Author Topic: differerent front and rear tyre combo?  (Read 180 times)

pondweed

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differerent front and rear tyre combo?
« on: September 25, 2017, 10:03:27 AM »
Is anyone running with different spec front and rears?
I'm still weighing up between 2.0 and 1.6 Supremes for Sherpa, but wondered whether anyone was running with larger front tyre for potential suspension effect or slackening geometry slightly? Or has experimented in this field?

mickeg

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Re: differerent front and rear tyre combo?
« Reply #1 on: September 25, 2017, 04:23:23 PM »
I always use the same size up front as the rear but sometimes use a different tread. For example, I have done several trips on my Sherpa with a Marathon Dureme up front and Marathon Extreme in the rear, but both were 50mm wide.

I consider sizes to be the same if they are within 2mm of each other, thus I consider 35mm and 37mm to be the same.  On my 700c touring bike I am contemplating mixing an old Marathon XR (35mm) on the rear with a Hutchinson Globetrotter (37mm) front.

I might on future trips run a Marathon Dureme (55mm) in front and Marathon Extreme (57mm) rear on my Nomad.  To date I have run the Extremes on both front and rear for touring, but I have a 55mm Dureme that I might use instead.


martinf

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Re: differerent front and rear tyre combo?
« Reply #2 on: September 25, 2017, 06:10:24 PM »
I've run different size tyres on 2 bikes, just to use up tyres.

32 mm front and 38 mm rear on a Brompton.

44 mm front and 50 mm rear came on my old mountain bike, it now runs on 50 mm front and rear.

Didn't notice any geometry issues with either setup.

Danneaux

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Re: differerent front and rear tyre combo?
« Reply #3 on: September 25, 2017, 07:34:21 PM »
Quote
Is anyone running with different spec front and rears?...Or has experimented in this field?
Well...sure. This is one reason why I wrote the article on trail and handling (the other reason is I am a hobbyist framebuilder and I enjoy the design aspect as much as building).

Not all that article was theoretical; I have indeed mixed tire sizes on the same bike and would suggest it to you as an empirical way to gain insight to how tire size affects trail and handling. One shortcut is to mix large differences in tire size, as small differences don't make much ehm, "difference" to handling, as George and Martin have noted. 26in wheel tires are generally available with big "jumps" between tire sizes, so sure...try a 2.0in tire at one end and then the other while keeping your preset 1.25in tire at the other and see what happens.

One major note here: It is desirable to adjust pressures to suit the tire sizes, so narrower tires with less air volume get higher pressures while bigger, fatter tires should get less pressure. This pressure difference can cloud the effects of mixing tire sizes, so I would suggest putting in pressures that result in the same "rim drop" under identical load (keeping in mind the caveat to avoid overpressurizing fat tires, as this can adversely affect rims by resulting in jacking forces high enough to crack or split a rim).

I will note, having tried mixed tire sizes in the past, I returned to two tires of the same size...notsomuch because I didn't like the resulting and slight change in geometry, but because I did not like the difference in tire contact patch shape and size (one tire patch is longer and narrower, the other shorter and wider). I felt that affected handling and cornering more and made a bigger (and unpleasant) difference in bike response than I wanted.

Sheldon Brown discusses mixed tire sizes about 2/3 of the way through the article at this link:
https://www.sheldonbrown.com/tires.html

If you are still having handling issues with your Sherpa, the I would do two things:
1) Try fitting two wider/fatter tires of the same size and see if that solves your problem...the present 1.25in tires you mentioned using in an earlier post are considerably smaller than are generally used on a Sherpa. Looking back at my pre-purchase notes, SJS Cyles staff told me my Mk2 Sherpa was designed around 1.6in tires, my Nomad was designed around 2.0in. I could certainly notice a difference when I replaced the 2.0in Schwalbe Duremes used on my Sherpa and Nomad with the 1.5in road slicks I borrowed from my tandem.

2) I would suggest some of your handling difficulties may relate as much to your (used) bike's size and/or how it is fitted as it does to the tires.

In the scheme of things, buying and trying one or two different tires than you have now or fitting a different stem or seatpost are relatively inexpensive and can go far toward dialing in the feel of the bike and making it closer to what you prefer.

Have you sent your dimensions to SJS Cycles to check you are indeed riding a size within range of your body type and preferred seating position? Though we may each fit a variety of frame sizes, individual variations in body part size can make a big difference to comfort, setup and on-road handling, i.e. whether you have long or short arms/legs/trunk for your height and need to accommodate for same.

Hopefully helpful.

Best,

Dan.
« Last Edit: September 25, 2017, 10:27:58 PM by Danneaux »

pondweed

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Re: differerent front and rear tyre combo?
« Reply #4 on: September 25, 2017, 08:53:50 PM »
Interesting stuff from all thanks.

I think Dan has brought up the real issue. I was wanting to try the 'trail' variation thing (and it seems that the most interesting thing would be to put a biggie on the front) but the comment on contact patches and affecting handling makes me realise that it really would just be experimentation. I will definitely put the biggie on there from my mtb, if it will fit with the mudguard, as a 1.25 up to a 2.1 might trigger some form of geometry difference I'd like to experience. I read on some Single Track forum some chap mentioning wanting to 'slacken the angle'...

Dan's other point on the sherpa being designed around 1.6s makes me realise I've got to get those over the 2.0s too, as a datum.

Andre Jute

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Re: differerent front and rear tyre combo?
« Reply #5 on: September 26, 2017, 12:26:15 AM »
Interesting thread. In my book DESIGNING AND BUILDING SPECIAL CARS (I'm not linking it as it was sold out the day before it was published so the only copies available are second-hand and oily from pawing by hotrodders, and anyway it betrays an earlier fascination with automobiles I have grown out of) I show characteristic graphics of tyre response to various inputs from car, driver and road. The significant thing about these graphs is that they are not a straight line, a convex or concave line, or even a normal distribution evidenced as a bell curve. Instead they are invariably, whatever you can think of measuring, the curve of a complex algorithm, seen visually at a glance as an S-shaped response line lying at various angles to the axes and across the axes. In short, tyre response is the least predictable of all the vehicle reactions.

A bicycle is a simpler mechanism, but dynamically the tyre and its response is still by far the most complicated thing about it. Fortunately most bicycles don't ever go fast enough for the rider to be faced with the ultimate limit of his tyres. At In praise of riding low pressure tyres fast, I describe the behavior of Schwalbe's Big Apples in extreme road conditions, and at the edge of handling (handling is how well the bike recovers when the rider is foolish or unlucky enough to go beyond the natural roadholding limits of the frame/tyre combination/load distribution).

Before you even get to dynamics, tyres have compounds that are deliberately engineered to respond in a different manner. My fave Big Apples at any one time has three or so different compounds, three or so different anti-puncture bands, several different sidewall constructions, and clearly, by this sample of what is known, probably as many unknown internal differences. All these matters are inputs to a major uncertainty about how tyres of even the same name and size and external appearance will respond to rider and road stimuli. Jan Heine's attempt to overcome the impossibility of full understanding by a "15% rim-drop at the right inflation" rule-of-thumb has been mentioned by Dan but is a bit inconvenient to apply once your wife takes her hemming measure back. It is more of a baseline than a final answer. You'd still need to discover empirically (that is, by riding the bike under a variety of conditions) how anti-puncture bands of different materials react with different inflation regimes.

I've explained the uncertainties inherent in tyres at some length, because it is to avoid aggravating them that I always fit tyres that are as close to perfectly matched as I can get both front and back; I achieve this by buying replacements and spares all at the same time.

Fitting the fatter tyre at the back of the bike is likely to make it livelier but also more dangerous at speed, as it will tend to overdrive the bike and thus make it oversteer. Jobst Brand, the Porsche engineer behind the Avocets which were the first everyday slicks to be sold to Americans, used to say that once you've lost control of the front wheel you're a passenger in your accident; nothing more you can do except pray. However, I have one bike with very lazy geometry that naturally understeers at speed, which is a safety factor at 70kph on badly surfaced, twisty downhills often with loose gravel swept in from the farmers' side roads; it was designed to do that, and everything I have done to the bike since it was new was intended to maintain this comfortable safety margin regardless of what we might meet. But let's say I want to turn this safe, fast tourer into a comfortable commuter. So, on this bike I would fit the fatter tyre at the back and nip in and out of the traffic on the thinner front tyre. You have to know how the bike holds the road and handles excessive inputs before you can start tailoring it. On a bike which already oversteers (bikes with excessively short wheelbases and unnaturally steep geometry: I have no such boy racer appendages), I would put the fat tyre on the front to tame the bike's road manners a little by slowing down the response.

That applies to a thin and a fat tyre of the same make and general construction from the same materials. Adding further complication besides width makes the whole thing too multi-varianced to conclude anything except that you have discovered a change but that you don't know in which direction to go (see the S-bends above) to enhance or reduce the effect. It could be expensive to buy enough tyres to control changes to one variable at a time, so it's probably smart to stay within one manufacturer's catalogue.

I caught "ceteris paribus" above or in one of the links. It's Latin for, "Remember to change only one thing at a time, keeping all others the same."

***

I've often wondered if there isn't a default case for making the rim and tyre on touring bikes wider at the back where the heaviest weight is carried. Not on the short wheelbase daytrippers but on the heftier long-range tourers. It would probably mean supplying two rear wheels with the new bike, one for touring and one for commuting and short rides, which would make most manufacturers cringe.

martinf

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Re: differerent front and rear tyre combo?
« Reply #6 on: September 26, 2017, 06:12:32 AM »
If you're looking for suspension effect, it would make sense to go for the fattest lightweight tyre that will comfortably fit, on both front and rear.

For the Sherpa, that would probably be the 50 mm Supremes. Or for a tyre more capable for off-road riding, the slightly heavier 50 mm Duremes as recommended by Thorn.

On the old 650B wheel utility bike I use for survey work I recently went from 44 mm Marathons, a rather heavy but still quite easy-rolling tyre, to 1.6 inch Supremes (about 40-42 mm) and finally to 50 mm Supremes.

On-road, the 1.6 inch Supremes felt much faster and slightly more comfortable than the 44 mm Marathons.

Going from 1.6 inch Supremes to 50 mm Supremes slowed the bike very slightly on-road, but increased comfort and off-road capability very significantly. I can now ride sandy stretches on paths where I would previously have had to push the bike.

The 1.6 inch Supremes went to my wife's bike, where they replaced the 44 mm Marathons she had before (not enough clearance for anything wider than 44 mm). She also finds the Supremes a very significant improvement, and has consequently done much more cycling than previously since the tyre change.

Tyre pressure is very important. I generally experiment with different pressures to try and find the optimum for any given width and model of tyre. Mostly I end up with relatively low pressures.
« Last Edit: September 27, 2017, 08:17:19 PM by martinf »

John Saxby

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Re: differerent front and rear tyre combo?
« Reply #7 on: September 27, 2017, 03:28:58 PM »
Martin, a detail question on your 50mm Supremes: what is their actual inflated width?  My 1.6 Supremes come in at 39 mm inflated, not the nominal 40.6 mm. 

PH

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Re: differerent front and rear tyre combo?
« Reply #8 on: September 27, 2017, 04:59:42 PM »
An argument against is the way many riders, myself included, maximise tyre wear while always maintaining a good tyre on the front.  That is to let the rear wear right down, then put the part worn front onto the back and a new tyre on the front.  I simply wouldn't wear down a front tyre to the extent I would a rear.

mickeg

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Re: differerent front and rear tyre combo?
« Reply #9 on: September 27, 2017, 05:49:52 PM »
An argument against is the way many riders, myself included, maximise tyre wear while always maintaining a good tyre on the front.  That is to let the rear wear right down, then put the part worn front onto the back and a new tyre on the front.  I simply wouldn't wear down a front tyre to the extent I would a rear.

I always want my front tire to be my best tire, I do not look at it from a tread maximization perspective.

martinf

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Re: differerent front and rear tyre combo?
« Reply #10 on: September 27, 2017, 08:44:07 PM »
Martin, a detail question on your 50mm Supremes: what is their actual inflated width?  My 1.6 Supremes come in at 39 mm inflated, not the nominal 40.6 mm.

Actual inflated width measured with callipers:

- 49 mm on a fairly wide EXAL ZX 19 rim.
- 48 mm on a fairly narrow Mavic 221 rim.
- 46 mm on an Andra 30 CSS rim, width intermediate between the first two.

All at fairly low pressures. I didn't check, but normally between 28 and 35 psi. I suppose the 49 mm tyre might just make 50 mm when inflated to the 60 psi max pressure printed on the sidewall (though this might damage the rim).

The tyre that measures 46 mm was bought more recently than the other two, maybe the width was reduced slightly between batches ?

John Saxby

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Re: differerent front and rear tyre combo?
« Reply #11 on: September 27, 2017, 09:05:41 PM »
Thanks, Martin, that's helpful info.  I'm thinking of moving to 26 x 2.00 Supremes in the next year or so, and wanted to check actual their inflated width vs. fender (mudguard) width. I'm thinking of using Velo Orange 60 mm fenders (external measurement), so the  26 x 2.00's should fit with ample clearance.