What is Motion Ratio? [Suspension Simplified] (Daily 011)

[@theohlinsguy4649]

I go over this in my video on motion ratios. You are right, I should have done that.

[@fujiki5507]

Very detailed info, thank you.

[@PaulOtlewski]

No problem, I am happy to help.

[@off-roadingexplained8417]

Would you say that independent suspension will tend to run stiffer because of less suspension leverage compared to solid axle , as a rule ?

[@vaibhavkanojia7189]

Thanks, man! this was really helpful.

[@yashkhedkar1823]

What is the difference between installation ratio and motion ratio?

[@vighneshdeshmukh738]

Can you please make a video on how to determine the stiffness of ARB in terms of motion ratio when we have a specified amount of body roll rate

[@veron732]

Good video man

[@kevincorrea3563]

So basically you’re able to run a stiffer spring on a double wishbone than a strut style suspension?

[@PaulOtlewski]

It's not so much about ability. Cars that have motion ratios that reduce a springs leverage on the wheel REQUIRE higher spring rates to have a similar wheel rate as a strut based car. When choosing spring rates, the motion ratio must be considered. That's why most coilovers for miata's (double wishbone) have very similar rates as coilovers for an s13 (Strut) despite the s13 weighing 500-600 lbs more. The Miata wheel rate is actually much lower due to the suspension design. Now, don't take this to mean that either design is inherently better or worse. It's just something to be cognizant of when choosing spring rates.

[@theohlinsguy4649]

The wheel rate with a .5 motion ratio is not half of the spring rate/damping on the shock/strut.

[@sreeramanil3902]

How can I calculate motion ratio of push rod suspension?

[@Jakeserra]

hey man I know this was posted in 2018 but maybe if you see this you can help me out, I'm a freshman at UofM dearborn and I recently joined the Formula SAE EV team. I was trying to ask the suspension leader on the team why he has the suspension mounted so high up ( I think that doing that is bad for weight distribution and CG as well as it disrupts aerodynamics) and he seemed so so set on making the MR as close to 1:1as possible, I understand there may be a benefit but I think that you could easily solve the problems it would create with MR if you move the shock lower and more inboard like an older f1 car or just a well designed cantilever suspension. he's pretty much in full disagreement with me and being a freshman vs him a grad student It's hard for me to back my claims up with math all I have is experience racing cars and karts as well as just a deep love of racing cars and engineering. also I'm sure having more suspension travel than wheel travel would be beneficial rather than a 1:1 deal. anyways hope you can provide some insight, thanks.

[@PaulOtlewski]

Well like anything with Suspension design it's all about balancing trade offs. Mounting the shock further inboard would be better for aero but the control arm taking the load needs to be stronger to take the cars load and depending on the design might increase the weight. A push rod style Suspension can alleviate this but you are adding more components and complexity of design. I am not familiar with the exact design constraints of FSAE. So I can't make any recommendations but I don't think there is anything inherently wrong with your argument.

[@akashtyyadav]

Thanks for making such an informative video. I just think you have said the opposite while giving example value of motion ratio. The motion ratio should have been 2 instead of .5.

[@PaulOtlewski]

Akash Yadav Thank you for you kind words. In relation so how we use motion ratio where the wheel rate is considered baseline as 1, so a 1:1 motion ratio would be one inch of wheel travel results in 1 inch of spring compression. So the wheel rate side of the ratio is always 1. In a 0.5:1 Ratio the spring is inboard halfway between the pivot and the wheel at the end of the lever. So 1 inch of wheel travel results in 1/2inch of spring travel. This gives the wheel more leverage over the spring making it less effective and halving its rate. If the ratio were 2:1 that would indicate that the wheel is inboard of the spring and the spring would be more effective at a given rate. So in respect to wheel rate yes it is a 0.5 motion ratio

[@theohlinsguy4649]

Motion ratio can be shown both ways. It is the same thing. The math to calculate wheel rate would just be different with the same final result

[@that.schamp]

@@theohlinsguy4649 The way I have seen this described is that the "motion ratio" is the inverse of the "leverage". So the CT9A rear has an average of 1.41 leverage over the arc of travel for an average motion ratio of 0.71. Which is why, if you don't get a custom spring and valve rates on your Ohlins, you will need large rear bar, sacrificing the ability to rally over curbs to close on an F80 =)

[@sr20dett1000]

E36 8k front is still to stiff

[@PaulOtlewski]

Just an example of the typical rates that most companies provide as standard on their coilovers. Sure there is always fine tuning of rates that can be done from there. This video is just an over view of motion ratios. I'm not advocating for a certain combination of spring rates as that is going to highly depend on a host of factors like weight of the car in question, the weight balance, motion ratio and discipline of competition. What works well in a stripped down drift car probably would not be optimal on a Aero Heavy Time attack car, or an Autox or drag car. There is no one solution that works for every situation. Everything works as a system and I am trying to give a general explaination of how the different aspects of the system work and affect each other.