Rising motion ratios, how much is too much?

[Sausage Sandwich]

Evening!

I have 30mm droop and 70mm bump, contacting the bumpstop at about 45mm bump.

My motion ratios are:

Full droop: 0.859:1
Static: 0.93:1
45mm bump: 1:15:1
Full bump: 1.38:1

Is this too much, because the wheel rate at full bump is over double that at static... although I'll never achieve full bump due to the bumpstop so the 45mm figure is more realistic.

In summary, what is the rule of thumb for the ideal amount of rising rate?

Cheers,
Sausage

[Sausage Sandwich]

Evening!

I have 30mm droop and 70mm bump, contacting the bumpstop at about 45mm bump.

My motion ratios are:

Full droop: 0.859:1
Static: 0.93:1
45mm bump: 1:15:1
Full bump: 1.38:1

Is this too much, because the wheel rate at full bump is over double that at static... although I'll never achieve full bump due to the bumpstop so the 45mm figure is more realistic.

In summary, what is the rule of thumb for the ideal amount of rising rate?

Cheers,
Sausage

[Denny Trimble]

My team talks about rising rates in terms of the wheel rate, i.e. installation ratio squared. And I think you're talking about "installation ratio" (spring motion / wheel motion, Milliken), instead of "motion ratio" (wheel motion / spring motion, Smith), since you think you have a rising rate.

There have been discussions on this forum about rising rates in the past, and most teams keep it small because of the roll stiffness change due to pitch.

Corner entry = nose down, tail up, = understeer with rising rates.

Corner exit = tail down, nose up = oversteer with rising rates.

[Sausage Sandwich]

Yep, my ratios are wheel:shock. So 1:1.38 means 1 inch of wheel movement = 1.38inches of damper movement.

Interesting though thanks. Anyone got any figures? Ie 10%, 20%? Mine is 40%+.

Cheers,
Sausage.

[kwancho]

The design I'm working with now has 1.5% per inch. *shrug*. I don't see why any at all would be good for our application.

[Psychosis]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Sausage Sandwich:
Evening!

I have 30mm droop and 70mm bump, contacting the bumpstop at about 45mm bump.
</div></BLOCKQUOTE>

Isn't that quite a lot of travel? gives you a ride height of at least 70mm. just curious because i thought most teams designed to meet the minimum rule requirements. or am i being daft and this isnt an FSAE car your building?

[Sausage Sandwich]

Yeah, sorry, it is another single seater, not an FSAE and it has a rideheight of 100mm.

But thanks for your help!! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Cheers,
Sausage.

[Marc Jaxa-Rozen]

As Denny suggested try plotting your roll stiffness distribution under load transfer through corner entry/exit, that should give you a measure of what to expect. I'd try to keep it under 10% (looking at wheel rates) for FSAE, probably more than that with aero.

Marc Jaxa-Rozen
École Polytechnique de Montréal

[Sausage Sandwich]

Thanks guys. I've had a look through the archives and read all relating to rockers etc.

I think I might tone mine down a bit and perhaps aim for 1:1 rising to 1:1.1 at most. Play it safe, it is easier to add a rising rate rocker later than it is to add a linear one later in my case.

Cheers,
Sausage.