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Old 11-09-2011, 07:19 AM   #1
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Default Confused about chasis flex!

So I seem to be confusing myself as to what is actually needed as far a chasis flex is concerned.

I have been told that with rubber tires you want a lot of chasis flex as they have less traction than foams.

I have also been told that you want a stiff chasis on carpet as it is higher traction than asphault.

My confusion is I run on carpet with rubber tires! Do I just pick a middle setting and play from there?

How does one determine the level of traction as a particular track or is is subjective. I have no basis for comparison.
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Old 11-09-2011, 08:16 AM   #2
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Originally Posted by Mera'din View Post
So I seem to be confusing myself as to what is actually needed as far a chasis flex is concerned.

I have been told that with rubber tires you want a lot of chasis flex as they have less traction than foams.

I have also been told that you want a stiff chasis on carpet as it is higher traction than asphault.

My confusion is I run on carpet with rubber tires! Do I just pick a middle setting and play from there?

How does one determine the level of traction as a particular track or is is subjective. I have no basis for comparison.
Depending on how many options you car has for flex, you will likely tune which ever end of the car is giving you issues. A lot of flex can hurt highspeed steering, to much can cause a low speed push, to much front and not enough rear can lead to oversteer, etc.
Most of the newer cars are very balanced flex wise, and you can tune by adding/removing screws on the top deck.
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Old 11-09-2011, 10:38 AM   #3
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I am confused about chassis flex in general. Why would you not want the car to be perfectly stiff (consistent and predictable), and then rely on the shocks/springs/damping to adjust traction?

It seems counter-intuitive to me to have what could be and unpredictable 'spring' or possible inconsistency of flex, in the middle of the car to add traction - then twiddle with the other suspension bits to also adjust traction????

Thoughts on this? (vehicle engineering types - please pontificate! )
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Old 11-09-2011, 10:44 AM   #4
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This idea of tuning "one end" of a chassis' flex always perplexes me.

The end of a chassis you are tuning is only flexing relative to the tyres on the ground at the opposite end of the car (the middle of the car is not attached to anything), so it's basically impossible to isolate the actual flex from one end of the car to the other.

I can see how you might get some additional effects if, say, the front end of the chassis is flexing relative to the steering mechanism; but fundamentally, a flexy chassis will generate more grip and be less responsive, and a stiffer one will generate less grip and be more responsive.

I'm of the opinion that if your car stops getting quicker as the grip level increases at a track, then the chassis is too soft. I used to run a TOP Photon, and the standard 6-cell chassis was awesome on the first run of the day (rubber carpet). By the fourth run of the day it was grip rolling. Going one grade stiffer on the top deck made the car slower in round one, but better in the finals, which is where it counts.
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Old 11-09-2011, 11:19 AM   #5
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Here's my thoughts on chassis flex. We have driven the state of development of R/C vehicle chassis to an extremely refined level. The tuning options and resolution of them is incredible.

Stiffer was better for a long time in following with standard race car practice from full size thinking. This was especially true during the foam tire touring car era. Grip available was so high that a flexy car was pure evil if not set correctly, or at the very least off pace.

Now that rubber tire is the standard we have to factor in tire loading and heat along with setting the car for driver and circuit. Load the tires too hard, traction goes away. Not enough, car is off pace or breaks traction easily. Traction level going up over the course of an event (primarily carpet but asphalt builds in relation to weather) means you are always chasing the correct setting for optimal tire loading.

With the highly refined level of tuning now possible from most chassis, finding the correct setting can be difficult. Getting right up to that 'magic' point where the car is truly neutral the entire run is very difficult. With a stiffer chassis, driver input is also much more direct and if you're off enough on settings, a car that is good early in the run may 'go away' by the end of the run.

One other factor that complicates what we do with model racing cars is feedback. Quite simply, we do not sit in our cars. We do not even 'feel' what the car is doing. Drivers tend to use that term to describe how the car reacts to input on track but there is no physical feedback from the chassis. Without that feedback we do not know when a car is near the limit of traction. We simply react to it after our eyes see it and it's processed by our brain and hands.

Long story short, a more flexible chassis with finer tuning results in better performance. This is because it is easier to drive near the limit of traction without going too far and overloading the tire. Think of the flexible chassis as a two way filter, both for input from the driver and the racing surface.
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Old 11-09-2011, 11:35 AM   #6
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I am running a Xray T3'11 with 17.5, stock timing, and rubber tires. I had some trouble getting around the tight 180 degree turn last week. The only scres not installed are the steering post standoffs.

I am also running a T3'10 VTA setup. and that car seemed too loose on the rear end at mid corner/corner entry. No steering standoff or rear chasis braces on that rig.

If that helps.

So you basically setup a car for the surface you will be running on and fine tune from there? I guess my reason for asking was the inital confusion on what intial setup is recommended for carpet/asphault.

Additionally, Xray has so many options for stiffness, chasis flex, hard arms, hard hubs, etc... I just can find a reference on how each part changes the car. If someone told me that with a carpet car, you need to set it up stiff. I would do it and vice versa.
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Old 11-09-2011, 12:05 PM   #7
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If you were to ask a full scale designer about chassis flex they would say it is bad. But they have hours in simulators and sophisticated suspension designs to over come compliance problems. As racers we have very little time to absolutely dial in the ultimate set up and rather old fashioned suspension design so we can use chassis flex to help. We can be 1lb out on the spring 20 cst out on the oil 0.5mm out on the position of the shock for the ideal configuration, but chassis flex will overcome all that and get a car that we can drive around the track in the quickest possible way.

So, chassis flex is bad but it is a compromise and a way of achieving balance easily.

An interesting point is that now MotoGP are talking about bike frame flex...so maybe someone is looking at RC
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Old 11-09-2011, 01:58 PM   #8
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I am also "frustrated" with chassis flex, as a former 1/1 scale person, but do agree about the true feedback and adjustability we lack when compared to full scale.

Here's another thing I am wondering about, 3* or so of rear toe. Why so much? never came close to that in full scale. Anyone have a "good" explanation, other than its possible, it has a positive enough effect, vs the negative drawbacks, so use it.

Thanks.
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Old 11-09-2011, 02:55 PM   #9
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Quote:
Originally Posted by LitemodzUK View Post
If you were to ask a full scale designer about chassis flex they would say it is bad. But they have hours in simulators and sophisticated suspension designs to over come compliance problems. As racers we have very little time to absolutely dial in the ultimate set up and rather old fashioned suspension design so we can use chassis flex to help. We can be 1lb out on the spring 20 cst out on the oil 0.5mm out on the position of the shock for the ideal configuration, but chassis flex will overcome all that and get a car that we can drive around the track in the quickest possible way.

So, chassis flex is bad but it is a compromise and a way of achieving balance easily.

An interesting point is that now MotoGP are talking about bike frame flex...so maybe someone is looking at RC
You my friend are exactly right.

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Originally Posted by oldrcr View Post
I am also "frustrated" with chassis flex, as a former 1/1 scale person, but do agree about the true feedback and adjustability we lack when compared to full scale.

Here's another thing I am wondering about, 3* or so of rear toe. Why so much? never came close to that in full scale. Anyone have a "good" explanation, other than its possible, it has a positive enough effect, vs the negative drawbacks, so use it.

Thanks.
The main explanation for so much toe is this, we don't really need to worry about top speed. Very few of us run on tracks where we can even hit terminal velocity much less spend much time at it. Therefore, we run very high drag wings and lots of toe to maximize grip, corner speed and acceleration at the expense of reducing a top speed that we will never reach anyway.

Plus tire wear is much less of an issue and our power to weight ratios are much higher than most 1:1 racing.
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Old 11-09-2011, 04:38 PM   #10
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My understanding is the chassi flex is it helps in two ways. Firstly softer chassi just like softer suspension increases grip, and a combination of a softer chassi allows for slightly stiffer suspension which provides a more responsive car with the same level of grip.

The second advantage is a softer chassi helps deal with the high frequency vibrations which the suspension cannot absorb and it transmitted into the chassi.

If you go in a modern day very stiff car with poor shocks there are a lot of high frequency vibrations which are transmitted throughout the chassi and can be felt by the passengers. This can result in less stability under load in corners. With great shocks this is much less of a problem but we don't have that in RC as of yet. This could be the next RC evolution.

Also keep in mind most bumps on RC tracks are much bigger than they are in 1/1.
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Old 11-10-2011, 07:08 AM   #11
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Chassis flex has been apart of the RC design philosophy for as long as I can remember. Understanding how chassis flex can help is very important, but I have found that each chassis has different characteristics. In the case of a top deck and the chassis keep in mind that both of those items effect the overall ability for the chassis to flex. Flex in the chassis exists in real scale cars as well, but is exaggerated in these scale cars. It is hard to see it but a chassis that does not flex will break!

Chassis flex is used as a tuning tool and makes a very large difference in how the car reacts to many surfaces. Some cars have a very predictable amount of flex and the flex is very linear. Other cars (The ones with multiple top decks) tend to flex in a progressive and less predictable manor.

Most modern RC cars (specifically Touring cars) are design for lower bite surfaces using rubber tires. These chassis need to flex in order to roll and generate the traction required for the tires that exists in the market today.

I recently made a small change to my top deck and this resulted in a huge change in the flex of the car. I felt this change immediately and has been great. But these types of changes don't always work the way you might think. It is best to try different scenarios to find what works best for you on the surface you run on.

Keep in mind that the components attached to the chassis also effect the flex. This would include the bulkeds, motor mount, servo mounts, arm holders, and steering components. When you fiddle with these components they will effect the flex of the car as well.

Like any setup change chassis flex can go too far and start to create inefficiencies within the car. Even though you can make chassis changes in the front middle and rear this will effect the entire car.
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Old 11-10-2011, 11:44 AM   #12
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I agree and disagree. Certainly flex is an integral part of touring cars, but flex does not gain traction. It gains you compliance. Traction is a result of a chemical reaction between the tyre and the surface. That cannot be changed by flex.

For example, 2 identical TCX's but one has a flexible top deck the other as a stiff top deck. They will both have equal traction of the start line. In the turn it is the car that maximizes the potential slip angle of the tyre that will corner faster.

Flex allows us to achieve balance quickly.

Playing about with the top decks of cars is a very inexpensive way of dialing the car to the track and your driving style and is highly recommended to all racers.

Last edited by LitemodzUK; 11-10-2011 at 11:45 AM. Reason: poor punctuation
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Old 11-10-2011, 02:58 PM   #13
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Traction is a result of a chemical reaction between the tyre and the surface. That cannot be changed by flex.
Care to show that reaction in a balanced equation? The force of resistance between tire and road is called Friction, its a physical property not a chemical one. F = ma. 4 years of uni science degree at work here, my $40K was not wasted LMAO.

And friction can be changed by flex, in both positive and negative manner. Think of it this way, a steel ball and a rubber ball, one is going to have much more rolling resistance (friction) than the other because of material flex, the same goes with chassis, if a chassis flexes under certain conditions it can generate more friction in one part of the car and less in another.
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Old 11-10-2011, 03:14 PM   #14
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Care to show that reaction in a balanced equation? The force of resistance between tire and road is called Friction, its a physical property not a chemical one. F = ma. 4 years of uni science degree at work here, my $40K was not wasted LMAO.

And friction can be changed by flex, in both positive and negative manner. Think of it this way, a steel ball and a rubber ball, one is going to have much more rolling resistance (friction) than the other because of material flex, the same goes with chassis, if a chassis flexes under certain conditions it can generate more friction in one part of the car and less in another.
IMO its a bit of everything. The softening of rubber tire is a chemical reaction to the compound, but flex is a physical change.

Ff = µFn would be a better example then yours. µ changes as the tire gets softer, and the more the car flexes, the normal force increases on the outer tires.

How does this all work in terms of how much flex you want/need?

Don't ask me.
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Old 11-10-2011, 04:19 PM   #15
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Originally Posted by sosidge View Post
This idea of tuning "one end" of a chassis' flex always perplexes me.

The end of a chassis you are tuning is only flexing relative to the tyres on the ground at the opposite end of the car (the middle of the car is not attached to anything), so it's basically impossible to isolate the actual flex from one end of the car to the other.

I can see how you might get some additional effects if, say, the front end of the chassis is flexing relative to the steering mechanism; but fundamentally, a flexy chassis will generate more grip and be less responsive, and a stiffer one will generate less grip and be more responsive.

I'm of the opinion that if your car stops getting quicker as the grip level increases at a track, then the chassis is too soft. I used to run a TOP Photon, and the standard 6-cell chassis was awesome on the first run of the day (rubber carpet). By the fourth run of the day it was grip rolling. Going one grade stiffer on the top deck made the car slower in round one, but better in the finals, which is where it counts.
Could not agree more
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