Hawke DL11 - Suspension issue

[Hawke]

I've been running my DL11 for some years on 350lb springs front, 450 rear with bars and shocks set in the mid range. Using hard compound Avon treaded tyres. It worked pretty well.

Last Xmas, due to no racing with COVID, I decided to rebond and rerivit my belly pan. I cant recall the glue I used, but I can say it was used in bonding aircraft components, and body panels. I think I'll need a jackhammer to remove it.

Ever since then, I've been plagued with terminal oversteer. Ive checked the alligment. 1.5 degrees camber front, 0.5 rear, 3mm total toe out front, 1.5mm toe in front. Corner weights correct to within 0.5 kg. Ride height front 35mm, rear 45. I've dynoed all the shocks, and they are OK.

At last weekends race at Sydney Motor Sport Park, I could get the car drivable by lowering the rear springs to 400 lbs, front bar full hard, as also shocks. Rear bar full soft, along with the shocks. I was 2 seconds off my normal times, and just coudnt push hard on fast long corners, as you would get the front to wash out at the end of the turn, and any bump, just put you off line with the real hard front.

The only thing I can think, is that since the rebonding of the belly pan, the chassis is now less flexable in torsion, and this has "made" the rear harder. ??? Next I'll be trying 350 springs rear, and 400 front.

Any gurus offer a suggestion?

[mark elder]

There are real experts on this forum (I am not one of them) but I will throw my hat in the ring.

Tell me you have lots of rear droop? I belong to the school of 'you cannot have to much rear droop'. I use 45mm. I have had your problem when I (accidentally) had 20mm of droop.

Tell me the rear shocks are very soft when you jump up and down on the rear of the car. Tell me they did not run out of gas during Covid. I have had your problem with shock issues.

Tell me the problem is the same turning both left and right.

Tell me nothing is binding at the rear; such as bellcrank bearings, new (longer) bolts catching on something etc.


And when all this is fine, bonding a belly pan can add 20% to the resistance to twist. Therefore, the rear springs/anti-roll bar combo could do with being 20% softer (yes, I know there is all sorts of geometry stuff at play here). So 350 rear springs and a softer bar if you have one; maybe your wet bar.

I cannot see a reason to start paying with the front at the moment. Leave it alone. Just play with one end

[Hawke]

There are real experts on this forum (I am not one of them) but I will throw my hat in the ring.

Tell me you have lots of rear droop? I belong to the school of 'you cannot have to much rear droop'. I use 45mm. I have had your problem when I (accidentally) had 20mm of droop.

* I’ll measure it tomorrow.

Tell me the rear shocks are very soft when you jump up and down on the rear of the car. Tell me they did not run out of gas during Covid. I have had your problem with shock issues.

* The shocks are twin tube, so have no gas. I’ve got my own shock dyno, and they are better than new.

Tell me the problem is the same turning both left and right.

* Both.

Tell me nothing is binding at the rear; such as bellcrank bearings, new (longer) bolts catching on something etc.

* No belcrank, just conventional old style shock set up. No new bolts, but will be checking for binding tomorrow.


And when all this is fine, bonding a belly pan can add 20% to the resistance to twist. Therefore, the rear springs/anti-roll bar combo could do with being 20% softer (yes, I know there is all sorts of geometry stuff at play here). So 350 rear springs and a softer bar if you have one; maybe your wet bar.

I cannot see a reason to start paying with the front at the moment. Leave it alone. Just play with one end.

* Yes, good idea, but with a limited number of springs, its easier to just swap front to rear.

Many thanks

[tlracer]

Are you on the same tyres, ie. the same ones you would have used 6 months ago, has there been any racing?

If so, is it not simply that they have gone off in that time, when normally they would have been used, worn out and discarded long ago?

That would explain the apparent lack of grip and, factoring-in a possible increase in chassis stiffness, the reason why you're having to run the rear so much softer to counteract the end result of both these.

[Hawke]

Are you on the same tyres, ie. the same ones you would have used 6 months ago, has there been any racing?

If so, is it not simply that they have gone off in that time, when normally they would have been used, worn out and discarded long ago?

That would explain the apparent lack of grip and, factoring-in a possible increase in chassis stiffness, the reason why you're having to run the rear so much softer to counteract the end result of both these.

Yes, tyres are the same, with little use over COVID. Unfortunately, no new tyres here at the moment. But these Avon tyres seem to have a long life, and dont seem to age like others. My experience with them is that when not used for a while, a few laps seems to wear off the crappy exterior, and they come good. Even the front runners are having to use tyres of similar age, and not suffering the same issues.

[pooch776]

450 Lb springs in the rear? Wow, IMO that is way to much spring. Not sure I can explain why your car handled better before you put a new floor on unless you managed to switch the front and rear springs around.

Until recently I owned a Hawke DL17. I ended up using 300 Lb rears and 450 Lb fronts. I am not sure if your motion rations are the same as a DL17 but I would guess that rear springs between 250-350 Lb would be about right for your car.

I suggest you measure your motion rations (MR = wheel travel / shock travel) then with your chosen springs, calculate your wheel rates (WR = spring rate / MR squared). I usually try and get my wheel rates to be 40-50% of my corner weights using Hoosier VFF tires (Avon tires should be similar). You will also want your front and rear to be within 5% of each other.

I have raced outboard suspension club and historic Formula Fords with wheel rates between 35% and 75% of the corner weights. Higher spring rates (stiffer) usually feels better (most cars) until you have a wet track. Then softer feels much better. 40-50% seems to be a good compromise for these cars/tires with both wet and dry tracks

YMMV, Joe

[Hawke]

There are real experts on this forum (I am not one of them) but I will throw my hat in the ring.

Tell me you have lots of rear droop? I belong to the school of 'you cannot have to much rear droop'. I use 45mm. I have had your problem when I (accidentally) had 20mm of droop.

Tell me the rear shocks are very soft when you jump up and down on the rear of the car. Tell me they did not run out of gas during Covid. I have had your problem with shock issues.

Tell me the problem is the same turning both left and right.

Tell me nothing is binding at the rear; such as bellcrank bearings, new (longer) bolts catching on something etc.


And when all this is fine, bonding a belly pan can add 20% to the resistance to twist. Therefore, the rear springs/anti-roll bar combo could do with being 20% softer (yes, I know there is all sorts of geometry stuff at play here). So 350 rear springs and a softer bar if you have one; maybe your wet bar.

I cannot see a reason to start paying with the front at the moment. Leave it alone. Just play with one end

Mark, you may be on the money. When I rebuilt the car after refitting the belly pan, I set it with a 57mm ride height. There is only 20mm of droop. I'll drop the the height today to its normal 45mm, and see how that goes. Otherwise, I've got plenty of longer top eyes, and longer shocks I can use.

[Hawke]

450 Lb springs in the rear? Wow, IMO that is way to much spring. Not sure I can explain why your car handled better before you put a new floor on unless you managed to switch the front and rear springs around.

Until recently I owned a Hawke DL17. I ended up using 300 Lb rears and 450 Lb fronts. I am not sure if your motion rations are the same as a DL17 but I would guess that rear springs between 250-350 Lb would be about right for your car.

I suggest you measure your motion rations (MR = wheel travel / shock travel) then with your chosen springs, calculate your wheel rates (WR = spring rate / MR squared). I usually try and get my wheel rates to be 40-50% of my corner weights using Hoosier VFF tires (Avon tires should be similar). You will also want your front and rear to be within 5% of each other.

I have raced outboard suspension club and historic Formula Fords with wheel rates between 35% and 75% of the corner weights. Higher spring rates (stiffer) usually feels better (most cars) until you have a wet track. Then softer feels much better. 40-50% seems to be a good compromise for these cars/tires with both wet and dry tracks

YMMV, Joe

I've got a motion ratio of 1.9 front and 1.73 rear. So with 350 lb springs front, 400 rear, this gives a wheel rate of 97 front, 134 rear. With 213lbs front, and 323 rear, I'm getting a 45 and 42% of wheel rate. So that looks fine.

However, I think that the ride height I set it at, caused the shocks to top out in roll. I'll lower the rear, and lengthen the rear shocks.

[pooch776]

I've got a motion ratio of 1.9 front and 1.73 rear. So with 350 lb springs front, 400 rear, this gives a wheel rate of 97 front, 134 rear. With 213lbs front, and 323 rear, I'm getting a 45 and 42% of wheel rate. So that looks fine.

However, I think that the ride height I set it at, caused the shocks to top out in roll. I'll lower the rear, and lengthen the rear shocks.

You are correct, your wheel rates look very good. I am surprised your motion ration is that high in the rear but I guess the shock angle on the DL11 is more then most outboard rear suspension cars I have seen.

Limited droop in the rear is certainly a recipe for failure. Unloading the inside rear wheel will definitely cause oversteer.

Good luck, Joe

[Hawke]

You are correct, your wheel rates look very good. I am surprised your motion ration is that high in the rear but I guess the shock angle on the DL11 is more then most outboard rear suspension cars I have seen.

Limited droop in the rear is certainly a recipe for failure. Unloading the inside rear wheel will definitely cause oversteer.

Good luck, Joe

I have lowered my car to the original ride height. However, I still only get about 22mm droop. The springs are now just rattling when in full droop.

What I cant get my head around is, that if I have more droop - by a longer shock - it will get to a stage that the extra wheel travel will happen with no spring. So wont this be a bad thing?

One other thing. I changed out my Imp like rubber donuts a few years ago, and replaced them with far stiffer rubber BMW drive line things. Of course, during axle movement, these rubber things have to deflect, so they are contributing to rear end stiffness. What do you think?

[DaveW]

Mark, you may be on the money. When I rebuilt the car after refitting the belly pan, I set it with a 57mm ride height. There is only 20mm of droop. I'll drop the the height today to its normal 45mm, and see how that goes. Otherwise, I've got plenty of longer top eyes, and longer shocks I can use.

Is that 20-mm droop at the shock, or at the wheel? If it's at the shock, you actually have 34.6 mm (1.36") of droop at the wheel due to your 1.73 MR.

[Hawke]

Is that 20-mm droop at the shock, or at the wheel? If it's at the shock, you actually have 34.6 mm (1.36") of droop at the wheel due to your 1.73 MR.

That is at the wheel.