Hawke DL2 motion ratio and spring rate recommendation

[Brian]

Some one must know! I measured .56 front and .70 rear, but it was hot and all I was using was a tape measure. I figure 300 fronts and 350 - 400 rears would be in the ball park - but what do people actually run?

Brian

[Jonathan Hirst]

I have 120's in the front and 200s in the back. Does he job fine on Dunlops. the M.R.s are in a book at the shop.

Based on the Mosport shakedown, were I keeping the Crossle then I would put in Hawk pads, do the bump steer and that's it. The roll doesn't bother me but it might others.

Jon

[Shep]

Brian,

I don't believe the suspension geometry changed drastically from the DL2's through the DL12, and though it certainly wasn't scientific, I estimated .56 front, .77 rear on my DL12. So, your numbers seem reasonable. Unfortunately, I haven't gotten out there to establish a set-up yet, but I was going to start with rates similar to what you mentioned. I'll be curious to know what you end up with.

Cheers,
Erik

[Stu Pidd]

Gents, please bear in mind that these historic cars were NEVER meant to run with stiff springs. I think Jon is right on track with his spring rates, By putting on stiff springs it would likely transfer load to the frame which may or may not take it. These cars do fine with a bit of roll as proven by Jon's times at Mosport. They aren't go karts.

[Bob Coury]

Ditto on the stiff springs. I use to own a Hawke DL-2A. and was going to look at stiffer springs. The first thing I did was take the car to Jerry Mong of Bobsy Sport Racer fame near Cleveland. He told me that we had to check the torsional rigidity of the chassis before we attempt to do an mods. I held a piece of tubing several feet long perpendicular to the frame at the front bulkhead. Jerry went to the opposite rear and did the same. When we attempted to apply any resonable amount of torsion to the chassis, we found it to be like spaghetti. Back on the trailer it went.

[Rob McClelland]

Brian,

I'm in Jon's ballpark for the springs, 150 and 250 respectively. I've heard of owners of cars similar to mine going 200 and 300, but this resulted in cracks to the chassis.

Rob

[Brian]

If you don't know the motion ratios, the spring rates don't help much. Wheel rates are what count, obviously. We're putting in about the same wheel rates as I ended up with in the Merlyn, similar era, similar geometry, etc. I don't buy the stiffer springs equal frame cracking story as long as you stay around wheel rate at 50% of corner weight or so. The stiffer springs simply transfer the load more quickly (higher frequency and all that), they don't change the amount of load transfered. You see slightly higher peak loads. It lets you run less bar front and rear, and I never could figure out why you would want to run rear bar on a formula ford in the first place - no limited slip, so let's force the inside rear wheel off the ground so we get even less traction! Or that other favorite - let's pick the front inside tire off the ground and get power on understeer as we try to get on the power out of the slow corner before the straight!

On this car, with the springs it had (185# fronts), putting 230 pounds of load on the front of the car put it firmly on the ground, almost 2" of bump. Can you say bump steer? half that would be about right, and raise the rear rates to balance the front. Set it up so it's driveable and neutral and quick with both bars full soft, then add front if it's loose, add rear if it pushes. Or play with shocks, if it had real shocks...

Playing around with stuff like this is what makes it fun, if you don't get to drive the car!

Brian

[DOC]

I'm not sure if you measured right, but if you did and you want to try the spring rate range you are considering, I think you would want the 400 pound springs on the front and 350 on the rear. That would give you a front wheel rate of 124 lbs and a rear of 171.5. That's a little over 50% of corner weight and a good F/R ratio. My DL-2A has similar rates and is hardly "radical".

Just watch out lowering ride height. It's OK if the roll centers are above or below the ground, but if they pass through the ground plane during the normal operating range of the suspension (ie. not hitting a curb or flying through the air unloaded) the car will do ugly things that even Senna-like reflexes can't cope with.

DOC

[Hawke]

I could not find the motion ratios of my DL 11, but using 300 lb front and 450 lb rear gives me a good set up. Best result so far is 2nd outright aginst other Formula Fords - Up to 1988 - Formula 2's 3's and a 1 - up to 1968.

Had a great race against the Formula 1, couldnt see him in the straights, but caught him under brakes and corners.

[Brian]

Doc, your note about roll centers is right on, particularly with older cars that were suppsed to run either on different height tires, or with a higher static ride height. A large part of what I'm working on trying to prevent is so much front suspension motion that that the roll center goes below ground on bump. I plan to recommend NOT running the car so low that roll center migration becomes a problem. My simplistic way of estimating this is if the lower A-arm goes past level and the inner pickup point gets much lower than the outboard ball joint.

Each car is going to have it's own issues, but roll centers and camber curves are key. They are pretty much designed in and hard to change, so you have to work around them. Spring rates, ARB rates, static set-up, shocks are all adjustable, but if you build in a problem it's hard to work around it...

Brian