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IRS WHEEL TRAMP, CONTINUED

IRS WHEEL TRAMP, CONTINUED

IRS WHEEL TRAMP, CONTINUED

(OP)
thread800-286119: Wheel hop / Tramp in IRS
As an update to then original thread on this IRSweheel tramp [Americans incorrectly call IRS tramp as hop}. Live axles do indeed hop vertically, but irs tramp does not actually lift the driven wheels, hence tramp is more accurate.
Update on my experiments and I really hope Greg and Pat and other previous contributors read this and offer comment.
Greg, as a followup to engine torque and the mounts being a source, well after doing extensive gopro filming of all of the likely energy storage bits; engine, gearbox, diff and wheel joints, i found that as expected they all tense up and release energy when wheelspin occurs. No wheelspin = no opportunity to release energy = no tramp. At tramp, the engine rocks the most on its liquid mounts, likewise the gearbox is bolted to the engine and of course rocks to the samne frequency, about 8 Hz. Greg, i fitted a shocker to the engine, mounting it horizontally using one of the few shockers that can work horizontally. I used 2000 QUAD Mustang shockers, as other types get an airlock in their tube that limits use. Result was that the resonance in the engine bay was reduced by about 75%, and reduction at the wheel, where it matters, was also greatly reduced.
The most alarming movement was seen in the diff; mounted with two rubber mounts at the front and a single offset mount at the rear. Gopro movie shows massive vertical resonance with minimal sidways movement. Hence a vertical rubber “snubber” would be a good helper here. A BMW fix in usa seems to be a fabricated rod end as a fourth mount with a solid bush in the original 3 diff mounts, hence solid diff bush with std rubber mounts everywhere else in the rear.
Gopro at the wheels showed the same 8Hz resonance, no vertical lift, only slip/grip cycles with some movement as the rubber mounts compressed causing significant toe change.

My mods, all done separately. Engine damper; excellent mod and the factory should have done it.
Polyurethane diff bush in each of the 3 mounts; NHV increased slightly with poly stiffness 90. Tramp surprisingly little changed. BMW “Guy” confirmed that he also failed with poly but succeded with solid joints. My experiments showed that these claims are dangerous because my experience is that only testing on a wet road is by far the best way to test. When questioned, suppliers of various poly bushes or arms or mounts showed that wet road testing had not been done. This certainly applies to claims from usa suppliers.

The use of big/small diameter axles gave unclear results. I used the Camaro use of differing axles as a guide and built my own axles to a similar torsion ratio. Improvement was minimal and resulted in me following up with GM designers. They also found it didn’t fix the tramp problem and ultimately did what bmw and mercedes did, and that is to use traction control to reduce or even stop all wheelspin. The trick here is that TC is not cancelled when pushed and needs either a 10 second hold down time, or simply cannot be turned off completely. How disappointing that these mega companies cannot engineer a solution and use basically the abs sensors to stop wheelspin rather than fix the cause, I call it waving them white flag!

I have systemically changed rubber bushes for poly or rod ends. The Camaro uses nice rod ends in their toe links, and added another spherical joint to the wheel hub. I can only conclude that GM has failed to find a complete solution and the IT solution used by others really only hides the problem. Electric throttles are part of this stop the wheelspin “cure”. Despite the throttle being 100%, the engine cuts to idle positioning!

Conclusion: i think the “www.hopnot.com” fellow using a “horizontal damper at the rear wheels is offering the best and certainly simmplest solution, and at minimal cost too. He also found that he needed an engine damper for full 100% wet weather solution. However, the layout of the ve cOMMODORE,/G8 is so far, too difficult to mount a similar damper. It needs innovative brackets to fit whereas hopnot solution is for a Chrysler 300 type car, similar rear IRS but different enough to prohibit it on the Commodore/Pontiac G8

So there it is: use an engine damper, fabricate a diff snubber, mount dampers horizontally at each driven rear wheel if possible! Using poly bushes is dangerous as they can only be used when the geometry allows the particular arm to rotate precisely {rotate over each connecting bolt}. This is not the case when you have semi trailing arm setups, like Camaro, C ommodore and Bmw or similar designs. I found poly suppliers simply do not understand how polyurethane works compared to vulcanised original rubber bushes. The rebound frequency of poly is considerably higher than rubber. Poly also does not like being forced to squish at an angle to the bolt in the joint, and will form an oval shaped hole after a few months in service and can resault in considerable dangerous movement in an arm or link.

After extensive testing, i am sticking with my thought that tramp is a system resonance caused by stored system energy being released through the wheel when wheelspin occurs, that’s energy being again stored and then release resulting in broken slip/grip wheelspin as the car moves along the road. If full wheelspin occurs, this bypasses the tramp mode, but of course forward progress is minimal.

Perhaps I should join the white flag crew and use electronics to stop wheelspin. I look forward to reading your comments. Please foregive typos, hard going on my iphone at night. Cheers

RE: IRS WHEEL TRAMP, CONTINUED

Good work. Interesting that the engine damper helped a lot but didn't cure it completely. I doubt anyone even tries to cure this mechanically any more - with modern gearboxes any powerful car is going to be running torque mitigation in the first few gears anyway (limiting engine torque so the gearbox doesn't explode), so you might as well run TC as well.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: IRS WHEEL TRAMP, CONTINUED

Tramp, tramp, tramp, the boys are marching,
Cheer up comrades they will come,
And beneath the starry flag
We shall breathe the air again,
Of the freeland in our own beloved home.

Written in 1864 about Union army prisoners-of-war in the South. Tramp has been a military term describing the action of marching foot soldiers (One step up, one step down, get it ??). Tramp is typically used to describe the ASYMMETRIC wheel jounce and rebound axle roll usually in solid axle vehicles under very high drive torque application involving tire stick-slip, (usually) leaf spring deformation and drive-line torque variation due to a lack of Cardan joint geometric control. We call what you are describing HOP (as in the way rabbits run) because that's what the wheels do in irs and some solid axle powered suspensions (front and rear). Farm tractors with mechanical 4wd are notoriously prone to hop AND tramp mechanizations when pulling very large attachments (plows), so the selection of part-time hydraulic 4wA (assist) is a popular option.

Youtube videos of hop and tramp drive-line response show commercial solutions (Steede) to hop (Mustang solid) and BMW (irs) which do work but their constraint solutions ignore the prop-shaft Cardan phasing feedback in their package, when a CV joint at the pinion would help in that situation. Even though the drive-line is designed as 'CV' using double Cardan (transmission and pinion end), the excessive pinion nose up or down rotation takes that design WAY out of the marching field parade, so to speak.

Soldiers on your continent must surely look silly bunny hopping along when the Queen visits ! Tramping looks better. The Iranians have an even better formal parade march stride. Check it out !

RE: IRS WHEEL TRAMP, CONTINUED

BTW: To validate your fix(s), run the evaluation play in reverse (gear). Torque ought to be higher and the results should be about the same, all things considered, except for ONE major factor change.

RE: IRS WHEEL TRAMP, CONTINUED

(OP)
Keep comments coming please, as this IRS problem seems to be unsolvable by major manufacturers. Using electronics to stop wheelspin I think is a cop-out! To follow up BMW solid diff mounts, google DEFIB LOCKDOWN. The very good HOPNOT.COM solution is worth further use too. I really do want to comment on quite dangerous mods by some American firms. IRS systems have bearings, CVs and diffs bolted to a subframe. Under acceleration there is no torque applied to the connecting links, only axial force. Claims that the links twist are difficult to understand by suppliers of giant sized trailing arms and other links. The mere facts that the semi trailing arms, as indeed most IRS systems use, means that the pivot plane is along a line found by drawing a line from the midpoints of the inner pivots. This plane is at an angle to the bush in each link, hence as the wheel moves it must squish up each bush. Think of it as it wants to rotate over each bolt but geometry forces each link to move at an angle. Firms pushing poly inner bushes do not allow for this “misalignment” squish. Poly takes it for a short while then remains oval hence the links rattle in this oval shaped hole.

The front end design also uses a semi trailing arm with the “castor” arm inner bracket forming a pivot line at an angle to the “bolt” through this link. Hence the arm rotates not perpendicular to the bolt but about a plan, same as the rear IRS arm is moving. Using a stiff joint in this “castor” arm introduces a moment into that arm and joint that was minor previously with a std rubber joint. I knew of an after market firm in USA selling a “delrin” bush, and ,get this, a replacement “castor arm” made with AL to the same dimensions as the std steel arm. I rang this firm to warn them and was told “what would you know!!”. Result was a moment was introduced into the now 1/3 strength arm due to the stiff bush and the arm snapped!!!!

I query the design of semi trailing arms in this wheel tramp/hop issue. Properly designed wishbones would indeed allow the use of stiffer inner pivots/joints. Live axles transfer a moment to the whole axle unit and can “hop” by raising the wheel end, IRS systems work in bearings and cannot raise the wheel hub (except under braking due to ther calipars being bolted to the hub). I don’t care what the Queen thinks either!!!

Greg and others, please comment on “Hopnot”, his web site has great videos. Not treating the cause (my tuning fork theory of resonance!!), and dampers are treating the vibrations AFTER they they occurred, but it seems to work.

Last comment; easily the best way to test possible solutions is to experiment on wet roads. Firms claiming success only seem to test on dry roads, and using the same “solution” on a wet road soon highlights any deficiency. My changes listed previously cured about 90% of dry road tramp, but wet road tramp is only about 75% improved. I haven’t fitted horizontal dampers to the rear though, but wish I could!! Cheers R

RE: IRS WHEEL TRAMP, CONTINUED

What modern IRS design are you looking at that uses a "semi trailing arm"? What modern FRONT suspension design are you looking at that uses a "semi trailing arm"?

Current generations of Mustang, Camaro, Challenger all use some variation of multi-link suspensions with quite complicated three-dimensional link orientations. None of them appear to even remotely resemble a simple semi-trailing arm.

Wheel hop on front suspension is only a relevant issue with front wheel drive, and all of the front-drive wheel-hoppers that I'm aware of use MacPherson struts.

I understand your points about substitution of urethane or delrin bushings in applications that have motions other than pure rotation around the axis of the pivot bolt and agree with its inadvisability, and I also agree with the inadvisability of not allowing links to flex when the geometry of the particular suspension design relies on them flexing/twisting, but it would help with understanding if your explanation accurately describes the situations that you are talking about!

RE: IRS WHEEL TRAMP, CONTINUED

It's the old Omega suspension. Basically the cheapest way of getting an IRS into a vehicle. Not necessarily a terrible suspension but it certainly has kinematic challenges (which I'm not going into).

Anyway the 100% solution, as I've said before, is a rubber up tube propshaft, as described in the previous thread. This is a known solution, but expensive. Some OEMs know this, perhaps they just don't see it as a cost effective or possible durable solution.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: IRS WHEEL TRAMP, CONTINUED

(OP)
Steady on Brian. Yes the front suspension I mentioned, on a VE and VF Commodore, same as Camaro, same as Pontiac G8 does indeed use McPherson struts. I was referring to a pivot plane formed by drawing a line from the lower “wishbone” links being the “castor” arm inner pivot and the control arm inner pivot. This pivot line is not parallel to the centreline; hence as the triangle setup moves through its arc, the inner links rotate at an angle to the bolts in each arm. Not a drama if squishy rubber is used, but is a problem if stiff poly is used and is a massive problem if delrin is used. Binding occurs and in the example of the Camaro that used, stupidly, the aluminium undersized “castor” arm, the bind introduced a serious moment and snapped it! Brian, the front end issue is indeed similar to the rear end on the IRS car studied, and very similar to many other makes of this type of suspension...... and they all tramp unless electronics intervene. Front wheel drive tramp I will leave to other more qualified forum members to solve.

Greg, I still don’t understand why the Lotus Carlton sleeved rubber prop works yet the prop on the studied car that has a rubber connection at each end of its shaft clearly does not work. Vibrations seem to be not dampened by these prop joints. This system resonance obviously relates to the overall stiffness of the total of the components. Examples that used solid engine mounts still tramped. I understand that GM experimented with various solid mounts in the drivetrain, and rear cradle, and failed to fix the issue. Hence the abs sensors were called on to fight the battle, and won it too. The idea of driving an expensive enthusiast’s car at full throttle to find the electronic throttle has been closed to avoid tramp is not a solution I like but may have to accept. I do like “Hopnot” though and would like further comment .... pity Mercedes, GM or BMW never thought of it.

RE: IRS WHEEL TRAMP, CONTINUED

As I understand it, the move away from the old semi-trailing arm IRS to the new multi-link designs at the OEM level did not stop the wheel hop (on the rear - I'm not sure how the non-driven front suspension would play into this).

The aftermarket is filled with "solutions" to perceived "problems" that not only fail to address the real underlying issue but also introduce new ones of their own. The old Fox-body live-axle suspension is a favorite one. Ford called it Quadra-Link at some point, everyone calls it Quadra-Bind. And yet, the stock setup works fine for the parameters that it was originally designed for (and I fully grant the point that the original design of that suspension layout was for the 1978 Fairmont). Hmm, upper links are an open cross section, and they twist. BAD. Let's replace them with solid. Hmm, the links have compliant rubber bushings. BAD. Let's replace them with urethane. Hey, why is the suspension binding? Ummm because it was originally designed to allow those links to twist a little bit and for the pivot points to have a bit of give in them ...

I've had wheel hop on wet pavement in a couple of front-drive VW cars, and in view of plenty of stories about broken differential cross-pins in those transmissions, I tried to avoid it (via driver intervention!) but I'm not drag-racing them and they certainly weren't designed for that. Folks who put silly power through the front wheels at the drag strip would be the ones to ask.

RE: IRS WHEEL TRAMP, CONTINUED

RUT prop works because it is very compliant. The Jurid couplings are incredibly stiff, from memory the torque is actually transferred via internal nylon cords, the rubber is there to encase them.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: IRS WHEEL TRAMP, CONTINUED

(OP)
I have tried to attach a file of the layout of a Camaro and hope it can be seen, albeit modified by BMR with links shown in red. (I hope the picture is attached!!). Greg and others, it is not the more simple Omega layout, and has upper and lower links that pretend to be wishbones. As you can see, the rear lower setup resembles a wishbone that clearly pivots about a line connecting the inner pivots and this line is about 30 degrees from parallel to the centreline, hence squishing the inner joints. Brian, the front setup has the similar problem as wheel arc means that the similar inner pivots also must squish up due to the geometry, admittably not to the same “misalignment” as found in the rear. The rear uses a toe arm to correct for the toe out condition with the lower “semi” trailing arm geometry. I understand that the simpler Omega design seldom used an additional toe arm ( Commodores 1997-2005 did) and did not also use an upper wishbone setup either; very different than the suspension I am trying to fix. Interestingly, to actually work, the toe correction must indeed have a “squish” bush somewhere, otherwise it will bind if not lockup. Suppliers of stiff poly or even delrin fail to understand this aspect. Observing open wheel formula cars at race meetings shows that they generally understand geometry, the ones that don’t fail to appear in the winners list!!

Still seeking comment on dampers at the rear, like Hopnot. GoPro vision of a resonating/tramping wheel shows the “ horizontal” damper on the hub needs to be generally parallel to the centreline but angled more towards the trailing arm mount. I need help to figure out how to mount such a system on the Camaro type suspension, the hub mount being the tricky part.

Greg, can you please explain how the vertical link on a 2017 Mustang and a M5 BMW works? I see it being useful under braking, but what does it do under acceleration? This part is the first thing replaced to fix tramp according to the aftermarket who use spherical bearings to replace soft Ford rubber in this link.

RE: IRS WHEEL TRAMP, CONTINUED

Is the diff housing vertical resonance a translation (like a heave mode) or a rotation about an axis parallel to the axle (like a pitch mode)?

If a rotation, this mode could be damped with a ladder frame and coil-over like the 5th coil-over used in (live axle) speedway sedans. Link

je suis charlie

RE: IRS WHEEL TRAMP, CONTINUED

I did some work on the old (1980s) Camaro, which had an arm connecting the diff nose to the back of the trans. That is an excellent arrangement. MX5 does the same.

Sorry, yes, I'd forgotten that in the last couple of years the Holden Commodore went to a direct copy of the Toyota rear suspension. I don't think suspension architecture is really the issue here, since I've seen similar problems on 3 different architectures. It's basically torsional windup along the driveline. My project to install tonewheels along the entire driveline was killed, sadly.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: IRS WHEEL TRAMP, CONTINUED

(OP)
GRUNTGURU, the diff has two mounts in the front and one in the rear offset inline with the crown wheel. “Gopro” videos are fantastic as the diff climbs as per the pinion climbing the crown wheel then as resonance occurs it pivots madly about the rear mount. “Snubbers” or a long arm connecting the diff front to a frame rail or perhaps gear box mount would certainly stiffen it. GM tried this on Camaro prototypes and cracked the housing, hence the steel diffs were used in the more powerful models. I would think the extra NHV would be a deterent. The BMW guy, google bmw diff lockout, claims success with solid diff mounts, with std soft rubbers in the rear cradle.

Greg, the GM VE/Camaro design is a copy of BMW 5 series, not sure what Toyoto model you are thinking of. All IRS seem to have similarities, and tramp is a usual outcome too. That is why i think it is a system resonance issue. Extra rear shockers to dampen the wheel hub, on a lever arm, seems the best idea for road cars. Racing saloon cars seem to eliminate all rubber and NHV is of no concern. Observation is that the only problem is under braking where the rear wheels can indeed lift off.

RE: IRS WHEEL TRAMP, CONTINUED

"Extra rear shockers to dampen the wheel hub, on a lever arm" may work, but it is attacking a torsional problem in a very second hand way.

I assume the modern Camaro picks up the final Commodore suspension, which was a plug and play copy of a Toyota suspension. Since the problem is torsional the suspension architecture is fairly irrelevant.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: IRS WHEEL TRAMP, CONTINUED

“Snubbers” or a long arm connecting the diff front to a frame rail or perhaps gear box mount would certainly stiffen it. GM tried this on Camaro prototypes and cracked the housing, hence the steel diffs were used in the more powerful models. I would think the extra NHV would be a deterent. The BMW guy, google bmw diff lockout, claims success with solid diff mounts, with std soft rubbers in the rear cradle."

A long torque arm connecting to a forward mount (especially the gearbox) should not have as much impact on NVH since force required to resist the axle torque oscillations is reduced by the lever length. Further - a more resilient mount can be used - it is control of angular displacement you are after and again, the long arm is your friend. Last - if mounted to the gearbox the factory isolation works in your favour.

je suis charlie

RE: IRS WHEEL TRAMP, CONTINUED

Kinda odd that no one has implicated a major root cause of this problem. (Ok, it IS very odd). Preserving the old-school suspension layouts developed with 180 kPa high grip high rolling resistance tires and then mandating 341 kPa low rolling resistance tires saturated with carbon and silicon has all the ingredients for axle hop, skip, tramp and a jump. From a simulationists viewpoint with a Pacejka tire root kit, we have a weaker 'D' term (peak force) and a lame 'C' term (shape factor). while this helps the ABS and traction control side of the house, the steeper downside slope of the mu curve (a negative spring rate) produces positive feedback to an already marginally unstable powertrain system. Therefore, look at a tire solution to this problem if you are tired of wrenching blind.

FYI, in the case of the G8, the car was developed in-house on low pressure YOK and PIR tires with some of the worst rolling resistance and unusual Fy characteristic I can think of. Because of the weight distribution (51%), the amount of understeer present in the car was extremely low already without the soggy bushings in the boomerang parts up front. At 26psi, there was some decent Mz on-hand to keep the car legal. Now we get to the "Global Vehicle Requirements" and the shit hits the fan. Not enough of these 'special tires' with low Fz sensitivity could be sourced (to try to make use of the 1-2 % weight, durability issues with the tires because of the carcass recipe, and of course now, the 35 psi slippery tires with a mile per gallon fuel economy mandate improvement. There goes the Mz, here comes the traction thing, and now comes along a transmission that grenades when the motor guys find some extra power between the mounts. (easily fixed as pilot build starts, by a TC twiddle).

And there we were, feverishly looking for a tire supplier with enough volume, enough resources, existing tire molds and enough patience to throw down a construction that would survive the Legal system's need for a 'safe' car in spite of 'foreseeable misuse' conditions (dreadful aftermarket tires and pressure loss with age). Did I mention that a spare tire fiasco comes along with this abortion in the making? GM certainly learned a lesson about cars designed by welding class graduates instead of mega Adams modelers. Made me a lot of extra money with the overtime, though. My IRA thanks all of you very much !

RE: IRS WHEEL TRAMP, CONTINUED

Quote (Ron364)

can you please explain how the vertical link on a 2017 Mustang and a M5 BMW works? I see it being useful under braking, but what does it do under acceleration? This part is the first thing replaced to fix tramp according to the aftermarket who use spherical bearings to replace soft Ford rubber in this link.
The vertical link is necessary to prevent the upright from flopping over in side view regardless of the direction of the longitudinal force at the contact patch. The single upper lateral link cannot do that job, so the upper part of the upright was tied off to the lower control arm instead. I suspect that the precise inclination of this "vertical link" (or a construction line drawn perpendicular to it through its upper attachment) plays a minor role in defining the SVIC and the anti-squat characteristics.

FWIW on bushings. Not all of the aftermarket companies are completely dense about what it takes to use stiffer bushing material. At least one aftermarket company has used a 3-piece sandwich design that places a short length of firm polyurethane in between softer pucks, the idea being to eliminate some of the moment resistance. And they ran a series of tests that generally showed that to be true. But unfortunately, most aftermarket bushing suppliers still insist on making the poly "bushing" material longer than the inner sleeves that the bolt runs through, which manages to take a poor approach and make it worse in most respects. But at least polyurethane is a fairly soft material as far as removing bits of it to induce a bit more compliance in some directions is concerned. A crude approach to voided bushing technology, but it does work.


Norm

RE: IRS WHEEL TRAMP, CONTINUED

I had a look at the Hop Stop site. If that solves the problem then it isn't the problem I solved. Nothing wrong with that, often problem definition is the biggest part of an investigation.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: IRS WHEEL TRAMP, CONTINUED

FYI, the latest 3 generations of Corvette have the differential attached to the end of a 1-piece drivetrain assembly so it essentially creates a long "torque arm". The design also eliminates the issue of engine/transmission torsional movement. They still do it.

If you want to see something interesting, jack up the car, take off the wheels and re-install the nuts to hold the rotors. Then, let the car run in gear and watch the effect on the rear suspension as the traction control applies the rear brakes. You'll likely see the suspension moving quite a bit. Controlling that movement could explain why the hopnot solution helps.

RE: IRS WHEEL TRAMP, CONTINUED

(OP)
Getting near the end of asking this forum for help on this topic. “Cibachrome” could you please have another go at explaining what you mean re tyre design. The VE Commodore and the Camaro were both designed in Melbourne to use very grippy Bridgestone Pontenza 050 tyres. I don’t like low profile 40 series tyres on road cars, perhaps address this too. The VE rear used a BMW 5 that was dismantled and viewed to the nth degree. It formed the basis of the IRS design despite the knowledge that those cars were generally very poor in ride on typical Aussie roads. Greg, the VE is not an Omega simple lower trailing arm, nor is it a Toyota copy. The top L shaped pseudo wishbone was used to fit available space. The weird pivot planes top and bottom are not parallel to each other and are at about 30 degrees to the centreline. Videos taken by BMR simply show the geometry change arising from the mismatched top and lower “wishbone” planes as the hub moves vertically with some rotation with normal driving, yet the company uses it as a display of massive movement that needs their products to correct! A rear toe correction arm pivots in a different plane to keep toe out changes reasonable. This geometry, especially the use of a toe correction arm, requires some joints to compress at weird angles to allow for the pivot planes not being “along the wishbone bolts”. Using stiff joints like poly or delrin is clearly not appropriate everywhere.

I agree with Greg that the engine mounts are the major source of the problem as the powerful V8 has lots of energy to store in its fluid filled mounts. Similarily, energy is stored in the corresponding gearbox rubber mount. The prop may well twist too, and then to the diff that is rubber mounted to a cradle that is rubber mounted to the monocoque. The onset of tramp occurs when the rear tyres lose traction. The release of the stored energy adds to the engine torque and makes the engine violently resonate, similarly the gearbox as seen by the gear lever resonating inside the cabin. The diff resonates about its only rear mount in a vertical motion, and it is indeed frantic as per my GoPro vision. The cradle moves but really this is minimal in comparison to the diff and much less violent. The wheels do not “hop”, that is a live axle outcome as the irs axles spin in bearings with no rotation being applied to the outer hubs only to the wheels. The tyres grip for say 6 inches, then spin for another 6 inches and this slip/grip process continues until traction is fully regained mainly by the throttle being lifted out of sympathy. The wheels themselves resonate in several horizontal directions seemingly as the arms’ rubber links flex. The massive amount of thumping and banging from tramp must surely have made the designing engineers cringe when testing! Pity they didn’t fix it!

Greg has suggested that “his” Lotus Charlton solution of putting a rubber twin tube in the tail shaft to dampen things actually works. I contacted English owners and they confirmed it too. This solution is beyond a DIY owner, and no manufacturer is interested now that a simple electronic solution is available!

My trial and error experience has shown that my biggest gain was obtained by using a $50 damper in the engine bay. Helped greatly in the dry road tramp, but still present on wet roads. Solid cradle mounts in the rear also helped but did not cure tramp. Poly diff mounts made very little improvement, a fact that the BMW “diff lockout” fellow confirmed to me; he tried poly but he needed solid aluminium diff mounts to achieve any success. I agree with a previous contributing forum member that poly rebounds at a very high rate and can make the situation worse despite reducing movement. Axles; at great expense I made up some big/small high strength axles to a torsion ratio of 4 to 1 like the Camaro and CTSVs; result was only a minor improvement. Despite the advertising hype from GM, I later confirmed that they drew the same conclusion and used electronic TC control; no wheelspin = no tramp. Funny how BMW and Mercedes also reached the same non-engineering solution.

Disappointingly, I have found minimal ideas from this forum despite the purpose of having a forum to share knowledge.

I found that the aftermarket generally has poor knowledge of the cause let alone the solution. When I saw poly bushes being sold with their inner tubes shorter than the poly, I knew they had no idea how their own joints actually worked. Discussion with several firms both in AMERICA and Australia soon made me conclude they were more interested in drag racing traction rather than curing wet road tramp.

The best free thinker on this subject has been “Hopnot”. Treating the end product at the wheels and not the cause at the engine, was a great solution (he does use a simple engine steady though). Cheap $100 dampers at the wheels to dampen the wheel resonance works. Same as putting your finger on a resonating crystal glass. Funny how other manufacturers have spent millions and failed as they never thought of the easy option. Annoyingly, it is too hard, so far, to fabricate mounts on the subject car (Commodore 2007-2017) and the Camaro (2010-2018).

So what has the forum offered? Grippy tyres I think but remain confused by Cibrachrome, rubber prop by Greg, and reference to Mu curves and models by others, being of little actual help. I remain amazed that huge companies like Ford sell Mustangs with IRS that have horrendous tramp. Solutions being left to the aftermarket and as I have found, they don’t test on wet roads and hence I have no faith in their ideas. Hopnot, GM, BMW, Ford and Mercedes would like to speak to you. R

RE: IRS WHEEL TRAMP, CONTINUED

If your question to me is about the usage tire properties related to power hop and tramp, which one of these tires do you think has the likelyhood of a problem ? And the Oz tire selected for the GMX521 program submission were 245/50R19 Yokohama Advant S/T at 32 psi on the Goat but faded to the 'Stones needed for the global market. Too bad your concerns don't show up anywhere on the "Voice of the Customer" mission statement for the G8. The Market clearly doesn't feel your pain I'd say !.

RE: IRS WHEEL TRAMP, CONTINUED

(OP)
Norm, thanks for the modelling reference. I presume the main manufacturers have access to the same or similar models. The outcome is that their IRS cars all seem to tramp, so does this mean the models are of no help, or perhaps other factors like cost override. I see no value in this model in the situation of working on cars that have already been designed and are in production and tramp. I was hoping to obtain a direction from forum members.

Cibrachrome, thanks for the reply and the reference to the “Goat”, an Australian GMH “Monaro” sold in America as the Pontiac GTO that uses a simple semi trailing arm that Greg incorrectly assumed. The subject car is very different, the only common element being both tramp like crazy. I find it difficult to follow your posts. The subject cars, Australian VE Commodores (the G8 and GXP is the same vehicle except the G8/GXP is sold in America and is left hand drive), and the near identical Camaro since 2010. They use 245/45/18 and 245/40/19 Bridgestone Pontenzas in Australia and a variety of similar grip tyres in America. If you think that I am the only person worried about tramp, then you are mistaken. Ask BMR or gforce1320 for verification. If you have a clear suggestion to solve the problem, perhaps with tyre choice, then please give it in plain English. The Pontenzas are very grippy tyres with stiff sidewalls in the 40/45 profile. A low profile that I personally dislike for road cars. If you know of better road tyres, please list them. The simple rule “no wheelspin = no tramp” holds true, so grip is king. The link below shows the rear end in good detail.

RE: IRS WHEEL TRAMP, CONTINUED

Not sure what tire resources you have nearby, but I took a look at tire constructions and construction modifications that were going on at the time. Without a doubt, somebody was having trouble with the Bridgestone 245/45R18 and 245/40R19 submissions. (These are all on 8.5" rims).

If you want 'grip', there was a Goodyear and a Continental at the top of the list in the 18" size. In 19", there is a Michelin and a BFG that look interesting. If you want to go off the beaten path, There is a wizzbang Pirelli in 245/50R19 that showed up as the perfect Artgum Eraser. (Lots of real rubber instead of fake rubber.)

Now. Here is a cheap experiment to run on your car that may reveal the answer you seek. What I see in most of these tests is wayward Mx (Tire overturning moment). Not the stiffness so much, but the DC value. It's equivalent to the scrub radius you would think about on a steered upright. Under power, (as in MASSIVE power application) this design 'feature' is a player in torque induced steer (as in break your fingers from steering wheel fight). What I recommend is that you reverse your tires in the rims (outside sidewall now mounted inside). This may reverse the sign of Mx (unless its rotation direction dependent). The tramp may get horribly worse or get much better.Only need to do the rears. Next play, you would add wheel spacers to change the effective rear scrub radius and witness a change in tramp. You won't have access to spacers with negative thickness, but if the phenom is much worse, a change in wheel offset would make sense at this point. Tires and wheels have to fit the wheelhouse, though.

Then report back.

RE: IRS WHEEL TRAMP, CONTINUED

Hans Pacejka is one of the top authorities on tire behavior and how to model it. I suggest looking at the mu-slip curve more for its shape and what that might imply. It's more difficult to keep your tires operating within, say, 0.02g of a sharp peak than within the same 0.02g of the top of a wider one with more gradual curvature. I provided the link mostly to point you at how the "C" and "D" terms mentioned by Ciba fit into this.

Here's another Pacejka link that I only had to type his first initial and last name to find. https://books.google.com/books/about/Tyre_and_Vehi...


On "no wheelspin = no tramp".
In a front engine/RWD car with more than half of the static weight being carried by the front tires, engine torque (briefly assisted by sudden clutch engagement) times overall gearing in 1st gear and divided by drive tire rolling radius may be putting you in a situation where wheelspin with reasonably normal road tires simply cannot be avoided. Keep in mind that anti-squat for most IRS systems tends to be significantly lower than what is possible with a live axle system intentionally designed for hard launching. Meaning that you aren't going to get as much traction help from near-instantaneous load transfer to the rear tires. Nor would I expect tires with higher vertical stiffness to cope as well with sudden traction loads as more compliant tires. Wrinkle-wall slicks were developed to serve a very specific purpose . . . and they aren't much good for anything else.

I'm quite aware of the unhappiness over the electronic approach GM has taken in the 6th generation Camaro (Alpha chassis), most noticeably in the supercharged V8 versions fitted with manual transmissions.

Norm

RE: IRS WHEEL TRAMP, CONTINUED

I was obviously wrong on the suspension geometry. I'd observe that the traction forces are not directly resolved longitudinally but instead go through those red NVH bushes (they provide the longitudinal compliance and damping at the wheel) and the pivots. That setup works well for 150 hp front wheel drives. My experience is that a longitudinal arm (eg Chapman strut) that directly resolves the traction forces is a lot less trouble than messing about with nice packaging friendly designs.

Now as to solutions. I've told you what worked on the Carlton. You complain that an owner can't fit that solution. True-ish. Nonetheless that indicates that at least some manufacturers were aware of the problem and knew how to solve it mechanically. Since these days it can be solved with correctly calibrated traction control then you'll no longer see OEM mechanical solutions for it. That's life. You seem to have found an aftermarket solution. Great, go and fit that.

The reason you are getting little traction here is that there isn't a big bumper book of secret solutions with the answers written down, problems have to be defined and investigated before most solutions can be applied. Different cars may need different solutions. I'd estimate that a proper mechanical resolution of that problem would cost about 1000 hours of engineering time and add about $200 in parts to the cost of the car, or about $1000 on the selling price. That is not small potatoes.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: IRS WHEEL TRAMP, CONTINUED

(OP)
Thankyou to Cibachrome, Norm and Greg for the type of replies that I was seeking initially from an engineering forum.

Greg, GM invested huge dollars in seeking a solution, as did the others, way more than you suggested and still came up short. I suspect that they ran out of ideas rather than money. Seems the white flag solution using TC wins in the end! I bet future experimental models will have engine dampers and horizontal rear hub dampers, if they have read this forum that is. Pity the rubber prop didn't find its way throughout GM.

Ciba, I will try to find, borrow or steal different offset wheels and see what happens. Not much clearance to the bodywork but maybe a little will be enough to help. I am keen on Michelin Pilot Sport 4 but they seem to love the low 30/35 series profile that pleases the looks dept but not the usefulness dept. The weight of the huge wheels and tyres these days is massive (mine are 26kg). GOPro vision of a wheel tramping is scary as it wobbles around with great energy. The American fascination with drag racing means that little can be achieved with road going tyre options. I was banned from a web site simply because I pointed out to PeddersUSA that their poly joints were inappropriate due to geometry squish issues. Rule apparently is to never critise a forum sponsor, even if they sold inappropriate "bushes" to an American market that seldom questions authority. PeddersUSA went bankrupt soon after!

Norm, thankyou for your input. Yes likely that GM, BMW, Merc etc used these tools but all reached the same conclusion; to use electronics. Hotup offshoots like Aussie HSV, Harrop and Walkinshaw are good at bolting on extra horsepower like superchargers and soon found the tramp issue in spades. They simply used the "tune the TC" solution hence making the extra power of little benefit. Driving one of their rocket ships, I remember getting wheelspin at 160 kph on dry pavement, a scary experience. Their high HP mods mean the tweaked TC now eliminates wheelspin at a very early stage, and thankgoodness really for these cars are missiles.

Conclusions: don't believe the advertising hype from manufacturers or from (most) vested interest aftermarket. IRS has indeed great advantages over live rear ends, but the downside seems to be you must deal with tramp issues. I am aware that you could always manually lift the throttle, as if that will ever happen! R

RE: IRS WHEEL TRAMP, CONTINUED

When I worked with GM products we used to come up with solutions to the problems we found, and the respective GM division would say, "Oh we know that but we can't afford to put it into the car". Now that may be true, or it may be CMA. A RUT prop is an expensive proposition and at $200 would be a deal breaker for many programs I have worked in, especially if an electronic fix is available.

Holden in particular are not famous for over-engineering their cars.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

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