HOW TO DO AN ADVANCED SEARCH.

Mon Jul 08, 2013 3:56 pm by Admin


For the benefit of 944Hybrids users there are two search functions available for you to use.
The purpose of this sticky is to explain the "Advanced Search" function because it is much more powerful and is the best choice when researching information.

When you log on to the site a list of options is shown in a line at the top of the page. One option is labelled "Search", use this option (NOT the search box lower down on the right).

After you click on the upper search option, a drop down box appears. At the bottom of this box is a radio button marked "Advanced …

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Wheel hop problem

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Wheel hop problem

Post  Luke714 on Mon Apr 28, 2014 1:53 pm

Why? How Fix? Smile I would be interested in any stories people have on this

-Flex in torque tube shaft?
-Flex in CV axles?
-Worn shocks?
-Transmission mount?
-Tires?
-Spring rate?

Whenever I loose traction I get some hop. I have a Lindsey racing ultra mount, I used to have a solid mount, but the problem was even worse. Khumo XS 295mm 180 tread wear tires. 29mm torsion bars. Old shocks. I'm thinking the higher spring rate and older shocks might be my problem, but some people are saying that it is all because of drivetrain flex and the shocks have nothing to do with it. The problem used to atrocious with the open diff, the new OS Giken unit has made a good improvement.

Thoughts would be great!


Thanks,

Luke
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Re: Wheel hop problem

Post  948 on Tue Apr 29, 2014 12:11 pm

Axle hop on these cars is not unusual, particularity higher HP cars with stock like suspension. Mine has always done it.
Based on my limited understanding of this, solving axle tramp/hop is never a simple task; though one might get lucky so you should try to solve it.
This NVH problem has to be designed out of the whole system, which is difficult to do. There are many papers and theories written about just this one NVH topic. This link gives a good outline of the bigger picture and some other search topic keywords. [You must be registered and logged in to see this link.]

In our cars I would believe that most of it is due to half shaft deflection, shock/spring settings and mounting bushing movement, both the bushings and the body mounts. My car had worse hop with the current bigger torsion bars and with my Koni rebound at full stiff.
Maybe a larger half-shaft on one side would have an effect, like the new Camaro and CTS.
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Re: Wheel hop problem

Post  Luke714 on Tue Apr 29, 2014 1:30 pm

Thanks for your reply. I would suspect that the spring and damping rates would have a lot to do with it. With how the geometry works between the rear trailing arm and half shafts there would be some load on the suspension as the CV axles twist (in addition to weight transfer onto the back wheels from acceleration). You would want to choose the damping rates such that you would have critical damping based on your spring rate and quarter weight of the vehicle. The load from the twisting axles and weight transfer onto the rear wheels would be a disturbance force I suppose. Maybe even over damped would be good.

Were you saying that when you went to heavier torsion bars you would get worse hop the stiffer you made the shocks?

Thanks,

Luke
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Re: Wheel hop problem

Post  Luke714 on Tue Apr 29, 2014 1:31 pm

Also, I do have new delrin bushings all around, and monoballs on the trailing arms. Everything seems quite "tight".
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Re: Wheel hop problem

Post  Luke714 on Tue Apr 29, 2014 1:53 pm

Doing more reading it seems like it is a change of toe under acceleration that causes the hop. Although, tired shocks can allow hop to continue oscillating. Stiffer bushings around the spring plate is what comes to mind (I think they would have the biggest impact on toe changes), but with Delrin already installed I don't know how to go much stiffer than that. Maybe adding some sort of a 2nd link between rear trailing arms and the housing that holds the torsion bars would help.
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Re: Wheel hop problem

Post  944convert on Tue Apr 29, 2014 3:46 pm

948 wrote:[You must be registered and logged in to see this link.]
My brain hurts after reading it...but great info.
acorad wrote:Oh yeah, Xschop, regarding axles, I was reading something lately about new high HP cars with rear half-shafts, that were designed with the actual shafts themselves being different diameters such that wheel-hop was minimized. Sounds like with different axle sizes both wheels don't hop at the same time/amplitude/etc.

Anyway, just mentioning it in case you hadn't come across this idea before...

Quote is from this thread:

[You must be registered and logged in to see this link.]

It sounds as if driveline resonance has a lot to do with wheel hop.

And from the same thread:
xschop wrote:...The Chromoly "spools" and has a different feeling when launching than the stock axles, which will save the trans under severe punishment. Also I have a Billet-polyurethane mount that I designed and installed and that in itself keeps the CV's from binding and exploding. With the rubber trans mount I could push it by hand 1/2" to 5/8" side to side just hanging under the car.....

So do these Chromoly axles help prevent or reduce wheel hop? If not, might using just one Chromoly half shaft help reduce 'resonance'/wheel hop since the one half shaft would spool/twist while the other wouldn't?


Last edited by 944convert on Thu May 01, 2014 4:16 pm; edited 1 time in total (Reason for editing : fixed broken link)

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Re: Wheel hop problem

Post  948 on Wed Apr 30, 2014 12:31 pm

Luke714 wrote:Were you saying that when you went to heavier torsion bars you would get worse hop the stiffer you made the shocks?  

Yes, when I went to 25.5mm bars (from 23mm) and stiffened the rebound wheel hop was easier to induce but the same severity. I have delrin trailing arm bushings also.

I think the reality is that getting rid of, or even reducing, wheel hop in our cars is going to be difficult. I suspect that the lengths you would have to go to reduce the hop, or move its resonance frequency, would likely make the car a less than comfortable ride on the street.
Companies like Merc still struggle to rid their cars of wheel hop even with all the FEA advances, decades of knowledge and tricks like traction control.

944convert wrote:So do these Chromoly axles help prevent or reduce wheel hop? If not, might using just one Chromoly half shaft help reduce 'resonance'/wheel hop since the one half shaft would spool/twist while the other wouldn't?

My understanding is that CrMo axles are typically used in off-road trucks and buggies. The idea is that when the wheels lift off the ground, even if for only a moment, and return to the ground under power the more elastic CrMo axle will absorb that shock load rather than passing it along the drive line. I think the primary reason these are used for our 944s is that they are readily available with the spines that we need, since they are the same as the VW based sand rail drive-lines. EMPI carries these types of axles, CVs, and even drive hubs.
Only an experiment would show if CrMo axles would change the wheel hop characteristics.


IIRC, the car makers used to add a second set of dampers to live axle cars, which allegedly reduced wheel hop.....not sure where you would fit one though.

Another option is to add a proper traction control system. A friend is using Racelogic in his 375WHP Jetta turbo. It has transformed the car completely. Look on YouTube for examples.

Having said all that, there is no harm in trying and you (we) stand to learn something from any experiment that are done.....
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Re: Wheel hop problem

Post  Luke714 on Wed Apr 30, 2014 1:28 pm

I'll try some stuff and post the results. Sunday will be earliest I can get to my car to try anything
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Re: Wheel hop problem

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