Simple question for the tire experts,
I am trying to find out what driving characteristics to expect with radials on the back and bias-ply on the front of a 30's type roadster I am building.

 I know this is not an ideal mix but believe this combination is ok for a street driven car (no racing). The car is custom built and has parallel leaf springs on back and IFS (mustang derivative) on the front. I know IFS is not very traditional but if you have driven a car with parallel springs on the front you will know why I switched.

 I am trying to strike a balance between keeping the traditional narrow tire look of the period but having a predictable steering car I can drive at highway speeds. Rear tires are almost completely covered by the fenders but fronts are exposed and I have been unable to find a narrow high profile radial for an 18 inch wheel.  Tires will be 255/70 R18 on back and 750 18 on front.

Thanks for any help.

Duuuuuuuuude:

It should not be difficult to get radials all around.  And this simplifies the problem considerably.

Here's the problem in a nut shell:

Radials tend to have a "pause" between steering input and tire reaction at the footprint - compared to bias ply tires.  This means that you'd want the radials up front to promote understeer.

personally, I think you've got bigger problem than just bias vs radials.  You've got non-passenger vs passenger.

Duuuuuude!  Please rethink this!

Capriracer thanks for reply, I have a few comments about your post.
First where do you get your info about using radials on the front and bias-ply on the back? Here is a quote from the Michelin web site --

" If two radial and two non-radial tires are installed on a vehicle, the two radials MUST be installed on the rear axle and the two non-radials on the front axle."

and the RMA web site ---
"When two radial tires are used with two non-radials, put the radials on the rear axle."

from dunlop website
"When radial tires are used with bias or bias belted tires on the same car, the radials must always be placed on the rear axle"

and so on. If you disagree with this please let me know why.

Secondly if you can tell me where I can find a radial with a 32 inch diameter and a 9 inch cross section that will fit on a 18" wheel I would very much appreciate it.
I have spent weeks looking and cannot find one.

Thirdly what do you mean by passenger vs non-passenger.

Fourthly don't call me duuuude, please!!

Thanks for the reply though

I made a mistake with my tire size on the bias-ply.

It should be 700-18.

Scrimshaw,

My bad.

I always get confused about where to put bias tires - and since the early 1980's this doesn't come up much any more, so I tend to "go home to momma" so to speak and revert to the speed of reaction thing that is common for inflation pressure differentials.

But this isn't just about radials vs bias.  It's also about differing types of tires - which is the reference to the passenger to non-passenger.  

The 7.00-18 you found?  This isn't a "normal" passenger tire size.  Nor do I find any variants in the reference books I have.  Did the tire have letters before or after the size?  Where did you find it?

Also, why the restriction of 32" diameter and 18" rims?  This is an extremely odd combination.  And don't forget, the primary purpose of a tire is to carry load, so the weight on the tire is an important consideration.

And lastly, Sorry about the "Duude" thing - I was in surfer mode when I posted.  Do you prefer "Pirate?"  Arrrrrrrgh!
  

Thanks again for the reply.

700-18 is made by Coker tires specifically for the custom market. Here is a link to it.
http://store.coker.com/product.php?productid=16831&cat=0&page=1

The diameter and wheel restriction is simply because of aesthetic reasons.
While I am willing to compromise on tire width a little I do not want to change diameter because the wheel/tire combo will not fit in the fender properly and will change the whole look of the body. If I am advised against the radial/bias-ply mix I will go for the same 255/70/R18 all round.  

 Ideally a 19 inch wheel would be correct for this body shape but nobody makes a 19 inch spoke wheel (and finding the correct tire for a 19" wheel would be impossible). Total weight for this car should be around 3000lbs.

So in regards to my original question what kind of handling characteristics would you expect and do you still see a dangerous situation?

BTW Pirate is a little better but I haven't said "Aaaaargh" for a while.

You are advised against a mix.

A mix won't necessarily be dangerous, but it just adds another unknown to the equation.

Incidentally there si no reason why your choice of suspensions should give poor handling, IFS+Hotchkiss can give a cheap IRS a run for the money except for handling on rough roads.

Cheers

Greg Locock

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Front IRS and rear hotchkiss was the normal standard on virtually all American and a lot of British and European RWD cars from the thirties through to the early 90s and as mentioned by Greg, can work very well.

18" wheels are certainly NOT a traditional or classic hot rod look.

7.50 is not narrow.

As mentioned earlier, different tyres have different response rate or reaction times between steering input and chassis reaction due to different sidewall to tread distortion shape, time and degree. This happens to some extent even with fairly similar tyres, and can be used as a tuning device, but it requires experimentation.

I would keep the tyres as similar as possible, with maybe a smaller tread width relative to rim width on the front so as to reduce sidewall flex and keep the steering crisp.

In my limited experience, especially compared to Greg or CapriRacer, I would consider:-

Same tyre, different width a minor and predictable and obvious change.

Same tyre, different profile, a slightly greater difference but also predictable. Obvious is changed sidewall flex re degree, not so obvious is slightly changed response rate.

Same structure type, different tyre, varies from very similar to significantly different to the point that it is a full time job just driving in a straight line. Obvious differences are tread pattern, but differences in reinforcement fabric structure and placement and rubber compound type and placement are the not so obvious differences that can have a profound effect.

Different structure type. Differences are obvious and predictable in nature but not in magnitude..

Different tread width to rim width relationship. Difference is fairly obvious and predictable.

I would buy a suitable front tyre that has the desired rolling diameter and driving qualities, then buy something compatible for the rear as the rear choice has greater freedom due to being covered.

If you want more cornering grip with the rear with a hotchkiss, you can bend the diff to give some negative camber, but that reduces tread wear life and reduces drag racing traction and reduces straight line braking performance.

Regards

eng-tips, by professional engineers for professional engineers
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Thanks for the replies. I am not worried about my suspension choices I know this system will give me adequate handling for the use the car will get. I have experience with parallel leaf front and back and know it to be fairly unpredictable which is why I am using the IFS. (the fact the car weighed almost 5000lbs probably didn't help!)



Pat thanks for the detailed post and I would take your advice but I simply do not have the choice available to me.

" 18" wheels are certainly NOT a traditional or classic hot rod look. 7.50 is not narrow." --  not sure what you meant by this but this type of car had 19 inch wheels and 700-19 tires as standard in the 20's and 30's which is the reason I will not change the wheel diameter (18 is the closest available), also tire size is crucial to fill the fender and keep proportions correct hence my insistence on 32" total tire diameter. I don't want it to look like a hot rod.
700 or 750 on an 18" wheel is about 5.5" tread width and 7 or 8 inch cross section, while this might  not be the narrowest of tires in vintage years, relative to todays radials I would say it was narrow.

Interesting idea on the diff. I have never heard of that before but I don't think I am going to need that kind of roadholding or want the extra complication.

Thanks for confirming my suspicions that this might not be the best thing to do and unless I can find something different in the meantime it looks like I will be using the 255/70 R18 all around.

There seems to be wide variation in the 7.00-18 tires - from ~31.1" OD to 33" OD.  How ironclad is the 32.0"?

If you can consider Coker's Firestone series at ~31.1" OD, why not drop back to 17" front wheels with 235/75 tires?  Five hits on availability at Tire Rack just as a start.  30.8" - 30.9" OD and 9.3" - 9.4" wide on a 6.5" wide wheel.  I doubt that the 0.10" - 0.15" radius difference vs 31.1" would ever be noticed.


Norm

Norm

I did consider this at one point and could live with the slight drop in tire diameter as you suggest,  but while a drop from 19" wheel to an 18" might not be noticeable I wonder if going from 19 to 17 would show. I do agree the diameter vs. cross section is a lot closer to the older look. I will have to make a few more mock-ups of the wheel sizes and try this out.

Another obstacle which I didn't mention is the tire has to be capable of being whitewalled. I will have to check and see if this type is ok. As this tire is mainly an on/off road tire do you foresee any handling issues?
Thanks.

Radials tend to have higher cornering stiffness all things being equal - i.e. they generate peak lateral force at a lower slip angle.

If you put the radials on then back and the crossplies on the front you will get bigger slip angles on the front for a given level of lateral acceleration - i.e. understeer, ie. stable.

Radials on the front would be the opposite, i.e unstable oversteer.

Ben

Thanks ubrben that was the kind of info I was looking for, although I think I will take the advice of others and use the radials all round.