[Torbjorn968]

Talked to the fastest 951 driver in Sweden yesterday, has a 350 hp machine compeeting against GT3s in the Swedish Porsche Club championship. He has the same LEDA setup as I do with 265s forward and 295s at the rear. Uses 4,5 degree forward and 3,5 degrees at the rear. Says he wants more at the rear for the dry but compromizes for wet grip. Self designed arbs forward and rear.



For my application with 255s all around I ordered his forward arb. He also adviced me to think about a rear wing, which I have laying around so time to mount it!



/T

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[flash]

i am betting that none of them are using any chassis bracing down under the car, which dramatically changes the needs for camber



how big are his ARBs? i am using 31.75 front and 22.25 rear (same shape and lever length as OEM) - this has also reduced my need for camber



i am actually looking to nearly get rid of the rear camber altogether - i have less net contact patch area that i will when they are flat



remember, it's not about how flat the tire is on the outside tire - it's about how flat it is on the inside tire - transfer that weight back to the inside, and you will really start going fast

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[Torbjorn968]

[quote name='flash' post='55240' date='Jun 23 2008, 03:12 PM']i am betting that none of them are using any chassis bracing down under the car, which dramatically changes the needs for camber



how big are his ARBs? i am using 31.75 front and 22.25 rear (same shape and lever length as OEM) - this has also reduced my need for camber



i am actually looking to nearly get rid of the rear camber altogether - i have less net contact patch area that i will when they are flat



remember, it's not about how flat the tire is on the outside tire - it's about how flat it is on the inside tire - transfer that weight back to the inside, and you will really start going fast[/quote]



I had a look at http://www.968forums.com/index.php?showtopic=2677&st=60 but you are talking about "under car bracing"?



Arbs are M030



I am probably marking words but how would you transferr weight? Under a certain cornering G the weight transferr from inside to outside should be a function of center of gravity and the lateral accelleration..





//TL

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[flash]

yes, i am talking about under car bracing - the stock "frame" allows for a LOT of flex, resulting in large camber change - i am amazed at how flimsy it really is - bracing below between the castor blocks limits that flex



M030 are not large enough - this still allows too much body roll to effectively get the desired results, but is a limiting factor in many classes of racing - i ended up making bars that are as much stiffer than the M030 as the M030 is over stock - much better now



weight transfer is dictated by a number of factors, not the least of which is body roll - limit the body roll, and you effectively "pull" that wheel back down



if the chassis is prevented from flexing, and the body is limited on roll, the result is more of the inside tire on the ground - this is the "dragging" tire - this is what prevents understeer/oversteer



the goal in a perfect world is to have all 4 tires flat on the ground, and most of the weight evenly distributed across them



most people choose to increase negative camber, so that the outside tire is flat on the ground in a turn - on the surface, this makes sense - but, large camber angles cause the inside wheel to barely make contact - but, adding too much camber is like taking aspirin for a headache, when the problem is really the wall you are banging your head on to begin with



the solution is to correct the frame weaknesses and limit body roll - in other words, stop banging your head on the wall



however, they must be done in concert - stiffen the frame and fail to reduce roll, and you end up lifting off even worse - limit roll and don't stiffen the frame and you get a lot of twitching as the frame snaps back and forth - bump steer and sudden oversteer are common results



another problem is that many people lower their car too much - lowering also seems to make sense on the surface, and to a point it does - however, you can lower a car too far, and drop the roll center below ground (there is a big difference between center of gravity and roll center) - this results in very ugly high g-load handling - i haven't drawn it out yet, but from data i've been able to collect, 630mm from the arch to the ground, on standard outside diameter tires, is the limit - you can correct this by changing the mounting points of the control arms and steering rack, but this also is ruled out in many racing classes



hope that helps

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[Torbjorn968]

Talked with the owner of the quickest car in the slowest PCS (equivalent to PCA) class in Sweden, a class which is dominated by stripped 944 S2s. The wheel angles where 4,5 degrees forward and 3,7 degrees at the rear and thus approaching the touring car numbers i have quoted above. This gave the most even tyre wear on track with some impressive lap times,



This year I have run 3 degrees all around. Track miles vs street miles is about 20/80 (most of the street miles driven to the track). Since the middle of the summer I have new rears and the fronts are a couple of months old. Front wear appears pretty even with the expected bias of wear on the left side. The fronts are to new (and the Michelin Cups to durable) to draw clear conclusions after about 5 track meets (2 in rain).



The rears are more interresting. The left side is very evenly worn where the right is more worn towards the inner side. This could be due to rear camber wearing more on the inside on the tyre affekted by camber and heel angles on a clockwise track with more rigtht turns.



The car is very well balanced but I do have one little worry and that is one particular cambered lefthander where I sometimes experience snap oversteer. This sunday the car should perhaps be forgiven with me passing cars on the inside making a very tight corner with cold tyres so perhaps not something to draw clear conclusions from.



//TL

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[Cloud9...68]

This has been a very interesting and informative discussion, but it seems to have drifted a bit from the OP's question about the best set-up for equal sized tires all around. Do the recommendations you guys have made (chassis stiffening, thicker sway bars, minimal camber, especially at the rear) change materially on a car with equal tire sizing all around, vs. one with the standard wider tires in the back? Does anything else besides the three items I've listed have to change with equally sized tires? Thanks.

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[flash]

no - tire size really has little to do with geometry - it can make a difference in sway bar settings and shock settings, but that's about it - the basic rules for the order of setting up a car are always the same though



chassis and frame first, doing whatever needed to prevent suspension mounting movement

springs next, calculating for maximum g-load and estimated corner weight both static and loaded

corner weigh and correct

change springs as necessary to maintain desired balance

measure and set ride height based on suspension geometry and roll center

shocks and strut settings next

first alignment geometry setting

swaybars - these need to be matched to the springs - stiffer springs = stiffer sway bars and visa versa

second alignment geometry setting

then you start messing with tires

third alignment geometry setting

reset tires again



doing this out of order only forces you into chasing down things that may be fundamentally wrong in the first place - it also opens the door for one problem being masked by others



a typical example is trying to dial out understeer or oversteer with shock settings, when the problem was too stiff or too soft of a spring somewhere



same goes for trying to make a car hold a corner and not understeer by increasing negative camber, when in reality that can reduce the net contact patch area on the two front tires, and can easily INCREASE understeer and overheat the outside front tire at the same time



math first - parts second

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[Torbjorn968]

[quote name='Cloud9...68' post='61594' date='Oct 14 2008, 01:23 AM']This has been a very interesting and informative discussion, but it seems to have drifted a bit from the OP's question about the best set-up for equal sized tires all around. Do the recommendations you guys have made (chassis stiffening, thicker sway bars, minimal camber, especially at the rear) change materially on a car with equal tire sizing all around, vs. one with the standard wider tires in the back? Does anything else besides the three items I've listed have to change with equally sized tires? Thanks.[/quote]



First of all: all numbers I have stated from my own car and lately for the quickest Swedish class 1 car is for equall tire sizing all around. The numbers for the class 1 car is valid for my own setup since spring rates are simmilar. The car runs MO 30 sway bars which I had previous to the Tarret Sway bars. The Tarrets have not changed balance but the cars feels better.



I find the following interresting:



- if I understand correctly all standard 968 settings have stiffer springs forward than at the rear



- the current swedish norms are 800 - 1000 forward and 1000 - 1200 at the rear, I run 800 forward and 1000 at the rear. This is also simmilar to the preffered Nurnberg Ring settings.



- even with a change of ratio in stiffness between the front and the rear (which reduces understeer/moves the car to oversteer) the wider front tires are beneficial to the handling of the car



It seems that all the quickest cars uses large camber angles, much larger than I understand US norms although our cars seems to run higher spring rates. This also seems consistent with numbers available from touring car teams.



Some on the this forum has voiced the opinion that the rather large angles at the rear should give power down problems at tight turns. I do not experience that and tyre wear indicates that this is not a problem. The tyre wear of the left rear seams very even which indicates that the contact surface has been maximized. Because of right hand turns being dominant on a clockwise track there will always be more wear on the inside of the wheel which is normally the inside wheel. Note that I run a torsen style Quaife diff which is torque sensing and thus sends the correct amount of power to the rear wheel with most grip.



For next year I think I will have adjustable forward angles so I can change from 3 degrees on street to 4 degrees on track and run the rear at 3 degrees. The Tarrets sways might be stiffened a bit forward (now at 90%) and loosened a bit at the rear (now at 50%) to compensate for the extra forward grip I hope to achieve by this.



In our discussions Flash generally comments that the important thing is to regain lost contact surface on the inner wheel. This would seem logical if it is not on the expense of contact surface on the outer wheel. The friction coefficient is higher on the highest loaded wheel and therefore the outer wheel should be the most important one.



Loosing contact area on the inner wheel is due to:



- heal angels

- weight transferr



The heal angles are a function of suspension (spring rates and sway bars settings).



The weight transferr is a function of center of gravity which is dependent on ride height and heal angles (at heal the center of gravity goes up).



Since the cars I have as references (Swedish PCS race cars and touring cars) are very stiff both in terms of bodies (welded in cages etc) and suspension they will also have small heel angles. They also ride low. Yet they seem to benefit from large camber angles. When you see a touring car you wonder if something is broken in the suspension! I know that the touring cars runs slicks which increases weight transfer compared to R tyres but in other terms they are a good reference to our cars.

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[flash]

sigh - ok - i give up

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[Torbjorn968]

[quote name='flash' post='61649' date='Oct 15 2008, 12:17 AM']sigh - ok - i give up[/quote]



It would be interresting if this would be more than a "torbjorn vs flash" debate.



Flash, I am a beginner trying to undstand the subject (I know the tone is not exaclty "beginner" - berhaps even "besserwisser" - and perhaps I should have salted my post with a couple of "softeners" such as "in my mind" etc). All your comments are very interresting but the problem is that they run contrary to references that are easilly accessible to me: the Swedish PCS scene and the touring car scene. The moderate camber school is very attractive since it would mean that I could drive with a more "streetable" setup. The problem is that I find no backup in form of race results with cars setup this way or tyre temperature readings, wear analysis of the tyres etc.



I make a couple of theoretical statements in my post. There is one in particular which would be interresting to disect and that is where I say that the higher loaded tyre has a higher friction coefficient and that increasing the contact area of the less loaded tyre would be interresting only if it can be done without significant detrimental effects on the loaded tyre. It would be interresting to have this with a race reference. Likewise my statements regarding weight transfer would be interresting. Can weight transfer be due to anything else than lateral G forces and center of gravity? I.e a simple lever arm analys.



I am also aware that stupid setups sometimes can be masked by good drivers. This is a frequent occurance in sailing where I come from. Eric Twiname was a great tactician and bookwriter in the field. I remember reading that he once took an old laser dinghy to race. The bottom was badly scratched and Eric claimed that it was better for the waterflow. Eric won the race using superior tactics. After the race several sailors where scratching their heads with boats turned upside down wondering where to put the scratches. Or the practical joke played by one of the best offshore race teams ages ago. They painted the deck black claiming that the updraft had aerodynamical effects winning the pre-regatta race. In the evening lots of teams where seen painting their decks black where our jockers secretively painted theirs white.... The next day was a sunny one. If you ever sailed on a black boat you know it is not a good thing.



//TL

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[flash]

lol - no worries - i just seem to be repeating myself with no change in understanding



i realize that the guys you have around you are doing exactly the same thing as everybody else



that does not mean it can't be better



i've tried to lay out the issues and the solutions - i highly suggest stopping changing components until you do some reading on chassis and suspension design - then break out a tape measure and a calculator - i think you will find that all of those guys are missing some basic design limitations that need to be overcome, and are approaching the problem from the wrong perspective - it is very common with this car - i don't think anybody has ever really looked at what is going on with the chassis and suspension, and so they continue to do the same old thing, without really solving the problems



i learned this the hard way on another car, and spent a few seasons bolting on "what everybody else did" - when i finally took the advice from somebody who knew, i immediately ran faster than those guys - i never did catch the guy who taught me (clearly he held something back on me) - lol - but i was a LOT faster



the one potentially limiting factor is going to be the rules in your class - i have no idea what you can and cannot do legally - that could limit you - the class i am going to run in has no such limitations, so i can correct the problems in the design without consequence or penalty - with the new parts i've designed, i already have the inside tire much more planted, and that's on a 3000lb street car - i can't wait to get it down nearly square on a 2500 lb race car where i can give up some ride comfort

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[Torbjorn968]

[quote name='flash' post='61674' date='Oct 15 2008, 03:15 PM']lol - no worries - i just seem to be repeating myself with no change in understanding



i realize that the guys you have around you are doing exactly the same thing as everybody else



that does not mean it can't be better



i've tried to lay out the issues and the solutions - i highly suggest stopping changing components until you do some reading on chassis and suspension design - then break out a tape measure and a calculator -[/quote]



The tape meassure part is beyond my understading, as I rather frequently have bragged that I use a Swedish Touring Car engineer as advice (he is now in the British series) , just need to get the hold of him (he is on the track) and ask for a comment on the things we have discussed (big panic now with the end of season).



I think that the lack of change in understanding is partly due to not establishing what we agree on. We mostly spend time on what we disagree on. That is always a good way to start wars...



Let me try a few things:



- we agree that chassis flex is a bad thing



Ca we also agree on the following?



Weight transferr is tue to:



- side G forces



- center of gravity



If this is true then the preassure of the outside relative to the inside will be dictated by those 2 parameters. We then get into diagonal distribution of preassure.



Spring rates (and to some extent shock absorber settings in a dynamic sequence), sway bars stiffness and chassis flex will determine the distribution of inside preassure between front and rear inside tyres.



The distribution of preassure on the inside tyres will have diagonal effects (a sway bar lifting the inner front to reduce heel will also reduce the preassure on the outer rear).



If we can agree on some basics then perhaps an exchange of ideas might be more fruitful. Suggestion: agree on fundamentals one by one to see where our thinking starts to diverge or if we actually starts to understand what the other person says and which actions create which resuts.



//TL

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[flash]

no wars - no worries



i would really love to continue this one, and so to be fair to it, i have to postpone for a week or so - i am swamped right now with work, and can't spend much time on the site at all, and what i do spend really needs to be focused on maintenance stuff, and not conversations



but if you can wait that long, we can take this up again at that time

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[Torbjorn968]

[quote name='flash' post='61749' date='Oct 16 2008, 02:53 PM']no wars - no worries[/quote]



Peace, love and understanding <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" /> . Talk soon!

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[Rich Sandor]

I will add fuel to the fire:



Different tires are recommended by the manufacturer to run a "minimum" or "optimal" camber. Outside these parameters, you might not be getting the optimal contact patch.



In addition, driving style will change your camber requirements. A lot of camber does give you a lot of grip in the corners, but does also contribute to snap oversteer once traction has been exceeded.



You cannot drive a pro race car driver's setup unless you are confortable driving the car the same way they drive it.



Finally, it's important to know how all this stuff works, but the key is to make changes based on actual tire wear & tire temps, not what someone else tells you they think you should set it to. Look at how your tires wear, and then adjust the alignment and tire pressures based on that and how the car handles in certain corners where lots of time is to be gained.

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[Torbjorn968]

[quote name='Rich Sandor' post='62227' date='Oct 26 2008, 11:46 PM']I will add fuel to the fire:



Different tires are recommended by the manufacturer to run a "minimum" or "optimal" camber. Outside these parameters, you might not be getting the optimal contact patch.



In addition, driving style will change your camber requirements. A lot of camber does give you a lot of grip in the corners, but does also contribute to snap oversteer once traction has been exceeded.



You cannot drive a pro race car driver's setup unless you are confortable driving the car the same way they drive it.



Finally, it's important to know how all this stuff works, but the key is to make changes based on actual tire wear & tire temps, not what someone else tells you they think you should set it to. Look at how your tires wear, and then adjust the alignment and tire pressures based on that and how the car handles in certain corners where lots of time is to be gained.[/quote]



Hello Rich!



Do you have a idea behind the mechanics that would affect optimal camber for a given tire? Grip would be one since more grip = more weight transfer and more heel of the car. Perhaps also the stiffness of the tire would be a part of this as well? I would think that R tires (semi slicks) would be rather simmilar in these parameters and differences in optimal camber therefore small enough for it to be interresting to listen to guys with different tire makes.



Also interrested in how more camber would contribute to snap oversteer. Would that be due to giving a lot of grip when the car is pressed hard but as the car looses grip and therefore weight transfer/heel angle the camber is less optimal resulting in less grip? And simmilarly: would the pro-driver eaking out the very last bit of the cornering ability achieve higher avarage G forces and therefore have more weight transfer/heel?



The cars as I have as my main referenses are the PCS race cars (Porsche club of Sweden) running simmilar chassis setups to my car, the same tire maker (Michelin) and lap times that are where I want to be in a couple of years time: quicker but not a huge difference. I thinkt the cars with lap records are interresting references especially since many of them burn through tires quicker than I do and can motivate their findings with tyre wear and temperature meters when I discuss with them. These drivers are more consistent than I am with the possibility to test angles in a better way. I want to find a good setup and then learn to drive well with that, if I start to play around too much then I do not know if it is my driving or the angles that are the problem.



Personally I have been rotating tires to keep wear down which means that my references in this respect is not good. And I have not had a temp meter. Will for next season. I have however had a decent amount of miles at my current rears which is interresting in terms of this thread since the guys over here (and I) seem to run much more camber at the rear than common among the US racers on this forum. In my case 3 degrees with people going all the way to four degrees at the rear. I will take some pictures to illustrate what I state in a post above: the left tire - the one spending most of its time on the outside on a clockwise track - is evenly worn. The "inside" tire - the right one has more wear on the inside. I would think that the rears have 3000 miles of them with perhaps 500 of these on the track. This means that the inside of the tire will be worn more during regular driving and also during breaking and straights on the track (toe in).



The quesion is what to optimize for. Is it cornering grip alone or is it also breaking grip and accelleration out of corners where the car is flatter? I would not think breaking grip at the rear to be very important given that - especially with same size fronts - the front end is doing most of the work. Neither would I think grip on accelleration is very important given that there are only a few second gear corners where power oversteer seems to be an issue.



If this is true lets get back to my rears: the left rear is evenly worn which means that it wears more on the outside during cornering since it spends most of its time getting worn on the inside (straights, braking, street). If one would optmize for cornering alone then this would be an argument for even more camber even if the right is significantly more worn on the inside. On the tracks where I do most of my driving the power oversteer situations are in right handers which means that it is the right tire that will do the slipping which the LSD does not take care of.



It would be interresting if there would be a racer on the forum who had tried large rear angles and gone back and to know for what reason.



Best



TL

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[flash]

rich - sage advice - measure, measure, measure



torbjorn - guys are finding they can reduce camber once they start stiffening up the underside of the car, especially up front, where it is very soft - the lower frame area moves about 1/4" under chard cornering, which equates to about 10 degrees of camber change - getting rid of that gets rid of a lot of the need for negative camber



not sure what you have in there, but in the rear, getting rid of the rubber bushings at the control arms does a lot too



there is a lot to be said for maintaining a certain amount of flex and load transfer in making a car easy to drive, and easier to balance - that is one of the things that the 968 has going for it - however, there is a limit to how much of that is good, and when it starts to create problems



yes, tire side wall will play into the equation - it's been too long, with too many tire changes, for me to comment on the specifics of which one does what on that one though, and i would defer to others with more recent tire experience - i will get to do more measuring and see what's going on with the tires i choose for the track car



back on the subject of weight transfer, weight transfer is due to many things, tires side wall, entry speed, transfer and exit speed, as well as how much energy is transferred across the chassis are just some of them - this is where i think a lot is going on with your car



but, the first thing to do is MEASURE - getting advice from others is great - comparing setups is all fine too - i also realize that is seems like the easy way to get there - but you will never get your car set up for your particular driving until you stop trying to adjust things and get quick fixes, and figure out what the car is doing to start with



as rich said, style is a key factor, as is how fast you are driving - somebody that is aggressive, and often very fast, will need an entirely different setup than somebody who is more conservative and just wears you down by driving smoothly and doesn't make mistakes



i would look at how far off other 968s you are, where, and by how much point to point - are they regularly entering corners faster? why? are they exiting faster? why? rights or lefts? big ones or shicanes? get somebody to time you lap by lap, comparing to other drivers who are pretty consistent - that will tell you a lot about you and your car



as to the car, tire temps are critical - you can't do much without that - heck, you could have a toe issue (front or rear) that is screwing things up, and all the camber adjustment in the world won't help - tire temps will tell you how much weight transfer you need to remove from what tire because it's too hot, and add to what tire because it isn't getting hot enough - chalk only gets you so far - same with pressure readings - those are important readings too though



contrary to popular belief, making a car go fast is done on the INSIDE tire, not the outside - getting that tire to stay on the ground, and increasing net contact patch area, is the key to speed - the outside tire will find the ground all by itself - no, you don't want to be riding on the corner, but preventing that from happening from the source is far more effective than trying to fix it in camber setting



measure measure measure - THEN make adjustments, attempting to cure the source of a problem, and not just the symptom

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[Eric_K]

I have lost track of what the point of this thread is. An equal front/rear setup should work fine if the car is tuned for it. I ran 235's all around for USTCC since that was the spec tire. The handling was fine after a few adjustments. Comparing camber settings seems to not have a lot of value to me. The same car/track will require different optimal camber settings based on the tire being used. The tire will make a huge difference in what camber is needed and I'm not seeing the tire talked brand/type talked about when comparing camber numbers. The only answer is testing. What gives you the fastest lap times while maintaining some driveability and longevity.

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[Torbjorn968]

The tire used in Swedish Porsche races are Michelin Cups.



Tyre pyrometer next year.



Perhaps Flash and I should start a new thread...



But given that this thread has already drifted away perhaps Eric can comment on the following: to me the preassure on the inside wheels is dependent on side G forces and the center of gravity. What can be done is to distribute the preassure differently fore and with sway bars? Sway bars lifts the inner wheels reducing heel angles and thereby create some more preassure on the inside wheels seen as a pair, perhaps this is what Flash has been trying to tell me?



Flash, regarding the under the car brace what is its intended effect? to separate or to hold together? The brace is paralell to the ground so it should not help much in romboid distorsion of the body but maybee prevent camber on the outer wheel from being reduced by flex. If so it would do this by pushing against the inner wheel increasing the camber on the inside? I would think the beneficial effect being a more consistent working camber of the outer wheel? Thus reducing the need for excessive camber on the outside since there would be less flex?



Sleep tight! (1:55 in Sweden).



T

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[flash]

actually, the sway bar pushes the outer wheel down - it does not really lift the inner wheel - it's actually a spring in how it functions - more sway bar means more weight transferred back to the inner wheel as the sway bar now pushes harder to lift the outside corner



the under brace holds the castor blocks still relative to each other - the stock setup moves a LOT - it's only sheet metal - there is no connection from one side of the suspension to the other - this allows the geometry to change dramatically - this is ok for a street car, and actually makes for a "safer" platform for the average driver, but no so much for a track car

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