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Tire and alignment discussion


Cueball1

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On 1/31/2024 at 9:13 AM, Croc said:

 

Set up for camber and toe-in spec is where I would expect them.  It is toe in and not toe out?  You don't mention caster or rake.  Some positive rake of 10-15mm is always good.  I expect you would have caster in the 5-6 degree range. 

 

Even so, I am not seeing anything there that looks to be driving substantial tramlining.

 

The tire size on the front is a flashing red light to me.  Given the R888 sidewalls are very much stronger (less flexible) than the sidewalls on the equivalent Avons (which are a lighter weight construction of carcass), I think you have created the convex profile that is likely causing your tramlining experience.  The R888s were designed for a much heavier car than a seven.  Much heavier than even the Avons were intended.  So they flex less.  The common way people got around that was to lower the tire pressures to absurdly low levels (think 14psi).  So while I run 18psi cold on  Avon ZZR/ZZS (which translates to 24-25psi when at ideal target hot temps), the R888 would need to go lower cold psi in order to make the sidewalls flex more and generate more heat into the tire.

 

These cars are sensitive to tire profile squareness and this is a function of adapting a modern saloon car tire onto a car less than half the weight of the regular saloon car.  We normally adjust for that by lowering the tire pressures.

 

If I were you I would run an experiment.   Drop tire pressures all round to 15psi and (carefully) see what that does to your tramlining.  Try again (carefully) at 12-13psi and compare results.  If my theory is right then you should see less tramlining as the car weight is flexing the sidewalls to get to a better profile cross section.  The concern I have at a 12 psi you can induce enough cornering force to rip the bead off the rim.  But as you are testing tramlining (i.e. a straight ahead feel) the risk of that happening should be minimal as you should not be cornering hard for this experiment.  This should give some feedback on whether a wheel width size change is needed or not. 

 

If it does improve it then that gives us the insight that tire sidewall stiffness is the issue.  The its a matter of choosing a Avon for a more flexible sidewall or if you want to stay on the existing R888 front tire sizing then maybe a square set up with 8 inch wide wheels all round?    I definitely would not be going narrower wheels with your horsepower. 

 

 

 

 

Time for an update.  A bit of rain and a dry Saturday in the 50s made for a nice early February tire test.  Changing nothing but pressures, I dropped my Toyo R888s from 32/34 to 18/24 cold for the first run.  The improvement was immediately apparent and significant.  Tramlining was less frequent and less sharp.  Just the dulling of the effect was enough to make the car easier and more relaxing to drive; though the ability to set a line was only slightly improved.  After getting some heat in the tires, at which point the fronts had risen to 20.5, I lowered them to 15 for the second run.  Another step improvement with more of the same.  Steering still required vigilance and (less) frequent corrections.  For the third run I dropped the front to 12.  This was the dramatic step.  Tramlining as a persistent effect was gone.  There was still the occasional pull which could be attributed to the toe setting.  But for the first time, the front of the car would take a set and follow a line.  Finally you could point it rather than guide it with constant corrections.  The car was finally relaxing to drive.

 

One of the charms of a Seven is how they pull you out of comfort zones.  Let this experience be one of mine.  Despite experience running race tires at low pressures, I saw this as a street car and was stuck in tuning it as such.  Lesson learned.

 

So what is next?  @Croc asked about goals.  I want a car with predictable enough handling that I am confident inviting others to drive it.  I also want to run the same compound/construction on both axles.  The car seems to want the 235s in the rear and they fit well.  Given the difficulty putting power down, I am hesitant to go narrower.  I might be able to fit a 245.  In the front, a 205 has to be tried.  It will answer so many questions and is one logical next step in the tire test program (another being to try the 225s on a 7.5" wheel - some research on options still needed there).  Looking at Croc's tire catalog, the obvious option based on size availability is the Toyo R888R or Avon (providing there is availability).  A LL rated tire, even in a harder compound, would be a fantastic experiment.  But my research on that path is not finding good data or recommendations.  LL rated tires seem to be very uncommon and not made in 225 and wider section tires.

 

I still want to check rake and I might try the car again at 10 and with a hair more toe-in.  But now we know the real goal is to get rid of the sidewall stiffness and reduce the tire's spring rate.

 

Thanks to the group for the questions and encouragement; including someone who sent me helpful food for thought on checking  whether scrub radius was playing a role (the answer is maybe as an aggravating factor, but not a primary cause).

Edited by Marek
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8 hours ago, Marek said:

I also want to run the same compound/construction on both axles.  The car seems to want the 235s in the rear and they fit well.  Given the difficulty putting power down, I am hesitant to go narrower.  I might be able to fit a 245.  In the front, a 205 has to be tried.  It will answer so many questions and is one logical next step in the tire test program (another being to try the 225s on a 7.5" wheel - some research on options still needed there). 

 

I'm really happy that experiment worked.  It really helps you to decide on next steps.  

 

Given tramlining is primarily a front end driven problem, I am not convinced you need to prioritize the rear end first.  I would look to finding the better front end tire solution then work backwards to what the options are on the rear.  Fortunately this is the cheapest way too.

 

It makes no sense to me to reduce the rear width with your HP and you seem to agree as you think it works well.

 

Memory escapes me but I think you said you had 7 inch wide on the front and 8 inch wide on the rear?  Shifting from 7" to 7.5" is a waste - too little a change to make it worthwhile.  

 

This is the point where a solution starts to cost money.  Trying to do the minimal for cost plus leverage what you have for R888s now, and keeping in mind your statement that you want same rubber front and rear, then I am thinking your only options are:

 

1) Move to a 205 in R888 on the existing front.  Leave rear untouched. 

 

2) Consider an 8" square set up all round so two new front wheels.   This preserves the current rear tire solution which works well and probably adds enough to the 225 on the front to stretch the R888 for a better profile cross section. 

 

Both of these preserve the current gearing which is ideal. 

 

If cost was no object then you could change all wheels and all new rubber but I recognize that has potential divorce proceedings or a new kitchen renovation attached to it which you would like to avoid.  So I leaned to the cheapest options first. 

 

Both of these are still working on the presumption you will be running street temps at 15-17psi cold. 

 

 

8 hours ago, Marek said:

One of the charms of a Seven is how they pull you out of comfort zones.  Let this experience be one of mine.  Despite experience running race tires at low pressures, I saw this as a street car and was stuck in tuning it as such. 

 

Once a thoroughbred, always a thoroughbred.  Its why I like se7ens.  You can tinker with it and make it your own to suit you. 

 

 

 

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15 hours ago, Croc said:

Memory escapes me but I think you said you had 7 inch wide on the front and 8 inch wide on the rear?  Shifting from 7" to 7.5" is a waste - too little a change to make it worthwhile.  

 

Correct 7 on the front, 8 on the rear.  For matching tire and wheel widths, I have been working from this chart: https://www.tiresandco.ca/tire-equivalence-advice.html

 

15 hours ago, Croc said:

I am thinking your only options are:

 

1) Move to a 205 in R888 on the existing front.  Leave rear untouched. 

 

The logical next step.  And this doesn't have to be once and done.  If a 205 turns out to have shortcomings, I'll go hunting a set of 8" for the fronts.  After all, if I am satisfied with the rubber on the car that leaves one less thing to tinker with.

 

15 hours ago, Croc said:

If cost was no object then you could change all wheels and all new rubber but I recognize that has potential divorce proceedings or a new kitchen renovation attached to it which you would like to avoid.  So I leaned to the cheapest options first. 

 

Here is one of the few areas where this forum is tough for a Rotus (maybe non-Caterham?) - parts interchange.  With the 80s Toyota running gear my bolt spacing is 4x114.5 (4.5"), meaning all the wonderful wheels discussed and for sale are of no use.  Searching for wheels on AE86 and other JDM forums is an entirely different experience.  Detailed specifics and concern for lightness are not regular themes.

 

15 hours ago, Croc said:

Both of these are still working on the presumption you will be running street temps at 15-17psi cold. 

 

The answer is probably yes.  Minimum pressures deserve more research and I will do some testing with alignment marks on the tire and rim to detect any sign of movement of the tire on the rim.  Even with a narrower tire/squarer sidewall, I will want to experiment with low pressures.

 

I am waiting for someone to leap in here and say, HEY, WAIT, XYZ tire has a soft compound on a soft sidewall...

 

15 hours ago, Croc said:

Once a thoroughbred, always a thoroughbred.  Its why I like se7ens.  You can tinker with it and make it your own to suit you. 

 

This deserves to be used as a tag line somewhere!

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5 hours ago, Marek said:

 

For matching tire and wheel widths, I have been working from this chart: https://www.tiresandco.ca/tire-equivalence-advice.html

 

 

 

Like many sites, it is not entirely accurate. To use tire tread width as the only reference for determining acceptable rim width is misleading. Tire mfgs list specifications for each offering and they are not always the same rec for the same tire size. I find a 1/2 inch wider than the spec'd nominal rim width to be a good compromise for fitting, bead retention (low tire pressure works against this), and handling.

 

For a manual steer, street driven seven that may encounter pot holes, I suggest a 6 inch rim for less unsprung weight with a higher cold pressure and more offset/ET for the scrub and still a 15 for upper ball joint clearance.

 

 

Edited by MV8
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6 hours ago, Marek said:

With the 80s Toyota running gear my bolt spacing is 4x114.5 (4.5"), meaning all the wonderful wheels discussed and for sale are of no use.  Searching for wheels on AE86 and other JDM forums is an entirely different experience.  Detailed specifics and concern for lightness are not regular themes.


I'd forgotten that was the Rotus hub mount of choice.  Still at least there are plenty of JDM options that will work.

 

Isn't 4.5" 4x114.3?

 

This is a website that I use to find wheels from time to time on other moneypits.  Its good for searching a broad range of options.

https://www.fitmentindustries.com/store/wheels?sort=instock&dia=15&width=8&bolt=4x4.5%2C4x114.3&saleToggle=0&qdToggle=0&suspension=Stock&modification=No Modification&rubbing=No rubbing or scrubbing

 

Didn't know your offset but Enkei and G-Line have some wheels that give an old school "Minilite" vibe.  Plenty of more modern looks but they would not suit your classic British style look.

 

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Nothing has struck me yet on the new wheel front.  I did run across a local set of 8.5x15 wheels matching the current style.  That would let me run 8.5 in the rear and move my 8" to the front.

 

The left field option is to riff off the 80s Japanese connection of the car and pick up a set of Hoshino, Advan, or similar period wheels.  The are unlikely to be light, but the look would be something!

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  • 4 weeks later...

I just found that my interruption of the political food fight was turned into it's own thread.  Yay!

 

I found information in other tire discussions about a new softer compound for the avons. There was an astounding video posted by a user of an HSR historics run in his 420r.  Nearly all passes were under braking.  With the light weight car he simply braked long after the opposing cars.  Such fun.

 

Anyway, he was a fan of the new soft compound.  033 if i remember right?  With zzr tires and a moderate driver not looking to kill himself every time out should these last a full season or more if I'm doing 5-8 track days a year?

 

Still debating the 13" vs 15" only because of cost.  I have a bunch of light 15" wheels from my Miata that will fit my Birkin.  13" is the right choice but 15" is already on hand.  Makes spending a few hundred more on  avons vs other tires I'm looking at more palatable.  

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