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Rear suspension hop on drag Subaru

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I've been running my Subaru drag car in the UK for several years, its currently the Fastest 4 cyl Subaru in the UK and previously held the fastest 4 cyl 4WD european record running our first 8 second pass back in 2013 (8.66 ET).

Anyway to the point of the post, the car upto around 2018 ran a 4 speed manual shift Subaru Automatic trans with this trans the cars suspension was faultless on launch and gear changes, approximately 2 years ago I changed over to the XShift Subaru plus sequential gearbox because the transmission losses were far from favorable about 40% including the 6% slip through the torque converter best 60ft with this setup was 1.31sec with the new set up we done the flat shift set up on the dynodynamics rolling road and hit an easy 1088hp then the car was wheel spinning on the dyno and just under 900ft lbs which is about 200whp gain over the auto. When I arrived for the first test hit at the track you could not even drive the car in a straight line the thing was just dangerous! Most of this was down to suspension movement and over the next year or so I managed to get on top of most of this and on a bad pass where I had to lift in first and second causing me to be off the throttle for 0.9 seconds and also costing 0.9 seconds to spool the turbo again I managed an 8.50 ET this was not a great 60ft 1.35 sec, now the real point of the post is rear suspension hop and this I am suffering with, on a far from perfect launch I did manage a 1.28 60ft which is not a bad 60ft but the hop is costing me, so what I had noticed was the car unloaded of the line completely different with the auto compared to sequential with the rear spring rate @ 275 lbs the auto would drop about 1 inch and progressively rise again with the sequential the car does not squat at all and puts the tyre into a situation where it bounces adding to the hop in first gear after the launch ,my next step is to soften the rear springs to around 200 lbs and the AST 3 way shocks are in for a service and dyno test with 200 lb springs. I have not had the chance to test the car again with the event table being cancelled due to Covid 19.

Does anyone have experience with the hop issue any further ideas are welcomed, I'm aiming for our first 7 second pass this year.

Many thanks in advance

Sounds like a VERY well built car!

Not my field of experise, to put it mildly, but here's some thoughts.

Wheel tramp is usually caused by a torque spike overloading the traction, traction being gained, lost,... in a violent fashion. The whole transmission acting like a spring getting wound up to the point of release and then winding up again... Sometimes it just needs a small change to something for it to be be made better, or worse. In short, possitive feedback.

Some things that can cause this are suspension bushings being a little too soft, or worn; engine mounts, if used, ditto; flexing of suspension or chassis parts - it's a good idea to give all those parts a good check over as tramp can put some very high loads on parts, like welds; tyre flexing frequency co-inciding with the rebound frequency of the suspension, etc.

Damper settings may help reduce the severity, but don't address the actual problems causing it. Changing spring rates may help, as may tyre pressures (effectively changes spring rate of the tyre) and/or tyre brand or design; if you're not using them, CF propellor and, maybe, driveshafts will help dampen transmission spikes and the reduced rotating mass will also aid acc'n. A slipper clutch will reduce the initial shock tot he transmission and tyres, etc. What are you logging for the engine rpm when the tramp is triggered - was the engine operating in that rpm range with the auto'?

Is there some how a change in anti-squat from setup to setup? It sounds like both suspensions are the same, however going from 1 inch compression on launch to none could easily be done by anti's. Changing springs and damping wont do anything here if all of your force is going through the control arms.

This might be the same as gords idea; Engine torque could be creating a lateral force loading 1 tire more than the other, causing it to give out earlier. Upon losing the resistance its pushing against, you lose that lateral force created by the engine rather rapidly, which then shocks the other tire(s). I believe drag cars simply raise the ride height of 1 wheel, or side to combat this and keep things equally loaded for some specific amount of torque. As you shift up it'll be sub optimal, but help keep first under control. I can pull a better explanation out of book if needed, I've never gotten to play with jacking to that degree.

With the beam type axles as found on most American drag cars, there is, indeed, an uneven loading due to the torque reaction, and sometimes it will be just as you suggest, with a VERY violent left-right shaking.

However, with independent suspension it's reaction is through the differential housing, which is attached to the chassis, and the tyres 'should' be equally loaded, if not immune to the left-right twisting if the traction is different enough to trigger it. It does bring up something I missed, though - the mounting and/or isolatators, if used, can also allow a deflection that can oscillate like a spring, increasing the chance of tyre shake/tramp.

Thanks guys, some good thought process there that is very similar to where I am with the car at the minute, here is some design points of the car which you will see translates to what you guys are thinking as I have developed the car.

• Engine and gearbox are solid mounted the engine is solid mounted to the OEM subframe however a tube type front subframe is being made at the moment, the gearbox is solid mounted via a rigid tube frame.

• The propshaft, trans to diff is carbon fibre.

• The diff was solid mounted but I cracked a rear housing in the knockouts at Doorslammers at Santa pod which was a real bummer as I was no1 qualifier at the time

For some reason I can only post half of my post? Strange.

Steven, it's been a while, any updates on your problem?

I was wondering is

a/ you had considered a slipper clutch - these are getting quite affordable as they gain popularity.

b/ what sort of engine logging are you doing - I was wondering it something, ot things, are developing a conflict that is setting off a positive feedback that is rapidly oscilating between full torque and low torque before the engine rpm moves out of that range?

Heck, something like a bad earth, power or data connection may be shaking from the launch, breaking and making the connection, before settling down with a good connection.

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