Can the jump at Grattan get ugly

[JRMarchand]

I will be running at Grattan this weekend and just wondering how to approach the Jump.
In the past (with an FM) I always enjoyed the feeling of shooting off the "jump" and then landing..... I'm not sure how this car(Novadiemen) will like the punishment.
Because of the circumstances, I need a guaranteed start this Sunday for an invite to the big dance(wont be practicing or qualifying to reduce the chance of something going wrong)

With so little droop, I expect the tires to come off the ground, maybe. I'm a little scared that the car might flip backwards, is it possible at those speeds(i think its under 95mph) The car has a wider floor that also extends forward of side pod opening.... its an FB
Any comments on how to approach the jump would help, I dont want to hurt car from not being mentally prepared, thank you.


Marchand Juan R

[Chris Livengood]

Approach it flat out. Report back immediately.

[kea]

No difference than how you would do it with the FM.

[SStadel]

Flat out until your rear wheels leave the ground, then back off the throttle. Don't hit the brakes until you've got all 4 back on the ground. One of the reasons I love Grattan!

[revtom]

Just remember to brake before the bottom!

[JRMarchand]

No different, really??? hahaha you are f@@@@@@@ with me Keith. The car has little travel and i think it will bottom out like crazy.

Ok I need numbers, the car is at 30mm ft/46mm rear, should I go up to 34mm/48mm for the weekend or leave it low and let the car touch(thats a rough track ya know)

With 400lbs front springs, you want some preload yes? is two turns enough. The rear has no preload. How important is adding preload and will it help the car from bottoming out after the jump? thanks

[JRMarchand]

I wonder if the F500 guys really take off with those hard suspensions. My friend scared me yesterday, because he flipped a 250SuperKart there while back. they modified the bottom of the car to have some reliefs or flaps and then it would fly straight.

Not going flat out anywhere with this car yet, its well above my skill set.

[Mike Devins]

what rear springs are you running

Jay if you see this can you share the Motion Ratios for the back of the car.

The front if it has the original rockers is only .7 to 1. 400# springs ends up with less than 200# wheel rates which is really soft, should make for great mechanical grip, but you may want to raise the ride height.

[JRMarchand]

Mike, 750 rears

great info, I will check which rockers the car has, if they are the "originals" then I should look at an alternative, better ratio out there? that is really soft 200 lbs, what the hell, the car pushes on the high speed , its annoying, I need to start testing this car with people that know them.

Jay when you come to GingerMan? let me know so I can test with ya if its ok.

ok i have to go shake down a crappy radical , later. thanks for the help. I knew something was off and the front felt too soft. maybe more preload for Grattan as I dont have anymore springs on hand....

[JByers]

Juan,
I have driven FF, FC and FA over the jump and the approach was the same in all three. Flat out until lift-off.
One weekend I witnessed a FC with a bent front wing that was developing lift going over the jump. The bent wing caused a slightly slower entry and prolonged air time of the front wheels. I'm sure you will find it very similar to the FM.

Jason

[JRMarchand]

Mike, is the motion ratio stamped on the rocker arm or do i need a special calculator and a degree to figure it out

this thread helped me out a bunch, I was wondering why the car bottomed out so much, I kept telling myself , "its ok , its the downforce", hahaha nope, my front springs at 400# are for newbies. Damn, I have training wheels for ft springs lol.

[Brian.Novak]

What Mr. Byers said is correct. You shouldn't have any trouble, just approach it in your normal manner, I did not have any trouble with it last year in my F1000.

I've had all 4 in the air in both my FE and my F500. The F500 wasn't a problem

/edit - If you want to take it easy, just make sure you don't pitch the car forward just before launch, it makes the landing much worse. ie Don't lift heavy early or brake hard.

[JRMarchand]

Brian, I totally get what you are saying now, thats why everybody says to take flat out and not off the power, it would nose dive on the other end.

[Mike Devins]

just left a message for Jay, for now assume that it the same as stock which is about .73. There were alternate rockers for the front which were .9

Only good way to check is to measure. Remove the shock and measure the distance from the rocker to the shock mount with the wheel at ride height. place 1" shim under the wheel and measure again, that is the shock travel for one inch of wheel travel.

Note that this really needs to be done in small increments say .2"s at a time but this will get you a rough idea.

To get the wheel rate square the MR X spring.

.7x.7 = .49 X400# spring = a wheel rate of 196#'s

Feel free to call - 586.623.3055

[Mike Devins]

Just spoke to Jay rear MR is .74

[JRMarchand]

Is there an easier way of telling the ratio, like with a ruler a measuring from the pivots out to the holes, and dividing by x. I'll definitely look into it ASAP

[TimW]

Is there an easier way of telling the ratio, like with a ruler a measuring from the pivots out to the holes, and dividing by x. I'll definitely look into it.

Remove damper, put floor jack under upright. Lift upright in increments, measure the wheel change and the damper change. Plot and curve fit in Excel, the resulting curve equation will something like y=mx+b, m is the motion ratio (y is wheel displacement, x is damper displacement, b is irrelevant). If the calculated line in Excel is not generally through the data points you've measured, start over.

[JerryH]

Hi JR,

Totally agree that the front is to soft.....but don't forget that the spring change will affect the balance....and need some track time to sort out.

I had a 97 VD FC on the pole at the runoffs in 2000. It has the same suspension and bellcranks as your 98. I have a pretty good handle on what makes the car work.....give me a call.

Jerry Hodges
JDR Motorsport
636-399-7060

[Dave Harmison]

What's with the choice of picture you posted?

[Mike Devins]

Hi JR,

Totally agree that the front is to soft.....but don't forget that the spring change will affect the balance....and need some track time to sort out.

I had a 97 VD FC on the pole at the runoffs in 2000. It has the same suspension and bellcranks as your 98. I have a pretty good handle on what makes the car work.....give me a call.

Jerry Hodges
JDR Motorsport
636-399-7060

Jerry still best to measure what he has - there have been rockers produced from .7 up to .91 that I have measured. BTW the .91's were on a RF-98

[JRMarchand]

Dave thats you man that picture is from 1999 I believe, there is a whole page about that race, must've been a good one.

Jerry, thanks so much, I will give you a call next week, for now I'm planning on driving the car as is and not make too many changes.

Mike, correct me but I believe my car is a RF98/2, does that mean that I have the .91 ratio rockers and so the wheel rate would be higher than what we figured out earlier. btw, the rockers on the car look like the ones on the pictures you sent me. thanks

[Mike Devins]

Sent an email with lots more info and an updated wheel rate calculator - looks like your rockers are the .7 MR that was standard with the 98.

[Westroc]

Juan just a note to keep in mind perhaps especially as you will no doubt be in a mixed race group. As Jason B said have no fear of a flat out approach. However if you are passing approaching this section be mindful mentally that the car is arrow straight for the "launch" as it will continue on that trajectory. With even crap pads it will stop but with crap pads the turn in may be another thing after the jump for the right hander. I would be hesitant w/o seat time to diagonal the jump in overtaking immediately at least. You will not believe how much you will be able to take advantage of that cambered turn after with an FB. Have fun.

[Rick Ross]

Remove damper, put floor jack under upright. Lift upright in increments, measure the wheel change and the damper change. Plot and curve fit in Excel, the resulting curve equation will something like y=mx+b, m is the motion ratio (y is wheel displacement, x is damper displacement, b is irrelevant). If the calculated line in Excel is not generally through the data points you've measured, start over.

This equation is correct if you define motion ratio as the ratio of wheel dispalcement to damper displacement (y/x). However, the inverse definition is more common in the industry, and I think that the 0.74 MR mentioned in this thread refers to the ratio of damper to wheel displacement (x/y). Using this second definition of MR, your equation would need to be:

x = my

where:
y = wheel displacement
x = damper displacement
m = x/y = motion ratio (MR)

With 400 lb/in front springs, and a motion ratio (x/y) of 0.74, the front wheel rates are only 219 lb/in........pretty soft for a car with that much downforce.

[JRMarchand]

Thanks everyone for the help, glad to have found the info on the rocker arms, thanks MikeDevins. I'm thinking about keeping the stock rockers for now and testing different springs.
The jump at Grattan was lame, I didn't have much fun there, someone at that track decided to use sealer or something like it on most of the turns, it is very slippery in the turns because of it. The track was repaved not long ago and now it has sealer on most of the turns, I heard that is used to keep the track surface from breaking up. Cant't drive fast with that stuff on the surface Maybe I was the only one who felt that way.

Jerry thanks I'll call you soon.

[kea]

Yes Juan, it was just you !

[S Lathrop]

I suggest that you use a single, small diameter, red, Dynamics bump rubber on each rear shock. Get them from Carl Haas. I would set the packers so that you engage the bump rubber when you still have about 1/2" of ground clearence.

Your spring rates for FB need to be the same or heavier than for FC. Even though the car is 200# less, with the higher speeds of FB, you will make a lot more down force. The diffference in speed and down force is dramatic in fast corners for FB vs. FC. Your speed compared to the FM might be much higher because of the faster exit speed from the preceeding 2 corners.

My bet is that the FB is going to be a real eye opener at Gratten. You might be looking at the overall track record as a goal.

[JRMarchand]

Keith, perhaps and maybe, but the other two or three racers that I talked to after the race said the grip wasnt on the sealer line, a lot of turns have the sealer now, I'm just saying it sucks, everyone else likes the jump so there, its still fun mix of road racing and acrobatics. I wouldn't make up that i felt the track has very little grip where the sealer is. I tried different lines, the sealer line is not my strong arm.

Steve, I havent been able to go faster than any FC's, maybe some, but it'll be a while. Just called Hyperco and I'm trying the 4 inch(down from 5inch) the new rates will be 650ft and 850rears, maybe should be higher? I dont want to buy too many springs and not be able to tell the differences between them etc. If the car now has 400ft and 750 rears, would it make sense to stiffen the front and rear at the same time or do one end at a time.

Any good idea on rates for RAmerica 650/900? or 700/1000? which would you choose I would hate to go there in september and fumble around with a ton of springs. hahaha

[JRMarchand]

Packers? ok, I had no idea packers, its probably for the landing after the jump. I'll use that if I ever brave out there again, to Grattan.

[JRMarchand]

I went to my video cam and found a lap that looks kinda hmmm... I've never been good here and i loose a lot of time in some areas. During the race the monster machine of Jean-Luc caught up to me to lap with a waive bye. I decided to go faster for a few laps and the best time was a low 1.16, I believe thats not far from the top qualifiers were doing in Q1.

In the video you can see clearly how the surface of the track has the crazy sealer from turn one and on. Turn one used to be a little different before, you can see the DSR in front taking a late brake.

http://youtu.be/5mXGMjailYw

[S Lathrop]

Jay can probably give you a better place to start for springs than I can.

I would suggest that you set the front and rear spring rates at some percentage of the weight on the corner. Say the car loaded has 200 front and 300 rear on each wheel. I would start with springs at say 2 times the weight on the corner or 400# front and 600# rear. I have used 3 times for some situations. Then have spring in 100# increments above and below those numbers. To find out the actual spring rates, you would divide the wheel rate by the square of the motion ratio to calculate the spring rate.

You should expect to need springs in 50# increments for racing competitively.

You have to do the testing to get the car the way you want. You should always go on the track with some change just to learn what happens. Start with the big changes, springs up or down in 100# increments. Wings with enough change to feel something. Change rake by 1/8th inch increments. Just changing the tire pressure by 2 psi will make a big change Most tires will see a spring rate change of 40 #/in for a 1 psi change.

A trick for setting up bump stops is to use enough rubber to support the car at ground level at both ends. That may be too much at the rear but I have done that at the front.

[Rick Ross]

Juan,

You need to carefully follow Mr.Lathop's advice here.....he knows this stuff better than most. You need to be thinking in terms of wheel rates instead of spring rates, so you must first measure your motion ratios. Once you know the motion ratios, the wheel rate is equal to the spring rate times the square of the motion ratio (just as Steve and others have pointed out here).

Here is a real simple example for a wheel rate that is twice the static corner weight:

Front corner weight = 200 lb.
Front motion ratio (damper to wheel) = 0.75
Target front wheel rate (2 X corner weight) = 400 lb.
Required front spring rate = 711 lb.

If you are running the Hoosier tires in your car, then Steve's advice is right on target regarding tire rates. A 1 psi change will result in approximately a 40 lb/in change in tire rate at typical FB loadings. Hoosier can provide more detailed data if you need it.

[Brett Lane]

All the technical stuff aside( I'm learning a lot with this thread, so thanks), the video is way cool!

[JRMarchand]

Brett , thanks for the comment, I got lucky with this thread and thanks to Mike Devins and Mr Lathrop, I have a better idea where to start with the spring rates with my FB car. I've been driving the car for the last month or so and the spring rates are backward, the car felt better than anything i've driven before but cant wait to see what it feels like as it gets the proper spring rates, have a good weekend everyone.

[S Lathrop]

I have thought about the air time over the jump. Given that you have a hard brake as soon as you settle, air time is important.

My guess is that it will be somewhat less for an FB than an FM because of the higher down force and lower mass of the FB. But the tendency to pitch nose down might be more because of the loss of rear down force. Wings work well but the diffuser will drop to nothing as you get air born. A lot of the braking effort required for this corner is helped by the car compressing as it settles to the ground.

I would think that you want to reduce your rebound settings for Gratten in general because so many of the corners have a elevation drop in them. You need the tires to stay on the track and rebound does not help that.

Gratten was one of my favorite tracks when I was driving. It is very technical and demanding.