Front suspension design input needed

[Ron Sutton]

Originally Posted by 358Mustang

So in order to couple with the rear setup I have got.. I am running a home made 3 link in the rear, roll center is about 8". coilover shock behind the axle, with a 300# spring. It needs more roll stiffness in the rear I feel, maybe due to the angle I have my shocks kicked in at... So my plan was to add a rear sway bar to that.

I just mention the rear because obviously I need them to work together. I am thinking I probly want to go with a little stiffer of a spring up front to limit front travel. I am trying to get the CG down and planned on having the motor fairly low. SO I guess that would fall towards the stiffer spring, softer bar category unless you can say why the other way is better..

I am unsure exactly on spring rate, I would thinkg of starting in the 450-500# range... Coilover mounted as far towards the tire as I can within reason

Gotcha. You're going the conventional route.
Based on what you said, it looks like you're looking to travel the front end in the 3/4" to 1-1/2" range (at the crossmember). I should have asked this earlier ... will you be autocrossing, running track days at full size road courses or both?

Regardless, for a low travel/high roll suspension set up …
• If your priority is autocross or the low speed corners of road courses, your target RC should be around 1” in full dive.
• If your priority is the mid-speed corners of road courses, your target RC should be between 1.5” to 2.5” in full dive.
• Of course, you can set it for anything in-between.
• The higher the RC is within the range 1” to 2.5” favors the mid-range corners. Lower favors the low speed corners.
• If optimum lap times matter to you, the answer of what is the optimum roll center will come through testing.
• If you're not that hardcore & just want to have fun running the track hard & fast, pick what makes sense to you & go run it.

P.S. I concur with Mike's tips above. He & I both favor high travel/low roll set-ups, so that influences our designs & geometry settings, including lower roll centers ... and long control arms which tame the geometry changes. The farther you travel the front suspension ... the more the longer control arms help.

I attached an illustration of my Track-Star front suspension which we travel in the 4" to 4.5" range at the crossmember (more at the outer front wheel). I design these custom for each client's particular car, but the LCA's end up in the 23" to 25.5" range & UCA's in the 11.75" to 13" range. This allows us to travel the front end that 4-4.5" and have the optimum geometry throughout the travel.

If you're traveling the front end only a little, then control arm length is less of an issue.

[Ron in SoCal]

^ Secret Sauce.

[358Mustang]

Originally Posted by Ron Sutton

Gotcha. You're going the conventional route.
Based on what you said, it looks like you're looking to travel the front end in the 3/4" to 1-1/2" range (at the crossmember). I should have asked this earlier ... will you be autocrossing, running track days at full size road courses or both?

Regardless, for a low travel/high roll suspension set up …
• If your priority is autocross or the low speed corners of road courses, your target RC should be around 1” in full dive.
• If your priority is the mid-speed corners of road courses, your target RC should be between 1.5” to 2.5” in full dive.
• Of course, you can set it for anything in-between.
• The higher the RC is within the range 1” to 2.5” favors the mid-range corners. Lower favors the low speed corners.
• If optimum lap times matter to you, the answer of what is the optimum roll center will come through testing.
• If you're not that hardcore & just want to have fun running the track hard & fast, pick what makes sense to you & go run it.

P.S. I concur with Mike's tips above. He & I both favor high travel/low roll set-ups, so that influences our designs & geometry settings, including lower roll centers ... and long control arms which tame the geometry changes. The farther you travel the front suspension ... the more the longer control arms help.

I attached an illustration of my Track-Star front suspension which we travel in the 4" to 4.5" range at the crossmember (more at the outer front wheel). I design these custom for each client's particular car, but the LCA's end up in the 23" to 25.5" range & UCA's in the 11.75" to 13" range. This allows us to travel the front end that 4-4.5" and have the optimum geometry throughout the travel.

If you're traveling the front end only a little, then control arm length is less of an issue.

Thanks a lot for the diagram of your suspension. That helps me a lot to visualize things... This is a strict road race car, that I am trying to build it to be as fast as possible (isn't that the point? )

Here is a picture of my car and what I have mocked up, which is basically an engine and the start of some frame work, so like I have said I more or less have a clean sheet. I wanted to go with high backspace wheels to keep the scrub radius down. I plan on using an 18" wheel, will obviously be custom due to the bolt pattern...

[Ron Sutton]

Originally Posted by 358Mustang

Thanks a lot for the diagram of your suspension. That helps me a lot to visualize things... This is a strict road race car, that I am trying to build it to be as fast as possible (isn't that the point?

You're welcome.

I don't think you should pull ideas from my design if you're building a suspension for a conventional set-up. Mine is optimized for a high-travel/low-roll setup and would not be optimum for a low-travel/high-roll set-up. It's not that it can't be run that way, but the geometry is not correct for a low travel/high roll set-up. For example, in a high travel set-up like mine, I have the geometry designed to be optimum for 4.5" of dive at the crossmember and a roll angle of 0.8°. Everything is different for low travel.

You will need different:
A. Anti-dive percentage
B. Roll Center location (static & dynamic)
C. Camber gain
D. Caster Gain
E. Bump Steer

If you build yours like mine & travel the front end 1-1/4" and roll it 3.0° ... the camber gain is too small, the caster gain is too small, the roll center is not optimum, etc. I suggest looking at examples in road race suspension design books printed before 2005.

Best wishes & don't hesitate to share or ask for help. Lots of good guys on this site.

[GregWeld]

Brian ---


Just get it over with --- sell your stuff and buy one of Ron's custom track cars with a Mustang body.

When I get done with my '40 Ford Pickup project.... that's what I'm going to do.

[Mike Maier]

This project remind me of my yellow mustang. When I first started I built my own suspension, several times. I think beyond purchasing kits. Which please understand this is no dig at your set up Ron but more of a state of mind of a craftsman. For example my father in law had plenty of funds to buy an old boat but he decided to completely restore a Chris Craft instead. It looks like this might be a little of that. So when I was researching set ups I looked for the fastest sedans I could find ( which are very similar to Ron's set up) Trans am cars. This became My inspiration in a lot of ways. They have very efficient ways of mounting things, Frame rail locations and sway bar packaging. If you do this on your own make it tunable. Design in the ability to change things up when you learn what you did could be a little better.

[mfain]

Originally Posted by Mike Maier

This project remind me of my yellow mustang. When I first started I built my own suspension, several times. I think beyond purchasing kits. Which please understand this is no dig at your set up Ron but more of a state of mind of a craftsman. For example my father in law had plenty of funds to buy an old boat but he decided to completely restore a Chris Craft instead. It looks like this might be a little of that. So when I was researching set ups I looked for the fastest sedans I could find ( which are very similar to Ron's set up) Trans am cars. This became My inspiration in a lot of ways. They have very efficient ways of mounting things, Frame rail locations and sway bar packaging. If you do this on your own make it tunable. Design in the ability to change things up when you learn what you did could be a little better.

I agree with Mike -- fabricating it yourself has a lot of rewards and you get an end product that compliments (fits) the vehicle you started with. But, you need to be realistic about how long it might take and cost. If you have a big shop. lots of time, excellent fabrication skills, and good tools, you might have a fighting chance. If you don't, then you might be money ahead to go with a professionally built set-up, like Ron's. He has the latest technology and years of experience in suspension design. If you do a lot of research, trial-and-error fabrication, and tons of experimentation, you will end up where he is anyway, because it is the right answer. I have been working on the same car for 39 years -- 4 rear suspension designs, and I just finished the 4th front suspension install. I have thousands of dollars in "new", unused parts that were superceded by the next evolution. Although I have better equipment now, I started with a hacksaw and a file, and the time factor pain was excruciating. I now have what I believe is a very workable suspension, but as Mike suggests, I made every suspension pick-up point adjustable in case I am "off" a little, for tunability, and to compensate for things like changes in tire technology that may affect the set-up down the road. Good luck.

Pappy

[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.

[Cris@JCG]

There are many rewards of building your own front suspension but it comes with a cost of frustration & compromise! I just spent 2- 1/2 developing my own front suspension & sometimes I wish I would have just taken something off the shelf that already existed & worked with it.. But as Mike has stated adjustability is key to making an optimal performance front & rear suspension. The only reward of making your own stuff is the level of self satisfaction you get when you see the end results.. I have gone thru 4 revisions on my design & each time enhancing it to make a better .. The only reason I made such a big investment in time & money on this project is because I can control how I make a chassis to my desired ride height, width on track, & personal challenge to see if I could do it.. I was making chassis's left & right here @ the shop for builds but had not tested any of it on the street or track.. SO I built a test car that proofed the concept of design & function before I proceeded to consider it good functional piece with great relief that it works.. A couple of weeks ago I put a shoe driver in my test car & did a comparison with another car that had another brand of suspension underneath it & the results proofed that ours had an advantage on the front grip vs the other car.. So we are still on the hunt to better the rear.. ours has great rear grip but still looking for forward bite.. It is a personal challenge to keep optimizing our design!

In the mean time we use other brands of suspension even though we have our own.. This week we had the task of grafting DSE stub on to a chassis with a build we have here @ the shop.. We stick with stuff we know that works.. Even have a few with just bolt on stuff from RideTech.. The Mustang stuff I just refer over to Maier.. There will be many opinion on what is best out there.. For a complete race car stub.. I know who I would personally work with.. Maier! (Spell Check)

[358Mustang]

I was at the historics this weekend and definitely payed close attention to the roush pro-fab imsa mustangs and capri's.




I notice the rack position was fairly far forward. Hard to tell in the picture though, I was expecting the arms to be closer to parallel.