13 replies to this topic

[bmarshall1]

I was talking to our local builder and asking if he could add the brace from the door bar to the rocker panel.  He stated he doesn't do them. I questioned why and he stated it's possible to get the car too stiff-  I asked him to expand further on that.

 

He stated one time he bought one of his old cars back and after prepping it he could not get it to handle.  After repeated attempts he realized the only difference was the additional cage bars giving additional stiffness.  He said he cut the bars out and viola', instantly better.  Sebring is our home track if that would make any difference.  

 

My friends car has the 'angle bar' from passenger footwell to rear section and the dash bar (under dash connecting side to side).  He recommended to cut those out.  Has anyone else experienced this? (mine has neither so to me it's a moot point)

I can conceive of the added flex of the body being beneficial, but always assumed stiffer = better.

 

Secondly, another racer I know wanted to put poly bushings on his Miata and I recommended against it as old bushings are stiff anyway.  I also recall reading somewhere (maybe here) that the rubber bushings give certain movement to the suspension for added stability (perhaps rear toe under cornering or braking, something like that).  In other words, does the additional compliance of rubber bushing help in any way or would the stiffness of poly be best.

 

I would also imagine in a small hit to the wheel the rubber would give maybe saving a bent control arm, plus the additional compliance of rubber would save some fatigue cracking after a while.

 

Lastly, once tightened, the rubber bushings act as torsion springs, how would eliminating this affect handling. Thoughts?

 

Has anyone tested both and what was the result I want to be right but have no way to back my claim.

 

This is more of a mental exercise than anything and trying to satisfy both my curiosity and theories.

 

 

 

[bmarshall1]

Come on, SOMEONE has to have some input on this, anyone? Satisfy my curiosity, unless it's a secret...

[Alberto]

I'm just another schmuck on the internet but I'm not buying the stiffness thing at all.  I've raced against plenty of SMs with the bars you seem to refer to and they handle just as well during the race.  Go somewhere else that focuses on Spec Miatas for that work.

 

Poly bushings are not allowed in SM but there is more compliance in the stock rubber bushings and I speculate that it will help in impacts.

[Michael Novak]

Cage--You need to talk to others but some parts might not be legal for the class And I would argue cage design is some of the reason why you pay race shops like East Street to cage/ or fully build a car for you. 

 

Bushings -- you have to use the Mazda version. It can be part of the tuning of the chassis but I believe most setup their cars in the neutral state--- car on ground sitting at correct level then tightening all of the alignment bushings. 

[TylerQuance]

Novak is spot-on for the bushings installation question. Using a rigid offset upper control arm bushing is one opportunity for us in SM to get rid of 4 rubber torsion springs. Allows you to run the car closer to level front & rear.

I can comment on the poly bushing question since it sounds like your friend is not using his car for SM. I agree with you, I would also advise against. Polyurethane is not a suitable material for sliding suspension bushings for a couple reasons:
1. Polyurethane has a slip-stick tendency where it wants to hold on to the steel sleeve until it starts moving, then it slides free. This causes unpredictable handling. You can grease them to remedy this, but polyurethane unfortunately is excellent at pushing the grease out. =mess.
2. Polyurethane is not strong enough a material to withstand high loads without breaking or deforming. E.g. Lower control arm bushings on a race car

Lots of poly out there because it's cheap to manufacture, but it's not good enough for race/track use.

[bmarshall1]

Hey Guys, thanks for the input.  We both have cars and are not in the position (or need) for a new build at this time so no changes (on mine) will be made, maybe his?  All I know is what I read (Car&Driver: The New BMW M3 is 47% stiffer and it's GREAT!), and what I'm told (e.g. The car was too stiff).

 

When/if the time comes, I would have to pay a builder for the cage (safety/knowledge/experience) but maybe could do most of the rest myself, of course I'd be missing the hundreds of little things that makes their cars winners, but I am the limiting factor anyway.

 

I'm pretty familiar with suspension set up- tighten the bolts on the ground (loaded), I was just curious how this added to the spring rate and how poly bushings eliminate this 'tortional spring'.  Some say the spring rates are too much for the car and eliminating the rubber 'spring' by installing poly could be a step in the right direction.  We are moving to STL anyway, I'm tired of getting punted off the track, last race cost me about $1,000 in repairs (Sebring Turkey Trot).

 

I am using EBJoints on the front, but would consider offset bushings if the benefit is there, at my skill level it is likely not.

 

A lot of the original post is theoretical and trying to satisfy curiosity (poly vs rubber and can a car be too stiff).  I'm not good enough to tell but I was hoping some that has done both could share experiences.

 

Stick/slip, in the motorcycle world we call that stiction (front forks are a prime example).

[Dave D.]

When starting out in racing or track day activities, in my opinion, there is a required reading list. As in school, reading the classics opens your mind up. My go to was/is the Carrol Smith collection with Engineer to Win, Prepare to Win, Tune to win,Drive to Win, and the valuable "Nuts, Bolts and Fasteners". You cant go wrong with any of them. What was written in the late 70s /80s holds up today. I've read/re-read mine so may times that pages are falling out. I think they are out of print, but worth it to track down a few of these. Instead of making wild a** guesses and going in the wrong direction wasting time and money, learn proper methodology and techniques.

[Alberto]

 

I was just curious how this added to the spring rate and how poly bushings eliminate this 'tortional spring'.  Some say the spring rates are too much for the car and eliminating the rubber 'spring' by installing poly could be a step in the right direction.  We are moving to STL anyway, I'm tired of getting punted off the track, last race cost me about $1,000 in repairs (Sebring Turkey Trot).

 

I am using EBJoints on the front, but would consider offset bushings if the benefit is there, at my skill level it is likely not.

 

A lot of the original post is theoretical and trying to satisfy curiosity (poly vs rubber and can a car be too stiff).  I'm not good enough to tell but I was hoping some that has done both could share experiences.

 

Stick/slip, in the motorcycle world we call that stiction (front forks are a prime example).

 

 

I've been curious about the bushing material impact upon suspension specs and feel as well.

 

Poly is stiffer than stock rubber but also suffers from similar stiction issues.  So there is less bushing flex and increased feel.  No idea how that decrease in flex / increase in precision impacts the spring rates you might want to run or shock valving.  Might not have a big impact.  They will likely wear out quicker than stock rubber though and might require some regreasing of the internal sleeve.

 

Next step is spherical bearings - no stiction issues with those.  Zero flex in the bushings.  Increased feel.  Increased vibration.  

Probably benefits from increased spring rate since there is no stiction / torsional spring action from the bushings.

Probably benefits from more precise shock valving.

Might add some additional stress to the control arms since there is no rubber or poly to cushion them from road shocks like curbs.  So you should probably check control arms and subframes for cracking more often.

 

Bummer to hear about the body work.  Maybe run a few STL races before you decide to change anything.

[Steve Scheifler]

Agree that for handling a stiffer chassis is almost always better. BUT, there is a caveat with SM that doesn’t apply to “real” race cars, or street cars when the manufacturer is designing the entire chassis. All the cornering and bump forces have to go somewhere, and less absorbed by the central part of the chassis means more transmitted to “weaker” areas. Since we can’t tie the cage into the front & rear subframes which carry all the critical suspension components there’s a limit to how much real benefit the cage offers, and the weld-popping loads are focused on what pass for frame rails particularly up front. We aren’t allowed to seam weld so it could be argued that a stiffer cage accelerates increased in flex and damage in areas that we can’t reinforce.

I’m inclined to reject the claims about a noticeable improvement in handling by removing those bars, but Sebring is a special case, an outlier in terms of being bumpy, so I won’t bet my house on it. Another possibility is that the large bar was under tension and literally held the chassis out of square but I doubt it could be noticeable.

[bmarshall1]

I've been curious about the bushing material impact upon suspension specs and feel as well.

 

Poly is stiffer than stock rubber but also suffers from similar stiction issues.  So there is less bushing flex and increased feel.  No idea how that decrease in flex / increase in precision impacts the spring rates you might want to run or shock valving.  Might not have a big impact.  They will likely wear out quicker than stock rubber though and might require some regreasing of the internal sleeve.

 

Next step is spherical bearings - no stiction issues with those.  Zero flex in the bushings.  Increased feel.  Increased vibration.  

Probably benefits from increased spring rate since there is no stiction / torsional spring action from the bushings.

Probably benefits from more precise shock valving.

Might add some additional stress to the control arms since there is no rubber or poly to cushion them from road shocks like curbs.  So you should probably check control arms and subframes for cracking more often.

 

Bummer to hear about the body work.  Maybe run a few STL races before you decide to change anything.

Yea, I'm still a little salty about the wreck, but time to move on. I find these subjects interesting, mixing theory, hypothesis, and real world results, all of your points are in the same line of thought as mine.  I was at a race shop the other day and he had metal bushings on his lowers and Delrin on his uppers.  I imagine the NVH levels were pretty high but likely very tolerable to 20 minutes at a time.

 

Instead of double-dipping on SM and ITS, maybe I can do STL and ITS?  Not sure if they run at the same times.

[bmarshall1]

Agree that for handling a stiffer chassis is almost always better. BUT, there is a caveat with SM that doesn’t apply to “real” race cars, or street cars when the manufacturer is designing the entire chassis. All the cornering and bump forces have to go somewhere, and less absorbed by the central part of the chassis means more transmitted to “weaker” areas. Since we can’t tie the cage into the front & rear subframes which carry all the critical suspension components there’s a limit to how much real benefit the cage offers, and the weld-popping loads are focused on what pass for frame rails particularly up front. We aren’t allowed to seam weld so it could be argued that a stiffer cage accelerates increased in flex and damage in areas that we can’t reinforce.

I’m inclined to reject the claims about a noticeable improvement in handling by removing those bars, but Sebring is a special case, an outlier in terms of being bumpy, so I won’t bet my house on it. Another possibility is that the large bar was under tension and literally held the chassis out of square but I doubt it could be noticeable.

All good and points and I agree.  One of my thoughts was that on a bumpy track like Sebring, eliminating chassis flex might bring a corner to the bump stops more often, or start un-weighting a corner compromising overall traction.  Perhaps a car like the Miata on a track like Sebring needs a certain amount of flex to keep all 4 wheels planted firmly.  It likely won't ever matter to me as I doubt I could tell the different btw an overly stiff chassis and 'standard' stiffness.

[Jim Drago]

I dont put petty bars in my cage.. Being you are in Florida, I have a feeling I know where the cut the petty bart out logic stems from.. As I said, I dont use them , but doubt very seriously the car handles any better( or worse) cutting that bar out

Jim

[bmarshall1]

I dont put petty bars in my cage.. Being you are in Florida, I have a feeling I know where the cut the petty bart out logic stems from.. As I said, I dont use them , but doubt very seriously the car handles any better( or worse) cutting that bar out

Jim

I'm not good enough to tell either way, but I found the topic interesting.  Is there a reason you don't add 'petty bars'?

[Bench Racer]

  Is there a reason you don't add 'petty bars'?

I'm betting if Jim answers I know why he doesn't put one in his personal car.