Aspect ratio and handling

[jknappsax]

I've searched on this, but can't seem to find any information on this. I'm curious about opinions on aspect ratio and handling in a GSXR wheel and suspension swap. Going from 19/18" front/rear to 17/17" front/rear obviously lowers the bike. If I went with higher aspect-ratio tires this would help make up some height difference, but would there be any downside in terms of handling? I would think that going to the 17" front would quicken the steering, but would the increased mass of a 90-series tire cause handling problems? Any thoughts and theories would be appreciated.

[Steve]

You asked this same question in THIS THREAD just a few days ago.

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[posplayr]

You asked this same question in THIS THREAD just a few days ago.

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Yea, I'm not sure what the point of getting 5.5" wheels is and then mounting a 90 series tire to it but maybe there is something worth discussing

[jknappsax]

Actually, it was in Projects

... and there were few responses (except you guys), so I REPOSTED a thread here in Tires/Suspension in hopes of a wider response. I'm concerned mostly with front-tire performance, and still wonder what the ramifications of trying to maximize the under-chassis clearance with a higher-profile tire might be on handling, the pros and cons. Having a wider tire with more surface contact, and the height, seem to be different issues, unless the spinning mass of the higher-profile tire will adversely affect handling characteristics. Lower-profile might not necessarily be better, but that's the point of this disussion. Any opinions would be welcome.

[Steve]

OK, since you did not respond to my request in the other thread, let's see if you respond in this one.

What specific tire sizes are you considering?

I will start by assuming that your 19" tire is the standard 100/90-19, but I have no idea what sizes the 17" tires are that you are considering.

Give me the sizes, I will plug them into my calculator and will be able to tell you exactly how much different they are in overall height at the axle, which is what will affect your ride height.

All I need is a list of sizes.

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[Nessism]

Steve's right. You can't figure tire height without knowing the width and aspect ratio. A wide tire may be taller as well, even though the aspect ratio could be lower.

[posplayr]

The only adverse effects I know of are:

• mixing radials with bias ply (the worst),
• using the wrong size tire for the rim (there is usually 1 step size either way that is OK)
• using stock bias ply tire (radials are much more secure)

[Nessism]

The only adverse effects I know of are:

• mixing radials with bias ply (the worst),
• using the wrong size tire for the rim (there is usually 1 step size either way that is OK)
• using stock bias ply tire (radials are much more secure)

Why is mixing radial and bias ply "the worst"? Having a hard time understanding why there could be a problem with mixing.

[posplayr]

Why is mixing radial and bias ply "the worst"? Having a hard time understanding why there could be a problem with mixing.

Well on a car it is complete NO-NO to put radials on the back with a bias ply on the front. The side walls flex more on the rear making the vehicle tend to fish tail(I have first hand knowledge).

I have never tried it on a motorcycle, but as for example a 180/55-17 radial uses a very wide wheel and so the side wall stiffness if probably even more so that any 90 profiles bias ply. So that might not be at issue.

Regardless I would not recommend any mixing unless you feel real adventurous.

[Agemax]

on a car, you can legally run radials on the rear and cross plys on the front, but it will induce understeer. you should never mix them on the same axle.

a few manufacturers, including Yamaha(on the big cruisers) and HD sold bikes straight from the factory with radial on the rear and cross ply on the front

[koolaid_kid]

adventurous = not afraid to die, well insured, papers all in order.
I too, experienced the mix with a cage. A very forgettable moment.

[salty_monk]

I ran a 120/65/17 Michelin Pilot on the front which is about as tall a tyre as you are going to get & it was fine....

On the rear I believe it was a 170/60/17.

Not sure where you are going to find a 90 profile front tyre to fit on a GSXR Rim?!

I had a RSU suspension. Overall there was a drop at the front from stock but not too much & a drop at the rear.

You need to give all the tyre details as requested above.

[posplayr]

on a car, you can legally run radials on the rear and cross plys on the front, but it will induce understeer. you should never mix them on the same axle.

a few manufacturers, including Yamaha(on the big cruisers) and HD sold bikes straight from the factory with radial on the rear and cross ply on the front

Are you sure that is not reversed. I distinctly remember radial in the rear (on a car) causing fishtailing not under steer (it has been about 35 years though).

For a motorcycle, the biggest issue for stability is having a rear tire that flexes the sidewall in excess of the front wheel. I'm pretty sure this will cause at least a feeling of uneasiness in the rear if not a weave instability.

I don't doubt that on a cruiser not intended for a lot of lateral force, that a big wide and stiff side walled radial, will be OK with a much narrower and taller bias ply tire. If anything it would probably under steer.

So Ed, to clarify, I'm not saying it is always bad to mix, but when exploring the worst things that can happen when mixing the wrong combination of tires and rims (The OP's topic) some combinations of mixing radials and bias plys can cause the worst that can happen in mixing and matching other that recommended tires/wheels.

[Nessism]

Radial's have short sidewalls, pulled tight by a wide wheel, to control sidewall flex inherent in the design. I don't think the overall flex is substantially different from that of a taller aspect ratio tire using a stiffer cross ply structure.

[jknappsax]

Sorry abour slow responses

I don't always get a chance to check in here every day. I must have missed your question in the other thread, Steve. I'm just starting to think about the options on various aspects of this swap, and though I'm generally thinking about 120 on the front, and maybe 130 or 140 on the rear, I haven't researched specific tires brands or models (and checked what IS available in those widths), so this is more of a musing on what might be factors to consider. The idea is to have a stable, good-handling sport-tourer. The idea of mixing radial and bias-ply tires is one I wouldn't even consider. And I'm probably going to find that I'll be limited in which aspect-ratios will be available in the tires I consider, but I haven't really narrowed down any choices; probably look for dual-compound tires, probably Dunlop or Michelin, maybe Conti. For right now, I'm running the original front end with Dunlop GT 501s, but I am trying to sort out some minor squirreliness at about 75-80 MPH, so this discussion is to give me some factors to consider when I get closer to doing the swap next fall. Sorry if my vagueness annoys you guys. I appreciate your expertise, insight, and responses