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Author Topic:   pull bar set up?
viper3244
Dirt Full Roller

Total posts: 57
posted June 24, 2005 08:58 PM  
Trying to figure out problems with forward bite on are car and I believe the pull bar is the problem. My question is how much preload? We run a progressive spring. Some one told me that run it up til it touches then a bit more so it is tight enough that you can spin the spring with both hands? And what does it mean if it is going over center? Our drive says it pulls only for a second coming out then spins the hole straight.

viper3244
Dirt Full Roller

Total posts: 57
posted June 24, 2005 10:47 PM  
Let me clarify a bit after doing a little studying. We run 4bar/z-link on a DW8. Progressive Hyper coil on the pull bar with no preload. Ran the car one night looks like the the travel marker moved atleast 3"s maybe more. The car skates bad (all the way to the wall) at entry, and has forward bite only or a split second the breaks 'em loose?

donslink1
Dirt Freak

Total posts: 318
posted June 25, 2005 08:14 AM  
what angle do you have your pullbar on, Too much angle will make that happen also.

JT54
Dirt Maniac

Total posts: 170
posted June 25, 2005 10:35 AM  
On my dual spring progressive I run as much as 3/4 an inch pree load ....it helps cut down on too much travle....we are getting about 3 1/2 inches.....3 link car hooks up to much sometimes but I would rather have that problem than not enough!

viper3244
Dirt Full Roller

Total posts: 57
posted June 26, 2005 07:49 AM  
Currently running 2cnd hole up on the frame mount and top hole on therear end. Running 8 degrees pion. 9010 shock. Not sure on the angle of the pull bar I am going to the shop to check today What angle should I try to get out of it? I still don't undertand what going ove center is? Or how much travel on the marker I should be looking for.

viper3244
Dirt Full Roller

Total posts: 57
posted June 26, 2005 06:45 PM  
Pull bar is at 12 degrees, and the shock is 12 degrees. (top hole on frame end) Both seemed to be centered. Where should the shock be mounted Half of its stroke static or almost retract al the way??

gould
Dirt Forum Champ
Total posts: 730
posted June 26, 2005 07:06 PM  
try 20 degrees downhill on the pull bar and 5 degrees uphill on the shock. If you mount the shock to the left it will have tendency to become looser under braking at turn entry, if its mounted to the right it will tighten under braking on turn entry. your pull bar needs to mounted in the chassis at the center of the weight mass of the car. The center of the weight mass is found by multiplying the rear track width by the cars left side percentage. example...60" track width and 53% left side is:

60X.53=31.8"

So, the center of the weight mass at the rear is located 31.8" to the left of the center of the right rear tire.

This was taken out of my Steve Smith book, this book has been the best $25 i've spent on my modified. Great stuff.

------------------
www.geocities.com/gouldracing11

viper3244
Dirt Full Roller

Total posts: 57
posted June 28, 2005 07:00 PM  
Alright here is my game plan let me know if you guys think I am way off on this. I am going to flaten the shock to 5 degrees(centerhole on the frame), Move the frame end of the pull bar down one hole to get some more angle and I am going to put a 1/4 inch preload on the spring. I hope this will give more forward bite quicker. We run a big half mile so I hope this works. I have not seen this on many cars but maybe thats what this one will like.

dirtbuster
Dirt Forum Champ
Total posts: 2007
posted June 28, 2005 07:44 PM  
dont get carried away with pullbar angle, especially in a 4 link car. I have noticed the trend seems to be toward less angle than in the past, especially on 4 link stuff. On the 2 link stuff I think you need more angle like 20* to help with bite, but 4 link Id stick around 15 or so. Thats with the car on the ground at ride height.

Also I would get rid of the progressive spring and go to at least a 1050 maybe a 1200, either that or mnaybe stick a sprign rubber in the progressive adn see how it acts that way.


viper3244
Dirt Full Roller

Total posts: 57
posted June 28, 2005 08:05 PM  
Thanks for the help guys. I can't wait to try some of this stuff. Just a couple more questions. Is it possible to run out of shock travel or compress the spring all the way? How much travel on the marker for the spring and how much on the shock am I trying to get? I have heard everything from 3.5 to 1.5 inches. I ran it last week with NO preload since it is a progressive could this cause a slaming effect because of the quick momentum of the rear end backwards?

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted June 28, 2005 09:14 PM  
it is VERY possible to run out of shock travel. I took my 90-10 off b/c it was hyper-extending(topping-out) during accel. and causing the car to lose bite as quick as it would get it, kinda like what yours is doing. But taking the 90-10 off will loosen entry and can cause wheel hop under braking. So if you decide to try that just keep those things in mind. if you plan on running the 90-10 then mount it to where, at ride height, the shock is at half of its travel.

I have also heard differnt things on the pullbar travel. I run an Afco progressive spring bar. I get between 2.5" to 3" of travel. Try running the bar with 1/4" of pre-load, thats what I do. The more preload the stiffer the spring.

[This message has been edited by FlyNLoIMCA17 (edited June 28, 2005).]

viper3244
Dirt Full Roller

Total posts: 57
posted June 28, 2005 09:33 PM  
Fly are you saying run it with out a shock? I can see your point but seems a bit scary, won't the car unload at the end of the straight?

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted June 28, 2005 11:17 PM  
Yes take the 90-10 axle damper shock off. well, I don't mean that will fix your car but I don't run one,mine was topping out. Also my pullbar has a rubber biscuit for braking forces. Taking that 90-10 off will have no effect on unloading down the straights. Its only purpose is to help control rear wheel hop under braking. It does nothing to improve or alter in any way the traction of the car under acceleration.

36modracer
Dirt Maniac

Total posts: 126
posted June 29, 2005 06:22 AM  
we had this very same problem with our Hoffman 4/z car recently. beginning of the season we were good on wet and dry track then as time went on we were not getting any forward bite on dry slick but still good on wet. we practically took the car apart looking for a bind or something bent etc and could find nothing. did this a couple weeks in a row with nothing. Then we decided to take the pullbar assembly apart and it was so bound up that even with the spring out of it you couldnt move it. we freed it up, cleaned it, lubed it etc and now it works great and getting 3 inches or so of travel and the car worked much better last week and we got 7th fast time and 7th in the feature here in central il among heavy competition. only other changes we made is putting a straight bar on instead of a jbar when the track went slick. we ran it on the left side of car of course at about a 14 degree angle and 2nd hole down on the pinion bracket, and lowered the pullbar down to the last hole down on the frame for more angle. oh yea, we used about 1/4 preload as well and are running a 1050 pullbar spring.since a hoffman car is very similar to a dirtworks i thought id throw our situation out there to ya. good luck.

rrrrick
Dirt Maniac

Total posts: 192
posted June 29, 2005 09:11 AM  
My 90/10 shock was topping out, so I just made a simple 2" extension on the shock.

viper3244
Dirt Full Roller

Total posts: 57
posted June 29, 2005 07:49 PM  
Ok I've done some measuring and heare is what I have trak width is 61 1/2, pull bar is at 30 1/2 from center of the right tire. And it is 6 1/4 above the housing, which seems low but it is in the top hole on the housing. The shock is 12 degrees up hill (top hole) and a little more than half way retracted when at ride height. The pull bar is also at 12 degrees up. Am I way off here? So what I am hearing is to move it to the left on the housing, move it down one hole on the frame put a min of 1/4 preload (I don't have a 1050 spring for now) I will check the marker for travel. I may try it for hot laps with out the shock to see if I get mor travel on the marker than usual that should tell me if my shock is restricting the travel. I also have a rubber buiscut so this should help when unloading. Will I run the risk out pulling the drive shaft out at the end of the straight? If so that shock is staying right where it is. Thanks again. I most not be the only one to struggle with this I see over 400 veiws.

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted June 30, 2005 01:36 AM  
You said your pullbar is at 12 degrees up? Up towards the rear or up towards the front? The pullbar should be pointed downhill towards the front of the car.

Also removing the 90-10 won't pull out the drive shaft. Only way it could is if your pullbar broke off!

viper3244
Dirt Full Roller

Total posts: 57
posted July 02, 2005 11:41 PM  
Well I moved the j-bar down one hole on the pinion side, moved up 2 inches on the frame, and moved the pull-bar 3/4 inch to the left on the rear end, put a 1/4 inch preload on the spring, and the car was pretty fast. Maybe a hint tight in the center on a slick track (not sure how I am going to tackle that? I'd say it was a top two car. We finished six but started in the back, and had to contend with alot of traffic. Thanks for the help guys.

irace74
Dirt Forum Racer

Total posts: 80
posted July 03, 2005 04:31 PM  
90/10 available travel is also a result of Pinion angle. Generally, too much Pinion angle will result in too little shock travel. One way we set pinion angle is from the 90/10 available shock travel. I personally think that our chassis builders should give us this dimension anyway because it is simpler to read then an angle finder.

Additionally, to figure the pull center or the center of gravity of the car the calculation which was presented is almost right. If you think about it, if the car has 53% left side would not the center of gravity be located closer to the left side of the car? The calculation should be :

Locating point from the center of the left rear track = track * (1-left side %)
=64*(1-.53)=30" in from the center of the left rear track.

AFCO generally recommends 5* up on the shock and 15-20* down on the pull bar. The up and down reference is from the rearend forward.

viper3244
Dirt Full Roller

Total posts: 57
posted July 04, 2005 09:51 PM  
Where should the front of the pullbar be mounted? Ours is currentlymounted on the driveshaft loop and is left of the drive shaft.Should it be centered or to the left? If so how much? I have a feeling this is contributing to a little push in the center.

rrrrick
Dirt Maniac

Total posts: 192
posted July 05, 2005 02:38 PM  
I use the single spring pullbar 1" of preload, mounted 20 degrees dowhill, aimed towards the R/F tire.
I also mount my shock the same angle as the pull bar.
Don't know if its right or wrong, but the car really seems to hook up great

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted July 05, 2005 09:02 PM  
Having the pullbar offset to the left side will tighten the chassis under acceleration, so yes that could be contributing to your push. My pullbar is centered with the drive shaft, but remember the pullbar location left-to-right is a tuning variable. If the car is tight on the throttle, then try moving the pullbar to the right.

rick: you say that your 90-10 is on a downhill angle like the pullbar? Normally the 90-10 is mounted uphill to help tighten the chassis under braking. I guess if it works then it works!!

viper3244
Dirt Full Roller

Total posts: 57
posted July 07, 2005 10:17 PM  
OK here is the results of testing. Our rear end is 61 1/2 center to center on back tires. I moved the mount on the rearend housing to the left (now 30 1/2) it was 30 I didn't want to get crazy. I put a 1/4 inch preload on the spring and REMOVED the 9010 driver said it was alot better. But now my pullbar is a little left of the drive shaft in the fron and alot in the back. should it be paralell with the drive shaft, either if it is directly above it or to the left or right? Also the pull bar has been barely touching the side of the driver cab, which is ok for now.. Thanks guys

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted July 08, 2005 05:08 PM  
Try to get the pullbar straight in the car. If one end is further to one side than the other, the bar will bind! I would try to move the diff. side over to match the front of the bar.

The bar doesn't HAVE to been in-line with the driveshaft, thats just usually the starting point, but the bar needs to be straight! Moving it left or right is a tuning variable. More to the left helps load the LR more on acceleration.

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