northern spotmod hopping

[nthdrt]

Im at a lose of what to do. Have 2013 BMS sportmod. scale # are left54.25,rear56.80,cross49.70and 47bite.These are real close to recommended builder specs. The prolbem is on a dry slick track the rear end hops like a rabbit.Being new to this class im not sure where to go to smooth this out . It works good in the corners until you hit the straights. A little more info, i have 4degree of pinion angle and the track bar is at 20 degrees. Any ideas, thanks

[north louisiana dirt]

what rear suspension is under the car? not familiar with bms sportmod..... does it have any type of pull bar ? What shocks ?

[UltimateRaceCars]

Is it hopping on entry or exit, on throttle or off?
Trailing arm angles?
Shocks?

[nthdrt]

what rear suspension is under the car? not familiar with bms sportmod..... does it have any type of pull bar ? What shocks ?

its a under slung car. just a solid bar for the pull bar have bsb black shock package

[nthdrt]

its hopping on exit down the straights,the lr arm isone hole down from the top and the rr arm isat the 5 hole from the bottom. it has bsb shocks on it

[JustAddDirt]

when car transitions from corner to straights, does it feel like the RR corner of car raises up. if so you are lifting car off springs, and topping out the drop in the suspension. so you have no suspension, but the sidewall flex in the tires. which will cause a hopping condition.

I am betting that if you took RR bar angle out of it, the hopping would stop.
could also chain the rr suspension and limit up travel.

[UltimateRaceCars]

Check to make sure that you are not over 45 degrees of left side bar angle when the car is hiked up. Adjust the left side chain so its no more than 45 degrees. Extreme LR bar angle can cause the car to pogo bad on corner exit and down the straights because the tire is trying to climb up instead of pushing the car forward.

[stock car driver]

its hopping on exit down the straights,the lr arm isone hole down from the top and the rr arm isat the 5 hole from the bottom. it has bsb shocks on it

Thats what bms mods do, A and sportmod, its very common.

I had a 14 a mod.

Ive raced at Salina and thats close to bms shop back then and almost every bms there in both classes did it.

It can be reduced by reducing bar angle but then youll have less drive, I lived with the hop it still went forward more than other cars on the track.

[langdonracing48]

Why only 4* pinion angle? I'd be bumping that up quite a bit. I run 9* in my bms.

[JustAddDirt]

Why only 4* pinion angle? I'd be bumping that up quite a bit. I run 9* in my bms.

with a solid pull bar?
Wow!?!

[langdonracing48]

I didn't catch the solid pull bar lol. Is that a lot for a solid pull bar? Nevermind on the 9* lol

[north louisiana dirt]

what is the ideal solid pull bar pinion angle and what are the reasons for so much less than a biscuit bar or spring bar? We have a class of mods that HAVE to run solid bars... I have never seen a whole lot of info about solid pull bars....thanks ahead of time.....

[langdonracing48]

I was wondering the same thing. I ran 7* on my solid pullbar stuff and it seemed fine.

[3dracer]

What would more pinion angle do for the car if it really doesn't lose much on acceleration? You can test all of this in the shop. I run just enough to keep it the same as the transmission angle when it is at full drop. (4* from trans to driveshaft, so run 4* from driveshaft to rearend housing) 5 degrees is plenty. With the biscuit bar, it loses angle because of the compression on the biscuits, and the solid bar won't compress.

[JustAddDirt]

A lot also depends on the length of pull bar, and the angle of the pull bar.
The arch in which it all swings has to be taken into account as well. 4-5* would be a safe starting point, I believe

[north louisiana dirt]

So the advantage is the HP gain/loss from an aligned/miss aligned driveshaft at full torque.....10-4.... I had a solid pull bar guy tell me his car "got more Forward Bite" when he reduced pinion angle..... do you think this is a true FB situation or just the effect of "reducing a bind" and slight HP advantage ?