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Hey Guys, This isn't a question about what width tyres fit what width of wheel - there is a mountain of info about this. My question is, how do you know which width will give your car the best balance of grip, steering feeling, avoiding bump steer and tramlining. If we assume that there is no limit caused by the suspension on the inside of the wheel, and the bodywork on the outside of the wheel - because it keeps the question simpler. Maybe this question is less relevant to a production car, because there are others who have tried and tested different options. In my case, my car is completely bespoke in it's suspension and weight. I will be getting wheels re banded to fit the right width of tyre, I just don't know for sure what width to go for. I currently have 205's on the front, and 225's on the rear. I get some tramlining, and I feel the steering is heavier than it should be, so I am thinking of going narrowerer. So, how do I know if a 175 or a 195 width tyre is best on the front for example, and then, what would be best on the rear? I presume it is directly related to the weight of the car, weight distribution, spring rates etc. I just can't seem to find what I am looking for - a kind of tyre picker calculator. How do I know without expensive trial and error?       |
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sparkyt
Posted a lot
selling stuff
Posts: 1,767
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Big question ..
but with all the car related questions .
Is the car for race, road ,style, drift ,fast road etc etc .. let's face it most cars we build are a compromise used for road
Which is it for you ?
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stealthstylz
Club Retro Rides Member
Posts: 14,960
Club RR Member Number: 174
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What does it corner like as it is? Understeer/oversteer/bit of both? How easy is it to provoke oversteer? The current tyres do seem pretty wide for the weight/power
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Big question .. but with all the car related questions . Is the car for race, road ,style, drift ,fast road etc etc .. let's face it most cars we build are a compromise used for road Which is it for you ? I guess I would say fast road..... What does it corner like as it is? Understeer/oversteer/bit of both? How easy is it to provoke oversteer? The current tyres do seem pretty wide for the weight/power Difficult to say really, I've done too few miles driving it in anger - mostly because of the previously poor brakes. It will be coaxed into oversteer fairly easily around a roundabout at fairly low speeds  There is a lot of feedback through the steering wheel over road undulations and bumps. It tends to get thrown off course by bumps, it feels like bump steer, but that is also related to the steering geometry....another variable. My gut feel so far is to go with 185 width on the front, 205 on the rear..... The car is front/mid engined, rear wheel drive and less than 800kgs. Maybe I should try and think of a car similar to use a reference. |
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Lots of seven replicas (weighhing ~600-700kg) use 185 at the front and 205 at the rear. Seems to be a sensible choice. Bear in mind that tramlining will also be affected by (IIRC) wheel offset, extreme camber, toe (more toe out = better turn in but worse straight-line stability), and perhaps caster. Is the bump-steer actual bump-steer, or is it just too-firm suspension causing the wheels to part company with the tarmac over bumps? If actual bump-steer, it probably won't be easy to fix. Feedback is probably a good thing. |
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Lots of seven replicas (weighhing ~600-700kg) use 185 at the front and 205 at the rear. Seems to be a sensible choice. Bear in mind that tramlining will also be affected by (IIRC) wheel offset, extreme camber, toe (more toe out = better turn in but worse straight-line stability), and perhaps caster. Is the bump-steer actual bump-steer, or is it just too-firm suspension causing the wheels to part company with the tarmac over bumps? If actual bump-steer, it probably won't be easy to fix. Feedback is probably a good thing. Thanks The more I read into suspension and steering set up, the more interesting it is, but the more I get confused by it all. Mind you - if it wasn't complicated, it probably wouldn't be very interesting. All I know is, I like this knowledge building stuff - feels like I am becoming less of a spectator in this car world. The other one I am trying to battle is if I reduce the width of the front wheel, but actually reduce the offset by more than the width, what will this mean to my KPI angle/scrub radius??  |
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stealthstylz
Club Retro Rides Member
Posts: 14,960
Club RR Member Number: 174
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Was the suspension custom built to suit the wheels? The scrub radius will always stay the same, and aslong as you use wheels of the same offset the KPi will stay the same too.
Matt
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Was the suspension custom built to suit the wheels? The scrub radius will always stay the same, and aslong as you use wheels of the same offset the KPi will stay the same too. Matt Offset affects both scrub radius and KPI because they're the same thing aren't they? |
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If you haven't got it already, get
"How to make your car handle" by Fred Puhn,
An interesting and 'old school' book on all aspects of suspension design including tyre choice etc.
I reckon it would give you enough information to allow you to make an informed decision.
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Old Fords never die they just go sideways
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rodney
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wheels look cool ,  , if your gonna drop sizes then 195 on the front and 205 on the rear but drop a profile , |
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facebook: rodney dean / rd transport  |
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Anglia68
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When Richard Parker,the original designer of the Olympic,built his Cossie 24v version in the late 90's he used 195/50R15's all round but he also re-designed the suspension,don't know if he had to or just wanted to,so maybe chatting to the current owner of the car might be useful. www.rochdale-owners-club.co.uk/phpbb2/viewtopic.php?t=188 |
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RobinJI
Posted a lot
"Driven by the irony that only being shackled to the road could ever I be free"
Posts: 2,995
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Was the suspension custom built to suit the wheels? The scrub radius will always stay the same, and aslong as you use wheels of the same offset the KPi will stay the same too. Matt Offset affects both scrub radius and KPI because they're the same thing aren't they? Not quite. KPI (King pin inclination) is the angle of the kingpin from vertical when viewed from the front/back. While scrub radius is the distance from the center of the tyre to where a line drawn through the king-pin would hit the floor (also viewed from the front/back). They both have various effects on the handling. What set-up is on the front of the car? is it e30 or something else? |
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stealthstylz
Club Retro Rides Member
Posts: 14,960
Club RR Member Number: 174
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Sorry I was mixing scrub radius up with ackerman. I'd check the scrub radius is correct before buying any other wheels. The KPI isn't adjustable without making new suspension uprights so makes sense to get the wheels right. Have you got any pics of the front suspension?
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Wouldn't it be more interesting to investigate the steering rack and its geometry?
Since your main problems are heavy steering (ratio to direct?) and a hint of bumpsteer.
I didn't recall much difference in the steering heaviness when I changed from 155/70R13 to 185/60R14 to 165/60R14 on FWD car, Although for spirit driving I certainly prefered the last one.
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 Click picture for more |
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If you haven't got it already, get "How to make your car handle" by Fred Puhn, An interesting and 'old school' book on all aspects of suspension design including tyre choice etc. I reckon it would give you enough information to allow you to make an informed decision. Yep, got that one, Cheers. A proper retro read, the photos are great. No relevant info on tyre width choices though. Wouldn't it be more interesting to investigate the steering rack and its geometry? Since your main problems are heavy steering (ratio to direct?) and a hint of bumpsteer. Yes, will be doing that also. I am changing the wheels to reduce the rolling diameter primarily, so just wanted to get the widths right as part of that change. |
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Sorry I was mixing scrub radius up with ackerman. I'd check the scrub radius is correct before buying any other wheels. The KPI isn't adjustable without making new suspension uprights so makes sense to get the wheels right. Have you got any pics of the front suspension? I need to get some photos when I get down to see the car next. Because this thread is very specific about my car now, I'll pick it up in my Readers Ride thread. Cheers |
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, if your gonna drop sizes then 195 on the front and 205 on the rear but drop a profile , 
