WHY do bigger alloys mean worse handling?

[Si_Crewe]

I said I'd post the link for suspension design when I got home to my own PC.
Here it is: http://www.rqriley.com/suspensn.htm

Well worth a read IMO.

[g1swx]

Hi all,

I am a new member and am waiting to take delivery of a 1990 Rev 1 GT Turbo T-Bar.

It is fitted with 17" Alloys (rubber size unknown at present)

Having read with interest all the comments/info on wheel/tyre combinations, I would appreciate any advice or comments on if the steering may be a bit heavy for my wife ??

She is only a midget at 5 ft.

Cheers

Chris

[Quigonjay]

Hi all,

I am a new member and am waiting to take delivery of a 1990 Rev 1 GT Turbo T-Bar.

It is fitted with 17" Alloys (rubber size unknown at present)

Having read with interest all the comments/info on wheel/tyre combinations, I would appreciate any advice or comments on if the steering may be a bit heavy for my wife ??

She is only a midget at 5 ft.

Cheers

Chris

depends if its got power steering really mate, it was an option on the rev1's so may/may not have it, she may struggle a little if not

[Big G]

Wow there is alot of info both correcty and not on here!! someone really needs to shorten it abit


"IF" you were to look into vehicle suspension design you would find there are maybe two dozen points of possible adjustment for a suspension setup.

HOWEVER the MR2 doesn't have all of these even the most well tuned suspension setup on a road going MR2 (with alot spent) will see only about 7 points of adjustment either side being

1. Rear camber
2. Rear tension rods
3. Rear toe arms
4. Rear Pillow mounts
5. Front tension rods (toe arms)
6. Front pillow mounts
7. and the stock steering rack obviously (negating the possibility of upgrade dual adjustable racks)

This is pretty much as much as you can expect to have, such things as the SAI and castor are not viable to adjust unless you have major monies so shouldn't really a concern tbh

All the above settings should you have the money to afford them all can easily (if you have the knowledge) be setup on a simple laser tracker


When discussing things such as change of camber in corners your wheel size is important kinda but you gotta think also about whether you have things such as strut bars, SAI is vastlychaged during cornering which allows the return to centre of the steering and also the rise you see in the vehicle as you turn to hard lock, strut bars change all that hugely so these new values if you wantt o get into it are ones which must be discussed too, not to mention the added chassis flex they induce to counter/cope the extra flex the body is under by holding the geometry still
Sttering and suspension is a huge subject and damn boring most the time



As for wheels and tires themselves, wheel size and respectively tyre size will play a huge part in handling increasing such issues as the scrub radius altering the steering efficiency and balance.

I read somewhere in this thread that wheels alter the gear ratio, this "they do not at all", the only bit that alters teh gear ratio is the final drive. These ratio's are set by the number of teeth on the driven an driver gears within the gearbox and not cahngeable by wheels size

Larger overall diameter wheels inc tires over that of stock will slow the power to the floor by maybe 5mph in every 80 or so (grows respectively to size increase)
the reason that people opt for smaller wheels for racing is that they will take less time to turn and are in many cases lighter

Tires, the thinner the profile ie 35's look good but as has been mentioned are pretty usless as a race tire, the wall skins are so thick that little compression is allowed hence equalling a bumpier ride, however larger depths aren't good either,a 50 wall in many cases will be to much giving way in the car to rebound from bumps in the road surface to much losing the footprint of the tire itself and obviously traction.

Again back to suspension, tires play a major part in the suspension and steering setup and if you want to setup correctly you need to take their compression states and footprint into concideration

[skinthespin]

read somewhere in this thread that wheels alter the gear ratio, this "they do not at all", the only bit that alters the gear ratio is the final drive. These ratio's are set by the number of teeth on the driven an driver gears within the gearbox and not cahngeable by wheels size

well, wrong really.....

everyone talks about power at flywheels, wheels and hubs, but in reality the only thing that matters is the tractive effort (measured in Newtons) at the driven wheels contact patch. This is the final force that moves the vehicle forward, obviously with the inefficiencies of rolling roads this figure is virtually impossible to measure accurately and repeatidly so we measure power/torque at the hubs or fly.

So changing the rolling radius of the driven wheels does affect the gearing as in reality this is the last 'gear' in the chain of events between the crank spinning and the power getting to the ground.

[skinthespin]

Larger overall diameter wheels inc tires over that of stock will slow the power to the floor by maybe 5mph in every 80 or so (grows respectively to size increase)

missed this bit.....

I dont think you really have a grasp of whats going on, if you increase the rolling radius of the driven wheels tractive effort will be reduced for a given road speed, tractive effort is acceleration/resistance (both rolling and aerodynamic).

It will however increase the roadspeed for a given engine speed, so in theory you will have a higher top speed assuming you can give the driven wheels enough tractive effort to overcome the resistances, so, if your car thumps into the rev limiter in top with ease the chances are by increasing the driven wheels diameter (which has exactly the same effect as increasing the final drive or gear ratio) the vehicle will have a higher top speed.

[Big G]

changing the rolling radius of the driven wheels does affect the gearing as in reality this is the last 'gear' in the chain of events between the crank spinning and the power getting to the ground.

Unfortunately you are very wrong matey, I can see the angle that you are coming from and why you're thinking it but its incorrect for the situation.

The "last gear" is actually called the final gear, I can give you a picture of one or a annotated transmission diagram should you require it.
You are right that the circumfernce of the wheel does donote speed of rotation to the ground but still it does not 'change' the actual gear ratio as all has been dictated prior to this point hence most manufactures use a hub dyno to find more acurate power of a vehicle as do many top performance tunners due to the ability of dialling out slight changes is rotations given by different size wheels.

I dont think you really have a grasp of whats going on, if you increase the rolling radius of the driven wheels tractive effort will be reduced for a given road speed, tractive effort is acceleration/resistance (both rolling and aerodynamic).

It will however increase the roadspeed for a given engine speed, so in theory you will have a higher top speed assuming you can give the driven wheels enough tractive effort to overcome the resistances, so, if your car thumps into the rev limiter in top with ease the chances are by increasing the driven wheels diameter (which has exactly the same effect as increasing the final drive or gear ratio) the vehicle will have a higher top speed.

I think you're trying to make newtons law of motion sound more difficult than it is matey

[skinthespin]

Unfortunately you are very wrong matey, I can see the angle that you are coming from and why you're thinking it but its incorrect for the situation.

The "last gear" is actually called the final gear, I can give you a picture of one or a annotated transmission diagram should you require it.

well, no, I put the word 'gear' in speach marks, to suggest even though it isnt an actual gear it does change the final effect the same as changing a drive gear would.

I don't need a diagram of a gearbox thank you as ive been involved in the design of gearboxes so have a rough idea how they work.

You are right that the circumfernce of the wheel does donote speed of rotation to the ground but still it does not 'change' the actual gear ratio as all has been dictated prior to this point hence most manufactures use a hub dyno to find more acurate power of a vehicle as do many top performance tunners due to the ability of dialling out slight changes is rotations given by different size wheels.

The actual ratio of a gear is only a stepping stone in the path from converting the chemical energy of a fuel into kinetic energy of the vehicle, the tractive effort is the most important figure in all of this as this is the motive force at the contact patch of the driven wheel, the radius of the driven wheels has just as much effect on this as the ratio of any gear and the final drive ratio.

Manufacturers (I work for one) measure power at the fly as the engine will be measured on a dyno, top tuners measure power at the fly as the engine will be on a dyno (I have associates at HKS europe for example so can varify this)

Other tuners use hub dynos as you dont have a tyre contact patch in the equation like you do on a rolling road, the tyre deforms and slip and can all sorts of odd things, so this variable is removed, although if you could measure this accurately it would be the most relevant figure.

I think you're trying to make newtons law of motion sound more difficult than it is matey

This comment seals it! I've not mentioned any of newtons laws of motion, all I said was the tractive effort which is measure at the contact patch is measure in Newtons as its a force, nothing more.

[luthor1]

Big_g,

castor on the front of Mk1 MR2's is very adjustable. Also on Revision1 Mk2 Mr2's it is adjustable also.

I think you need to raise your technical game a little, we are all very much aware of gear ratio's final drives and rolling circumferences, and to pick holes in semantics is not really necessary. We all know that fitting a bigger wheel doesn't increase or decrease the number of teeth on the gears or final drive etc so we can really quickly move on from that one!!!

You talk about "strut bars" and camber changes? I am not sure of your point here? Are you saying that unless you have a rigid body it is pointless concerning yourself with these points of suspension tuning? all chassis's flex, none are 100% rigid, at what point do you draw the line as to where you would worry about it?

[ed_arnold]

Hello! I've read the previous 9 pages and there is obviously a wealth of knowledge on here.

My question - I have a mk2 J-import with 195/50/15s on the front compared to 215?/50/15s on later revisions with the advent of PAS - how did this affect road holding and handling characteristics? And also did any chasis modifications occur with the T-Bar to further maintain rigidity or were strut braces front and rear the only changes. Is the suspension setup on an import '92 mk2 N/A any different to UK?

Thanks
Ed

[raptor95GTS]

front tyres are 195x55x15 for all rev2+, the rears being 225x50x15.

suspension wise I think all the UK models had the rebuildable struts whereas only the rev1 JDM models had this.

[baz8755]

Does anyone know what size tyres should be fitted as standard on a SW20 turbo revision 4 15" wheels.

The reason I asked is that my beloved rev 2 turbo was written off a few months ago. The car handled beautifully as was completely standard. I used to be able to drift it around bends and roundabouts and it always felt predictable.

However I now have a standard rev 4 turbo but the front end does not feel as well planted when I try and get the back end to break away. I do not know what size tyres my rev 2 had except that I was using F1s. However my rev 4 has front 205/50 and rear 225/50 15".

What can I do to get the front feeling as it should.

Also the wheels on this car are the anniversary diamond cut ones and are severly corrided, can these be refinished?

Cheers

Baz

[Leon.]

Does anyone know what size tyres should be fitted as standard on a SW20 turbo revision 4 15" wheels.

The reason I asked is that my beloved rev 2 turbo was written off a few months ago. The car handled beautifully as was completely standard. I used to be able to drift it around bends and roundabouts and it always felt predictable.

However I now have a standard rev 4 turbo but the front end does not feel as well planted when I try and get the back end to break away. I do not know what size tyres my rev 2 had except that I was using F1s. However my rev 4 has front 205/50 and rear 225/50 15".

What can I do to get the front feeling as it should.

Also the wheels on this car are the anniversary diamond cut ones and are severly corrided, can these be refinished?

Cheers

Baz

As said above rev 4 standard tyre sizes are:

Front 195/55/15
Rear 225/50/15

You are running 205/50/15s on the front which are supposed to reduce understeer. F1s are one of the better tyres on the mk2 so could be why you felt happier with them. Maybe you should just change your tyres, but note that certain tyre sizes in F1s are no longer available.

Yes you can get your rev 4 wheels refurbished. Costs £40-50 per wheel but make sure to go to a well recommended place as they're difficult to refurbish well!

[greglebon]

Phew......! That was a HEAVY read...!

So....to summarise: Bigger alloys DONT necessarily mean worse handling....its just that unless you alter other suspension geometry on your '2, you upset the Mr.T OEM setup.....

If you want better handling, get a better handling car.....

If you want your car to look cool, get HUGE alloys.......

[baz8755]

Hmmm.....

I still am having severe problems with understeer on my rev 4 SW20 Turbo.

As I have already said my rev 2 Turbo used oversteer beautifully and I spent nearly 10 happy years confidently sliding round corners.

My new rev 4 has 205 front and 225 rear tyres (people suggesting that 205 are bigger than standard), the fronts are michelin pilots (I am used to F1's but surely this is not going to make that much difference?).

I have read on wikipedia that later MR2's were designed to understeer as the oversteer of later models was considered too dangerous but my mechanic tells me the suspension is the same on all models.

The upshot is that I want my lovely oversteer back and at the moment the understeer scares the hell out of me. HELP!!!!!!!!!!!!!!!!

[Quigonjay]

have you had the geometry set up?
a bit of negative camber can help turn in/reduce understeer as can a front strut brace
its just the rev1's that had the oversteer trait with rev2+ having the built in 'understeer'

[jonb-]

have you had the geometry set up?
a bit of negative camber can help turn in/reduce understeer as can a front strut brace
its just the rev1's that had the oversteer trait with rev2+ having the built in 'understeer'

Good advice.

A decent 4 wheel can sort many problems out, failing that your fronts might be shagged.

These cars certainly understeer a lot, but it's nothing that can't be overcome with a dollop of right foot.

[skinthespin]

I have fiddled with my suspension a lot, and it was at a stage where it never ever understeered, in the rain, with bald front tyres, turn in was awesome, and thats with more weight removed from the front than the rear, all that was done on a bit of rear toe and camber on the front, ie running not very much at all!

This worked great on the road and pretty good on track, it did mean the car was a little unstable under braking and was prone to oversteer but thats how I liked it.

I have since backed off on the camber on the front (by adding more negative, the wheels were virtually upright before) and its prone to a little understeer in the wet but its still planted in the dry, its also a bit more stable and a bit more predictable, so its a good compromise.

Also dont forget when we talk about making handling 'better' its not a black and white thing, I like a planted front end a twitchy rear (ooer!) most people would find that horrible and undrivable, handling really is the most subjective matter after aesthetics. What you need to do is work out what you want your car to do, the chances are you can make it do it (most of the time anyway!).

[jonb-]

I have since backed off on the camber on the front (by adding more negative, the wheels were virtually upright before)

Not wanting to be picky but that's a contradictory statement. If i was to 'back off' my camber I'd imagine going closer to upright, not further negative.

Don't forget the day after you moved your camber the track was extremely greasy, my car pushed more than normal too.

[skinthespin]

That is being picky!

What I meant was I was running virtually no camber at all, which is quite extreme, so backing off from that extreme nature meant I was adding a bit of negative.

The track was slippy but the car did understeer more than it did before in those conditions, when it was dry later on though the car still didnt understeer and felt more stable under braking, and the dry set up was my priority anyway, so it was a success.

Before that track test I had only driven on the road, and you just dont lean on the car and tyres like you do on the track, so even though my upright wheel set up was good on the road, meaning no understeer at any speed it was a bit much on track where your pushing the car much harder.

Shouldnt you be outside fixing your car anyway so you can come to Donny tomorrow?!?!?!