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Mid-motor touring cars. Legit faster or fad?

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Mid-motor touring cars. Legit faster or fad?

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Old 09-10-2019, 08:13 AM
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Interesting... that is one heck of a serious setup for a 21.5 class. The battery must be pretty reliably and snugly maintained to the chassis with this mounting system, right?

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Old 09-10-2019, 09:31 AM
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Yes, USGT 21.5. The car will also be used for TC. The big battery was part of the reason for choosing the carbon Medius kit. The BD9 battery hooks and strap are very good.
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Old 09-10-2019, 09:44 AM
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Originally Posted by Lonestar View Post
I used to come from the same school of (full size car tech) though, but there is something else more upstream anyway: flex is a "pure" spring - shock springs and roll bars are hydraulically damped.... A damper filters high-frequencies out - a spring doesn't. you cannot do the same thing with suspension as you can do with flex. All in all - I'm clueless about this stuff, I can't set it up, I'm not sure I can even put a name on how it "feels" on the track - but I believe it actually matters.
Thanks for this perspective. But I have always thought that undamped springs are never your friend. Luckily the carbon fiber has some internal damping. The aluminum chassis does not have internal damping however they are so stiff that the natural frequencies are extremely high and probably don't affect the handling of the car. I will admit that I may be missing something that makes these little tiny cars different from full size cars. But I will also admit that the reasons I've read so far for using chassis flex, either I don't agree with them or more likely, do not understand them!
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Last edited by glennhl; 09-10-2019 at 10:08 AM.
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Old 09-10-2019, 10:48 AM
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The tub chassis TA-05 was a great car. Lots of steering. It was a fun car to drive especially in stock class. The way it handled corners was more "round". Many of the things discussed here (equal length belt, gyroscope effect, flex, etc) was mentioned in the TA-05 bible that tamiya published years ago. But that was when we still used nimh and brushed motor so things are a bit different today. Good to see old ideas come back like this.
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Old 09-13-2019, 11:36 PM
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This was my first mid motor yokomo😉
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Old 09-14-2019, 12:20 AM
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Here's my current Yokomo mid motor car.😎
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Old 09-14-2019, 01:53 AM
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I'm just finishing setting up my ta07 ready for some 17.5t spec racing indoors. that motor is there just to place everything.
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Old 09-14-2019, 02:56 AM
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It seems like there needs to be a distinction between "mid-motor" and "mid-shaft" vehicles.
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Old 09-14-2019, 07:43 AM
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Originally Posted by glennhl View Post
I also struggle with why RC car designers like chassis flex so much.
The answer is simple, they are found the cars are faster that way. Chassis used to be very stiff, some of the old touring cars had absolutely solid chassis. The chassis development these days means the drivers are given a range of chassis of width and flex, then test them to find out what's faster. It doesn't matter what either full size or small scale setup theory says, if the car laps quicker that's the chassis that will be on it.

Just look at chassis layout, the early touring cars had the electrics down the centre with perfect side to side weight balance, by it this meant a layshaft up above the motor. Now most of the cars have the heavy motor tucked into the left rear corner of the chassis with the battery a lot further forward on the other side. This is totally against any dynamic theory, but as they are faster because the layshaft is around 25mm lower that's what we use.
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Old 09-14-2019, 10:16 AM
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Originally Posted by terry.sc View Post
The answer is simple, they are found the cars are faster that way. Chassis used to be very stiff, some of the old touring cars had absolutely solid chassis. The chassis development these days means the drivers are given a range of chassis of width and flex, then test them to find out what's faster. It doesn't matter what either full size or small scale setup theory says, if the car laps quicker that's the chassis that will be on it.

Just look at chassis layout, the early touring cars had the electrics down the centre with perfect side to side weight balance, by it this meant a layshaft up above the motor. Now most of the cars have the heavy motor tucked into the left rear corner of the chassis with the battery a lot further forward on the other side. This is totally against any dynamic theory, but as they are faster because the layshaft is around 25mm lower that's what we use.
Thank you for the response. I am still looking for the engineering reason why the cars are faster with a flexible chassis. If it's because it lowers the roll stiffness of the front or rear, then why not run a super stiff chassis and then just lower the roll stiffness by softer springs and/or softer ARB's? I can understand that, I just don't understand why you want to decrease the roll stiffness using the chassis. I understand it on a kart because they don't have suspension, but we have a darn good suspension on our RC touring cars. I would love to do some testing with a top notch driver that can run consistent laps using his best setup with a flexible chassis. Then stiffen the heck out of the chassis and re-adjust the springs/ARB's to obtain the same results. Maybe it's not possible, but I'm betting that it is. I'm only skeptical because of all the tribal knowledge that seems to exist in RC racing. It reminds me of the guy at one of our local dirt tracks that showed up with helium in his sprint car's wing. He was the fastest driver and by the next week everyone was running helium in their wing. At least it was good for helium sales that week. But nobody went as fast as the same guy because that guy was the best driver. The helium was just noise. The other issue we run into is determining what is a real gain and what is just natural variance. I liked Mark Donohue's book, The Unfair Advantage. Mark said if you make a change, make a large change so you can tell the real difference.

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Old 09-14-2019, 07:59 PM
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Im not going to pretend to have an answer. the difference between a high flex carbon chassis and a stiff aluminum measured 1mm different from corner to corner in torsion. I can measure flex and Im faster on a stiffer chassis...sorry a little off topic. But springs on the damping system have a complicated life that based on what I have read and applied have a very tight window for handling high frequency and low frequency movement of two different masses. the chassis movement and the compression and rebound of the arms and wheels. it might be tough to add a 5th job for them and keep everything working reasonably well. you might be able to test rig something though.
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Old 09-15-2019, 12:16 AM
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Originally Posted by terry.sc View Post
The answer is simple, they are found the cars are faster that way. Chassis used to be very stiff, some of the old touring cars had absolutely solid chassis. The chassis development these days means the drivers are given a range of chassis of width and flex, then test them to find out what's faster. It doesn't matter what either full size or small scale setup theory says, if the car laps quicker that's the chassis that will be on it.

Just look at chassis layout, the early touring cars had the electrics down the centre with perfect side to side weight balance, by it this meant a layshaft up above the motor. Now most of the cars have the heavy motor tucked into the left rear corner of the chassis with the battery a lot further forward on the other side. This is totally against any dynamic theory, but as they are faster because the layshaft is around 25mm lower that's what we use.
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Old 09-15-2019, 09:22 AM
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Originally Posted by glennhl View Post
I understand it on a kart because they don't have suspension, but we have a darn good suspension on our RC touring cars. I would love to do some testing with a top notch driver that can run consistent laps using his best setup with a flexible chassis. Then stiffen the heck out of the chassis and re-adjust the springs/ARB's to obtain the same results. Maybe it's not possible, but I'm betting that it is. I'm only skeptical because of all the tribal knowledge that seems to exist in RC racing..
I'm pretty sure this exact scenario is how we got the flexible chassis.
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Old 09-16-2019, 04:12 PM
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One thing that I have noticed with using a stiff chassis (T3 - EU) is that the suspension has to work much harder, meaning overall it has more movement. This does not make a car slower, but harder on tires. Knowing how to adjust the cars roll stiffness by way of roll centre and weight transfer is key and important to ensure that grip is at the correct level, so that you can maximize corner speed and match track conditions.

The challenges with a stiff chassis, is that I found that a suspension can take quite a beating, and may require filling shocks more often because they leak shock oil. If a shock(s) start leaking (as most commonly do), they get unbalanced left to right, and the result is that you will likely be slower. This is where a racer needs to check and rebuild shocks regularly.

If you have a flex chassis, I believe it can hide / compensate for leaky shocks, and possibly compensate for a wheel that is unbalanced.
Put an unbalanced wheel on a stiff chassis i.e. front left, and it is likely that the front right end of the chassis will be affected by the vibration. The unbalanced wheel will also add unwanted movement to the suspension (high freqency oscillations), and could cause a shock to leak prematurely. The faster a car travels down a straight with an unbalanced tire, the higher amounts of vibration will occur resulting in loss of contact patch. Top level drivers pay attention to this by "listening" to the feel of the car while it is on the track, or they can visualize what is happening by placing themselves inside the car.

Basically, I see flex as a way to put a band-aid on a bad suspension setup, as it takes time to learn what works to match track conditions with different ambient temperatures.
If you don't have the time or patience to play with shock settings, or don't take the time to experiment with shocks, you will never know how good a chassis can really be for your driving style.
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Old 09-16-2019, 05:38 PM
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Can anyone tell me the roll stiffness of the chassis flex versus the roll stiffness of the spring/ARB's? Just curious of how much chassis flex are we talking about here?
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