So now your car is mid turn with an estimated ride height of 3mm at the right front.
You end up with less weight transfer resulting in less steering than the smaller bar.

My theory:
Putting a stiffer swaybar on one end of the car will reduce lateral grip because you end up with less longitudinal weight transfer.

Hey Josh,

Interesting theory..., but I don't believe it is accurate based on the research on vehicle dynamics and books I have read.

The reason you get more steering with longitudinal weight transfer to the front is that you are adding more downward force to the front tires and taking them away from the rear tires. This is not producing more overall grip...but it is producing more grip on the front and less grip in the rear...thus more steering and the illusion of more overall grip.

The key science behind this is that if you have four tires with equal vertical load and then you transfer that vertical load from some of those tires to the remaining tires you actually get less lateral grip.

As this little definition on Wikipedia points out
Quote:
"The maximum lateral force that can be developed does increase as the vertical load increases, but at a diminishing rate."
...backed up by this wikipedia definition, which states
Quote:
Load transfer causes the available traction at all four wheels to vary as the car brakes, accelerates, or turns. This bias to one pair of tires doing more "work" than the other pair results in a net loss of total available traction. The net loss can be attributed to the phenomenon known as tire load sensitivity.
Also if you look at the image below which is a screen shot of the Tire performance curve described in the "Chassis Engineering" book you will again see that research shows that as you increase the load on a tire, yes the traction increases, but at diminising rates. And this is why when you transfer weight from the two inside tires to the two outside tires you loose overall lateral grip. Stated more generically, when you have four tires with equal vertical load an you transfer that load from one or more tires to the remaining tires, you are reducing your overall lateral grip.


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Old 10-28-2014, 09:54 AM   #508
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Originally Posted by frozenpod View Post
Hi Martin,

What are you thoughts on varying the quantity of oil in a gear diff?

IE filling the diff with 50% oil vs 100%.
To be honest I have only ever filled my diffs 100%, so I don't have any actual testing data to conclude anything. Having said that, my educated guess would be that at 50% full you will have less resistance. What I am not sure about is how that resistance would change during the course of a race compared to a 100% full diff due to heat and viscosity breakdown.
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Old 10-28-2014, 11:21 AM   #509
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Originally Posted by Martin Crisp View Post
To be honest I have only ever filled my diffs 100%, so I don't have any actual testing data to conclude anything. Having said that, my educated guess would be that at 50% full you will have less resistance. What I am not sure about is how that resistance would change during the course of a race compared to a 100% full diff due to heat and viscosity breakdown.
I think the other pierce here is consistency. Even assuming you can fill the diff to exactly 50% (or whatever percentage) using a scale and weight of the fluid, the consistency would change drastically as you spun the diff during the race. The fluid would travel to the outside edge and you would have very inconsistent feel. If you fill to 100% every time you can control the diff performance with viscosity, which is much more consistent (assuming you stay with the same brand of fluid). That's just my take on it.
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Old 10-28-2014, 04:53 PM   #510
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bought the subscription
Good explanation of weight transfer and roll center. I saw the vid below before, but never paid too much attention to it. But after reading the explanation in the learnsetup, the concept of roll center in the vid all came together. The key is to think of the forces generated from the tire and how it affects the chassis.
+ YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.
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