Quote:
Originally Posted by Ron Sutton
We might be taking Brian's thread a bit off course. He is already planning to build his own front suspension. He's looking for advice on some key targets like roll center, camber gain, etc.
With a low travel set-up, Brian you will need more camber gain, caster gain, anti-dive & a higher RC in full dive ... than a high travel set-up like mine ... which is why I encourage you to research some road race books for suspension designs. You'll find the UCAs have more angle in them for caster gain & the LCAs have less or are reversed. Just don't get carried away with UCA angle or you will end up with too much mechanical leverage known as "jacking effect."
Brian, the best advice I can give you is to always think dynamically. You don't care what the settings are in the parking lot. You care what the settings are in dive & roll when braking, turning & accelerating in & out of corners. And on a road course ... those 10-12 corners are different. If you're using a suspension software to work out your set-up ... one of the first things you'll want to decide on is your suspension strategy ... which equates to how much are you going to travel the front suspension in dive & with how much roll angle.
Without those numbers worked out ... it's tough to run through different scenarios in your software. But if example, you say I'm going to travel the front end 1.0" (at the crossmember) & roll it 2.5° ... then you can plug in those numbers and experiment with all the suspension pivot locations until you hit your target RC, camber gain, etc. If your software also calculates FLLD & RLLD, then you'll be able to work out your spring & sway bar package for your target roll angles front & rear. The key thing the software won't do is work out how much front spring rate you need for your target travel.
From results gained from experience over the years, I worked out a formula for what wheel rate is needed to achieve a target front travel number. It's not perfect, because it is tough to account for some variables like front braking force, driver braking pressure, etc. ... but it will get you in the ball park. If you post up your target travel & roll angle numbers ... plus the CG height & anti-dive percentage you're planning to run ... I can give you a "wheel rate percentage" to start with. This way, when you know your corner weights & motion ratio, you can calculate the spring rate to start with for your baseline. Once you input the front spring rate into your software ... from there it's a lot easier to work out rear spring rates & both sway bar rates to achieve the target roll angle.
You'll need to know your target G-force numbers to arrive at your on-track "average" roll angle. If your combination reports you car will roll 2.1° per 1G ... then you can just do the math from there ... if you can somewhat accurately predict the g-forces your car will achieve. I better clarify that "average" roll angle does not mean the average from different corners. It means the average of the front & rear roll angles. If you get the FLLD & RLLD correct, the rear will roll about .35°-.40° more than the front.
So for example, if you were pulling 1.4G on a road course ... with the set-up outlined above ... the roll angle would be 1.4 x 2.1 = 2.94°. We don't want the front & rear rolling the same or the car won't handle well. So only as an example for discussion sake (and to keep the math easy) let's say we're rolling the rear 0.4° more than the front. So in reality, we'd be targeting a front roll angle of 2.74° & a rear roll angle of 3.14°. I find an FLLD % about 5% higher than the car's front weight bias provides the correct FLLD/RLLD ratio to achieve this ... which equates to a well balanced/neutral handling race car.
Anyone following along, please don't lock in on these travel & roll angle numbers as a recommendation. They are just an example to show the math.
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Great explanation of things Ron. Thank you. Unfortunately I lost my demo of the suspension software I was playing with.. The only thing I can find right now just gives basic camber gain and roll center changes.. Not sure I want to spend 250$ on a software I don't plan to use very often..
I will try and keep you guys updated with how it goes. Right now I am trying to mock things up and start figuring out the chassis lay out a bit. And then hopefully start playing with some numbers.
I'm also still up in the air about wheels. I am running 5 on 5 x 5/8" stud. Not sure where to go for wheels, my plan was Joengbloed 18" wheel...
Brian Hobaugh SCCA National Tour June 2014
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