Rubber Change

[ffrgtm]

So I soon will have an rf90 (before xmas even!) and need to order some tires however I have a problem. Because the ld200 is so low speed gearing challenged it's going to take a combination of cwp, gears, and reduced tire size to get the car where I want it (47mph 1st is the best I can do).

Now I assume the car was designed by van diemen to run a 20.5x6-13 (21" dia) front and a 22.5x7.2-13 (22.6" dia) rear. I need to drop that 22.6 down to the 21.0 size of the fsae intended 20.5x7-13.

I can see several problem associated with this: the lower profile rear tire definitely has a higher tire spring rate than the taller FF rear tire. I'm not entirely sure but I think this would raise the rear roll center a few inches. Possibly a change of rear tire scrub angle and slip angle.

Are these problems going to totally negate the benefit of lower gearing? Will I be able to fix these problems? I was going to have to calculate new spring rates anyways but I'm not ready to move mounting points.

Thanks for any suggestions

Ben

[Steve Demeter]

Best suggestion is to call Craig Taylor and see what special CWP or gears he might have available.

[Jim Garry]

Steve's right. Call Taylor. I have a friend who just obtained a later model VD and he was able to find a reasonbly good first gear without having to change his ring and pinion.

BTW, most autocrossers use the 7" front tire rather than the 6". Helps a bit with reducing understeer.


Jim

[ffrgtm]

I assume the 7" front tire is the fsae size? Do most cars in cmod use the spec 22.6dia ff rear tire?

[Jim Garry]

As far as I know, the entire grid at Nationals used:

Front 20.5 X 7.0-13 C2500 6.75" 20.8" 66.2"
Rear 22.5 X 7.2-13 C2000 7.25" 22.6" 71.0"


And of course, use the R25B compound.

Jim

[ffrgtm]

Thanks Steve and Jim, I know what I need to know now. I've also got an email going with Taylor, hopefully that pans out. I love apexspeed.. glad theres a community of intelligent like minded individuals.

Thanks,
Ben

Now to study for finals

[Jim Garry]

Thanks Steve and Jim, I know what I need to know now. I've also got an email going with Taylor, hopefully that pans out. I love apexspeed.. glad theres a community of intelligent like minded individuals.

Did you mean "intelligent-like minded"? Or "intelligent like-minded"?



Jim

[ffrgtm]

like-minded, intelligent individuals Made sense early in the morning...

So Craig explained to me that switching to the smaller dia rear tire would change the roll center a bit. The problem comes along when you try to return the ride height to the previous state which causes a large roll center raise, and a loss of rear end stability (glad i asked). He did mention that if I were to use a smaller front tire with a corresponding ride height decrease (.8") I could keep the geometry relatively similar.

However! There is good news. Taylor has an integral 1st gear that is lower than anything listed on the hewland site. In fact it actually will get the speed of 1st lower with just the cwp and 1st than it would have with the cwp and 1st I thought I had to use + the smaller dia tire.

So all is good, thankfully I don't have to mess with any critical geometry parameters.

From,
Ben

[teamfour]

Run the GTM.

[ffrgtm]

Oh I ran the gtm last year. It was just way too wide, and way too powerful. Plus you can't see jack out of that cabin.

Also, I have another question. By changing to the wider front tyre I was under the impression that I am not increasing front grip, just changing the shape of the contact patch (wider and shorter). This would increase lateral grip but decrease longitudinal grip correct? SO i am trading stopping ability for more front grip in the corner? I realize that these cars are very rear weight biased.. and it seems the tyre width ratio front to rear should reflect that.

Also, I will be running a hillclimb very soon. Will the wider front tyres cause me some problems there? (Intentially a vauge question)


ps: I was thinking a little about the fact that "every" cmod driver uses the wider front tyres. Could this simply be because the wider tyres operate at a smaller slip angle, therfore requires less steering angle in a car that already is not designed for high steering angles?

Thanks for any input,
Ben

[Dick R.]

Don't know about the steering angle idea.

I suspect that the road racers would run the wider tire also except aero drag is much more of an issue with them than with autocrossing. Even when the 7's had the same construction (as far as we knew) as the 6's autocrossers found that the 7's were better.

Bruce Foss of Hoosier used to explain that many of the FF tire construction changes over the years were to maintain grip while reducing aero drag by making the tires narrower.

FYI in regard to steering angle, I don't know how "tight" a turning circle your 90 VD will do. You need to do whatever you can to get the turning circle diameter down to around 40' (similar to a Mustang, Camaro, etc.) to work on many tight autocross situations. My 85 VD was at about 50' when I bought it due to rack limiters and general constraints of the various bits. Removing the limiters and doing a lot of intelligent filling/grinding completely solved the problem.

Also, one reason for the "wheel rates equal corner weights" springs is to improve transient response without having to try to do it with shocks. The car needs to respond NOW when you move the wheel, especially in slaloms but even most other corners. You don't have time for body roll

Dick
CM85
85VD FF1600

[ffrgtm]

Aha so I see. I suppose with that increased drag also comes increased lift as well. We'll I don't know I've confused myself into a corner. Time to go read some books on the subject I suppose.

Thanks for the bit Dick

[Jim Garry]

am i trading stopping ability for more front grip in the corner?

Don't worry about braking at autocrosses. Most courses will not require much of an effort from a FF. Besides, if you are late braking and then jumping on the pedal viciously and trying to get 101% from your brakes, your times are going to suffer a great deal.

Dick, as usual, provided good info and insight.


Jim

[ffrgtm]

I do really appreciate the insights I've gained form you guys, but I want to throw this out there one more time.

What effect does going to just a larger front tyre size have on the vehicle dynamics?

Aside from the obvious fact that wider tyres generally have a higher spring rate that I'll need to account for, the fact that I will be using this car at speeds higher than I originally thought, and the fact that I will be doing testing with my crappo spec tyres (which are the 6" fronts, not the 7" fronts I'll be using for actual competition) makes me feel like this is something that is important to understand.

[Dick R.]

What you will be doing is practicing and getting acquainted with the car when using the crappy spec tires. You cannot actually TEST until you have new/almost new 25's of the size you will run. For autocrossing a CMod the compound is the most important variable. Followed by tire size/version and surface temp and surface condition. Tire or spring rate, tire width, shock valving "fine tuning" comes latter. Believe me the difference between hot/warm fresh 25's and and anything else is night and day . . . not as drastic as wet/dry but still a major difference. When you get used to the car you will feel the balance change during a run as the tires warm and also change as the tires get more events on them.

Dick
CM 85

[ffrgtm]

Yes I definitely understand how big of a difference the compound will make. Just was wondering what things that I will be getting used to with the car I should remember will be changing once I put the new rubber on.

It just seems like I can get a straight answer from books until I start to involve transients, and at this point I'm hoping someone can just tell me what they noticed as far as a difference once they switched the widths for autocross who has specific experience in a formula ford. There are just so many variables to look at that it's hard for me to trust answers I get from people who are using street cars or even f500s... solovees ect.

Dick or Jim did you get a chance to note the differences once you switched over to the wider front rubber?

Also, I cannot find anywhere in the rules where it explicitly states you can change tire sizes, only compounds. I assume that its legal if you say everyone at nationals runs those sizes but it still makes me worry... And at that, you CAN NOT change rim widths correct? It seems like the tire width change would be no more legal than the rim width change but I haven't ever seen anyone move to a wider rim (which presumably would increase response, and lower the needed slip angles for more grip.. something good for autocross I think).

Maybe I am being too technical about this but with the big gap until I can get some testing (or making acquaintance with my van diemen) all I can do is bench race!


edit: Now I see that the fccs does not so much as even mention tyres, and the solo rules don't even say a thing about compounds as I had said. Whoops... so help me with this: If people run the smaller front tyre on FF for aero reasons, then why would they switch to a wide track suspension? I feel like there just has to be more to the story, and that the understeer problem is more of a suspension setting than a tyre issue, considering so much of the weight is rearward as it is. However it seems pretty hard to argue with people who have competed in this class for years.. considering I haven't even had my car out yet

[Dick R.]

Ben,

It is good that you are thinking about these things but, as you said, don't overthink them.

I have no idea how much Solo National level autocrossing you have done but keep in mind that MANY of the CM competitors over the years were National Champions in other classes and, of course, many repeated in CM. I'm an example of a National Champ in other classes (DS and ASP) who could do no better than consistent trophies in CM. Because so many of us had proven autocrossing and car set up skills/experience it was fairly easy to determine what worked and what didn't work. On tire size everyone quickly settled on the 7's for Hoosiers. Back when GY had a competitive tire, however, I believe the GY was narrower and at least as fast.

Regarding car setup, I've seen all sorts of setups (stiff, soft, old crock, fairly new) and driving styles end up with roughly the same performance . . . as long as the driver used what he had. Just "run what you brung" for awhile until your speeds/times against top competition are in the ball park. Use PAX to benchmark if there are no Mod cars handy. In fact, you could probably go to the events in the upper Midwest that the big bunch of DM/EM guys run and use them as benchmarks (you should beat them). Sign up for the Yahoo Groups DM/EM list to find out where they are running.

Unless you have experience driving a faster car at competitive Nationals speeds you are likely to find that "using" the car is the biggest challenge. Even coming from an ASP car it took numerous events against top competitors before I was used to the speed and back to driving "ahead" of the car rather than just reacting. I think it was halfway through my third Pro Solo with the car that I finally was comfortable with the speed.

Dick
CM 85

[ffrgtm]

Thanks Dick, that was definitely something that was cooking up at the back of my mind but helped a lot to hear it from you. I've known for a while that I should really just be testing the car before I make a lot of changes. I suppose I am in school for Mechanical Engineering and my current way of thinking is a tragic byproduct :-)
Before I get my car out I wanted to make sure it was setup in such a way that it would have as gradual a learning curve as I could manage to create. Also my car was previously owned by someone who's setup abilities I have little faith in, not to mention it was "set up" for road racing which is totally different. Overall I'm just worried about my spring weights and shock valving that I'm nearing a deadline for coming up with and wanted to make sure the wider front tyres weren't adding an extra variable to the equation that I'm not seeing yet.

Basically I was worried that there would be some forces involved that changed because the outer and inner edges of the tyre are now at different points. From everything I can remember all the kinematics I've ever seen ONLY involved the center point of the tyre, and so far just from looking through some basic text's that I have this is still true. I had thought I remembered seeing a certain variation of the 4-bar model that used the outer point of the tyre but I could be wrong.

Either way, one thing I've learned from fsae.. is even if you screw up a design parameter, a 400lb car is still going to get through the cones so quickly it's difficult to notice you screwed up. I suppose that fits in with what you're telling me as well Dick, thanks for helping quell my fears... just a little bit more

[Dick R.]

Ben,

Sounds like you have the right approach. One thing to keep in mind is that with bias ply tires that they really aren't "that" sensitive to the "optimum" setting . . . if there is such a thing with an autocross car that has such a huge range of corner radii. You will probably find that a front camber setting of 1.5 to 2.0 degrees negative and toe out of about 3/8 to 1/4 total will put you pretty much dead center in the ballpark. This assumes that ackermann is close to "standard" (i.e. the lines intersect at the middle of the rear axle line).

A really good low tech (i.e. no test equipment other than a tread depth gauge) of determining which portion of a bias ply tire is doing the work (on average) is to literally monitor tread wear and rubber buildup as the tires get some use. That said, front tire wear in CM is generally minimal so you will be looking at the inner and outer edges mostly. Note that it is common to have so much pickup rubber from all those street and R compound cars that you will end an event with more rubber than you started with . . . until you scrape it off. Managing the pickup rubber during and between runs can be a major handling variable.

Dick

[ffrgtm]

Yes! That's exactly the kind of info I was looking for. And when you say "managing the pickup rubber" I assume you mean for the fastest runs I should be scraping the tyres with an old hacksaw blade or the like between runs correct?

Thanks for that info.. I am worried about my ackerman being too low of a percentage, and haven't calculated it yet, I may still have to machine some new steering blocks.

Thanks!
-Ben

[Jim Garry]

So much good advice from Dick. But the best was to get the car generally set up and then go and run it. You will not find anything close to the best setup through garage work and research. You'll get something reasonable. And that will be enough. Don't sweat it. Get it reasonable and then go out and try it.

Unless you are a prodigy, you won't be able to drive near the limits of the car in the beginning. So for awhile you may feel the car handles great. Then as you begin to push yourself harder, the car may begin to show handling issues. That will be normal and that's when the fun begins.

As for which tires and what track ... everything is compromise and the direction you will go depends on what you are trying to do.

[ffrgtm]

So much good advice from Dick. But the best was to get the car generally set up and then go and run it. You will not find anything close to the best setup through garage work and research. You'll get something reasonable. And that will be enough. Don't sweat it. Get it reasonable and then go out and try it.

Unless you are a prodigy, you won't be able to drive near the limits of the car in the beginning. So for awhile you may feel the car handles great. Then as you begin to push yourself harder, the car may begin to show handling issues. That will be normal and that's when the fun begins.

As for which tires and what track ... everything is compromise and the direction you will go depends on what you are trying to do.

Nice thanks to you too Jim,
So far the only things I have done have been the obvoius autocross changes (installing a rad fan) and the safe (fixing things like a leaky fuel cell, leaking brake line, cracked upright ect.) things. I am definatly not Lewis Hamilton's secret child or anything, so I'll just stick to picking some good springs and valving the shocks for the springs and a fairly high ride frequency (as i've heard is autox specific). Other than that I'll let you guys know how it goes and I've been taking plenty of pictures along the way.. anxiously awaiting the apex speed build log feauture!

Thanks!

[ffrgtm]

Yes! That's exactly the kind of info I was looking for. And when you say "managing the pickup rubber" I assume you mean for the fastest runs I should be scraping the tyres with an old hacksaw blade or the like between runs correct?

Was I correct in my thinking here... or are we talking about something else such as spraying water on the tyres between runs?

[Jim Garry]

Rubber pick up is not as big a problem as it used to be. At one time, some of the stock category "race" tires left a gooey rubber down. People walking course would actually walk right out of their shoes sometimes. That hasn't happened in years. And at HPT there was no problem at all. Perhaps on the concrete, if it provide exceptional grip, we'll see some rubber pick up problems again.

If so, there is still some disagreement over how important it is to remove the rubber with one group saying it is critical and another group saying that on your next run you'll lose some rubber and pick up other rubber so what's the point.

I'm in the middle. If it's bad, I get rid of it as best I can. If it's just a middling amount, I don't bother.

I use this to scrape stuff off my tires. It's a steel curry comb for horses. Most of the grid at Nationals has one. I bought two (they're cheap) so my co-driver and I could work simultaneously to save time.

[Viperklr]

I follow Jim's practice as well. Unless there is some serious pickup on the tires I don't do anything between runs. I may clean them a bit between heats and I always clean them before the car goes back in the trailer. I prefer to get all the pickup rubber and stones off of the tires before they roll on my trailer floor and get tied down.

I have the same comb pictured in the previous post. Got it at Tractor Supply in the horse section.

[Dick R.]

Ditto on the rubber pickup comments even though I haven't been to Nats since 2001. I now have the pictured curry comb. My traditional scraper is a wide blade putty knife with the blade shorted about half way and the corners rounded to prevent puntures (tires or skin ). When possible it it desirable to get the big chunks/worms off before the tires cool completely. If they do cool put the car in the sun to warm them.

Dick