Dan's Vehicle Dynamics Corner - The importance of motion ratios

Summary

In 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

00:00

🏎️ 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.

05:03

📏 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.

10:08

📊 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.

Keywords

💡Motion Ratios

Motion ratios are a critical component in vehicle dynamics, particularly in racecar setup and vehicle modeling. Defined as the ratio of how the damper moves over the ratio of how the wheel moves, motion ratios are essential for determining the spring rate the tire sees, which is calculated as motion ratio squared times spring rate. In the script, Danny Nowlan emphasizes the importance of getting motion ratios right for a correct racecar setup, as it can greatly affect the vehicle's performance and how everything falls into place.

💡Racecar Setup

Racecar setup refers to the process of configuring and adjusting a racing vehicle to optimize its performance. This includes tuning various aspects such as suspension, tires, and aerodynamics. In the video, the focus is on the role of motion ratios in racecar setup, illustrating that proper motion ratio calculation can lead to a more effective and balanced vehicle configuration.

💡Vehicle Dynamics

Vehicle dynamics is the study of the forces and torques that affect the motion of vehicles and the relationships between a vehicle's mechanical and handling characteristics. The script discusses motion ratios within the context of vehicle dynamics, highlighting their impact on how a racecar performs on the track.

💡Damper

A damper, commonly known as a shock absorber, is a device used in the suspension system of a vehicle to absorb and damp shock impulses from the road surface, helping to control the motion of the vehicle's sprung mass. In the script, the movement of the damper is a key factor in determining motion ratios, which in turn affects the spring rate seen by the tire.

💡Spring Rate

Spring rate is a measure of the stiffness of a spring, defined as the force needed to compress the spring by a given distance. In the context of the video, the spring rate as seen by the tire is directly influenced by the motion ratio, which is crucial for the vehicle's handling and performance.

💡Wheel Movement

Wheel movement refers to the vertical displacement of the wheel relative to the vehicle's chassis. It is an important aspect considered when calculating motion ratios, as it helps determine how the wheel interacts with the suspension and damper movements.

💡Suspension

Suspension is the system of springs, shock absorbers, and linkages that connects a vehicle to its wheels. It is designed to provide a smooth ride by absorbing and dampening the impact of surface irregularities. In the script, understanding suspension behavior through motion ratios is highlighted as a critical part of setting up a racecar for optimal performance.

💡Kinematic Programs

Kinematic programs are software tools used to calculate and analyze the motion of a vehicle's components based on their geometry and kinematic relationships. In the video, programs like WinGeo and SusProg are mentioned as useful for calculating motion ratios, although the speaker advocates for physical measurement as a backup.

💡Instant Roll and Pitch

Instant roll and pitch refer to the immediate response of a vehicle to changes in direction or road conditions, which are critical aspects of vehicle dynamics. The script mentions that getting motion ratios right can lead to accurate instant roll and pitch correlation in simulations, indicating a well-tuned vehicle setup.

💡Simulation

Simulation in this context refers to the use of computer models to replicate the performance of a vehicle under various conditions. The script discusses how correct motion ratios can lead to accurate simulations, which are vital for predicting and improving a racecar's performance on the track.

💡Lateral Load Transfer

Lateral load transfer is the shifting of a vehicle's weight from one side to the other during cornering. It is an important factor in vehicle dynamics that affects handling and stability. The script implies that understanding motion ratios can also have implications for analyzing and improving lateral load transfer in a vehicle.

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.