Dan's Vehicle Dynamics Corner - The importance of motion ratios
Summary
TLDRIn this episode of Dan's Vehicle Dynamics Corner, Director Danny Nowlan emphasizes the critical yet often overlooked role of motion ratios in racecar setup and vehicle modeling. He explains how motion ratios, the ratio of damper to wheel movement, directly impact the spring rate perceived by the tires. Nowlan demonstrates practical methods for measuring motion ratios, including using a car jack and verniers, and highlights the importance of plotting wheel movement over damper displacement. He also discusses the implications of accurate motion ratios for simulations and lateral load transfer distributions, ultimately stressing that correct motion ratios are essential for optimal vehicle performance.
Takeaways
- 🏎️ The importance of motion ratios in vehicle dynamics, particularly in racecar setup, cannot be overstated as they are crucial for getting the setup correct.
- 🔍 Motion ratios are often overlooked but are a critical element in vehicle modeling and should be kept in mind for optimal performance.
- 📉 The motion ratio is defined as the ratio of damper movement to wheel movement, which impacts the spring rate that the tire sees.
- 👨🔧 To deduce motion ratios, one should plot wheel movement over damper movement from full droop to full bump to understand the vehicle's dynamics.
- 🛠️ Practical methods for measuring motion ratios include using a car-jack and a table with a level next to the tire for open wheelers and adding a pipe extension on the wheel nut for touring cars.
- 📏 Tools like Verniers are used for plotting wheel movement on damper displacement, providing a direct way to measure motion ratios.
- 🤖 Kinematic programs such as WinGeo and SusProg can calculate motion ratios, but manual measurement is recommended to catch any discrepancies.
- 📊 Approximation formulas can provide a quick estimate for motion ratios, such as using the pickup point to pivot length ratio for lower arms and the contact patch to instant center length.
- 📈 Plotting motion ratios on the fly can help quickly identify mistakes and ensure that the vehicle's setup is on track.
- 📝 An Excel spreadsheet can be used to document motion ratio data, which is useful for comparing actual and simulated vehicle dynamics.
- 📉 Accurate motion ratios are indicated by a linear graph and a close correlation between instant roll and pitch in vehicle dynamics simulations.
Q & A
Who is the speaker in the video script?
-The speaker in the video script is Danny Nowlan, the Director of ChassisSim Technologies.
What is the main topic of discussion in this episode of Dan's Vehicle Dynamics Corner?
-The main topic of discussion in this episode is the importance of motion ratios in racecar setup and vehicle modeling.
Why are motion ratios considered critical in racecar setup according to the speaker?
-Motion ratios are critical in racecar setup because getting them right can significantly impact the vehicle's performance, making everything fall into place more effectively.
What is the formula that relates wheel rate to motion ratio and spring rate?
-The formula that relates wheel rate to motion ratio and spring rate is wheel rate = motion ratio squared times spring rate.
How can one measure motion ratios in practice?
-Motion ratios can be measured in practice by plotting wheel movement over damper movement from full droop to full bump, using tools like a car-jack, a table for leveling, and Verniers for precise measurement.
What is the purpose of using a table and Verniers in measuring motion ratios?
-The purpose of using a table and Verniers is to provide a fixed level and precise measurement of how the wheel moves up and down, which helps in plotting wheel movement over damper displacement.
What are some software programs mentioned that can calculate motion ratios?
-The software programs mentioned that can calculate motion ratios are WinGeo and SusProg.
Why does the speaker advocate for measuring motion ratios even when software like WinGeo and SusProg are available?
-The speaker advocates for measuring motion ratios manually because there might be a one in a hundred case where the software could be incorrect, and manual measurement can catch such discrepancies.
What is the formula provided for motion ratio approximation in a push rod operated open wheeler arrangement?
-The formula provided for motion ratio approximation in a push rod operated open wheeler arrangement is motion ratio = A on B multiplied by sine theta, where A on B is the ratio of the length from the end of the lower pickup point to the instance center, and theta is the angle of the push rods.
How does the speaker suggest verifying the correctness of motion ratios?
-The speaker suggests verifying the correctness of motion ratios by comparing the simulated data with the actual data, particularly looking for instant roll and pitch correlation and ensuring there are no massive discrepancies between the two.
What implications does getting motion ratios right have on a vehicle's performance and simulation?
-Getting motion ratios right has implications for both the vehicle's performance and simulation accuracy. It helps in understanding the car's behavior, affects lateral load transfer distributions, and improves the reliability of simulations for setup adjustments.
Outlines
🏎️ Motion Ratios in Racecar Setup
Danny Nowlan, Director of ChassisSim Technologies, introduces the topic of motion ratios in this episode of Dan's Vehicle Dynamics Corner. He emphasizes the often overlooked but crucial role motion ratios play in racecar setup. Motion ratios are the ratio of damper movement to wheel movement, and they significantly affect the spring rate that the tire experiences. The correct motion ratios can lead to a well-balanced setup, as illustrated by a practical example promised at the end of the tutorial. Nowlan suggests plotting wheel movement over damper movement from full droop to full bump to understand the vehicle's behavior. He also provides practical steps for measuring motion ratios, such as using a car-jack and a level to measure wheel and damper displacement for open wheelers and touring cars.
📏 Measuring and Approximating Motion Ratios
The script continues with a detailed explanation of how to measure motion ratios for different types of vehicles, including open wheelers and touring cars. Nowlan describes the process of removing springs and using a car-jack to achieve full droop, then measuring wheel and damper movement with Verniers. He also discusses the use of kinematic programs like WinGeo and SusProg for calculating motion ratios, but advocates for physical measurement as a safeguard against potential errors. The paragraph includes formulas for approximating motion ratios in touring cars and push rod operated open wheeler arrangements, highlighting the importance of accurate measurement for achieving a linear and consistent motion ratio graph.
📊 Analyzing Motion Ratio Data for Vehicle Dynamics
In the final paragraph, Nowlan discusses the importance of analyzing motion ratio data to understand vehicle dynamics. He explains that correct motion ratios can quickly reveal discrepancies in simulation models, which is vital for refining vehicle setup. The script provides an example of a preliminary simulation of a Touring car, where the actual and simulated data are compared. Nowlan advises looking for instant roll and pitch correlation as an indicator of accurate motion ratios. He also warns of the signs of incorrect motion ratios, such as large discrepancies between simulated and actual pictures, and emphasizes the impact of motion ratios on wheel rate, which is motion ratio squared times spring rate. The paragraph concludes with a strong recommendation to undertake the motion ratio deduction exercise for a deeper understanding of vehicle behavior.
Mindmap
Keywords
💡Motion Ratios
💡Racecar Setup
💡Vehicle Dynamics
💡Damper
💡Spring Rate
💡Wheel Movement
💡Suspension
💡Kinematic Programs
💡Instant Roll and Pitch
💡Simulation
💡Lateral Load Transfer
Highlights
Introduction to the importance of motion ratios in racecar setup and vehicle modeling.
Motion ratios are often overlooked but are critical for a correct racecar setup.
Correct motion ratios can significantly simplify vehicle dynamics setup.
The formula for calculating the spring rate the tire sees based on motion ratio.
Practical advice on plotting wheel movement over damper movement from full droop to full bump.
Instructions for measuring motion ratios on open wheelers and touring cars.
Use of a car-jack and table for leveling when measuring motion ratios.
Technique of using Verniers to plot wheel movement on damper displacement.
Recommendation to measure motion ratios despite the availability of kinematic programs.
Explanation of motion ratio approximations for touring cars and open wheelers.
The use of Excel spreadsheets for motion ratio data collection and analysis.
Demonstration of plotting motion ratios on the fly for quick error detection.
Importance of linear motion ratios for accurate vehicle dynamics simulation.
How to identify discrepancies in motion ratios through simulation and real-world data comparison.
The impact of correct motion ratios on lateral load transfer distributions and vehicle performance.
Final emphasis on the significance of motion ratios in determining wheel rate and vehicle dynamics.
Transcripts
Hello, my name is Danny Nowlan and I'm the Director of ChassisSim Technologies. Welcome again to this
latest episode of Dan's Vehicle Dynamics corner. What we're going to be discussing in this episode
of Dan's Vehicle Dynamics Corner is the importance of motion ratios. Now when we think about motion
ratios in some respects sometimes I think of them as the almost forgotten bit of when you're doing
racecar setup and when you're putting together a vehicle model, yet even though they're sort of
forgotten and not thought about it's particularly sexy I can tell you right now getting your motion
ratios right is such a critical element of getting your racecar setup correct
because the motion ratios... if you get these right you'll be amazed at how everything falls
into place and I'll show you a practical example of that at the end of the tutorial so really this
is more really sort of even though what I'm about to discuss is incredibly obvious it's nonetheless
a very, very important thing for you to remember and for you to keep in mind. So let's get started
right when it comes to deducing motion ratios what we want to classify is the ratio of how
the damper moves over the ratio of how the wheel moves now from time to time you will
see race car manufacturers state that the other way around. But here's the kicker the reason it
is so important is because the is the spring rate the tyre sees is given by wheel rate is equal to
motion ratio squared times spring rate. This ladies and gentlemen is why classifying emotion ratios
is such an important job and it's actually one of the most overlooked jobs that you'll see but
if you get it right you will just be amazed how things fall into place and really what we want
to do is we want to plot wheel movement over damper movement from full droop through to full bump now
I say from full group droop to full bump because we need to get a really good idea of what it's
actually doing now in terms of the practicalities of doing it actually not as difficult as you think
this is what you need to do. If you'll running say something like an open wheeler what you if you're
dealing with something like an open wheeler what you need to do is simply take
you need to do is take the spring off and what you do get the car in the air and let it go down
to full droop get a car-jack and what you get a car-jack and what you do is that typically if it's
something like say an open wheeler you'll grab a table with the level next to the tyre so you can
get a good level if you're dealing with something like a touring car you do exactly the same thing
except what you would is basically put someone like a bit of extend it a little bit
like say a little bit of pipe on the wheel nut so you can go through but the whole idea is you
just want a fixed level so you can measure how the wheel goes up and down and typically what you'll
be doing is that you'll grab a set of Verniers and you will be simply plotting wheel movement
on the on damper displacement you also do the same thing with the bar movement and
what you do what you would do with the bars is obviously you disconnect
disconnect the bar but you would provide that you would leave the motion ratio linkage as the
same and all you're doing is that you're plotting how the motion is that you're basically plotting
how that motion ratio drop link is moving so consequently it doesn't have to be ultra
ultra advanced and indeed I know that there are a lot of kinematic programs out there that will
calculate motion ratios for them I mean WinGeo and SusProg cases in point and they do it very,
very well that being said I always do advocate and measuring this because ninety nine times out
of a hundred wins your programs like WinGeo and SusProg will get you right there however there
is that one out of a hundred case that can ruin your whole day so which is why I always advocate
measuring it. Now in terms of some motion rational approximations you might find useful if you're if
you're dealing with a touring car or something that's not mounted to a bell crank this is an
old trusted this is an old trusted little formula that you can use so effectively you measure the
pickup point from where the damper connects to the lower arm to where it pivots divided by the length
of the lower arm and the C on D is effectively measuring the
length from the end of the lower pickup point to the instance centre and D measures the length
of the top of the contact patch to the instant centre now if you're in a rush and you don't have
your kinematic programs to hand look you can approximate it with A on B but that's
if you're in the rush if you need sort of a rough rule of thumb to go okay I just need a rough
rule of thumb what my motion ratios are you can use A on B but if you really want to nail down
it's basically A on B multiplied by C on D the other motion ratio approximation that you can
use is if you've got a push rod operated open wheeler arrangement. Now in this situation what
you've got what you've got to do is you measured the angle that the push rods at and
you've you measure the rocker arms
and what you'll find here is the motion ratio is A on B multiplied by sine theta. Now please bear
in mind this is an approximation this is there to help you
really sort of nail down what the motion ratios are so you get some sort of rough expectation of
where you should be. Your next step after you do this is that you go through
and measure up the motion ratios properly and what we've got in this little Excel spreadsheet
is that you'll see this is a motion ratio stuff for some Star Mazda work I did a
couple of years ago and what we've got a what I did was that I was like as we discussed before
I took off the springs and bump rubbers I just left the Springs connected got the car in the
air and all I did was I just simply grabbed the table, moved it near the action it moved
it near to where the centre of contact patch is just grab the couple of clamps basically fit a
clamp to fix level to the table and what I did was I simply move the wheel up and down and as
you can see I plotted my wheel deflection and I plotted my damper deflection and that is what I
got left now the beauty about this is what and this is a little technique that's gold because
what it does is that you can do this on the fly so if you make a mistake it's going show up really,
really quickly now as you can see here the motion ratios here are actually very, very linear and
usually if you've got a really nice continuous graph that's showing you that you're really,
really on the money so this is why I always advocate
advocate plotting it as you go because if you screw up you're going to see
you're going to see a discontinuity so that's a really, really handy tip. Now in terms of what
to look for in the data when you get your motion ratios right you're going to get something like
this now what we've got here is a preliminary simulation of a Touring car that
that we did these measurements on. The actual data is coloured the simulated data is
black so this is a first cut but you know you're on the money when you're starting to get instant
roll and pitch correlation off the bat in terms of what to be looking for I can
tell you right now if we just take a look at this pitch comparison which is the two
second and third last bottom graphs they're basically spot-on straight away if you
get a spot on straight away that's a really good indicator that you're a that's a really,
really good indicator that your roles are that your main spring ratios are absolutely spot that
your main spring ratios are spot-on also too, just zooming back out zooming back out looking at
somewhere we had equivalent speeds we can also see that our role that our roll motion ratios
were also that we're also close now initially when you're doing this particularly when you run up the
first model for the for the first time you are not looking to a roll and pitch correlation within the
within the nearest nanometer but you're looking for to say with depending on
your spring race how big the car is rough rules of thumb plus or minus a couple of Mil on your
initial cut if you've got that you're laughing what you need to be really mindful of is a really
tell tale sign that you've got your motion ratios wrong is when you've got massive discrepancies in
how the simulated pictures look and the actual pictures look. The other thing that you need to
be and the other thing too when you got your roll motion ratio is wrong is when you've got really,
really big discrepancies in the roll I mean... Like a factor of two that is when you know that
you've got something wrong but really I cannot stress enough the importance of working through
an exercise like this because it's going to tell you so much about what your car is going to do
and it's really going to nail there and not only does this has implications for simulation stuff
but it also has some implications for things like doing lateral load transfer distributions
are really quantifying what you want to do so really I cannot stress enough the important
and really the important the reason this is so important is because you wheel rate which is
what your tyres are saying is effectively motion ratio squared time spring right so I'll leave
you with that thought and really and really get out there and deduce your motion ratio.
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