Wider front tyre?

timbooth

I am looking for new tyres and have been reading forums and reviews - the same thing keeps coming-up: "run a skinnier rear to reduce rolling resistance". Now, it's a long time since my physics exams, but rolling resistance affects both front and rear tyres, so why skimp on the back? Add to this that fatter tyres actually have LESS rolling resistance and the point is weakened further. See:

http://velonews.competitor.com/2012/03/ ... ren_209268

If you then add to this, that the rear takes more of a hammering, carries most of the weight and has an inherently weaker wheel (more dish) and it would make sense to have a fatter rear.

So, can anyone explain this trend?

cooldad

More grip on the front, theoretically.

gt-arrowhead

People like to bias their grip to the front to avoid wash outs generally. And because losing grip on the rear is alot safer than losing grip on the front.

cooldad

I'm not convinced.

The Rookie

TimBooth wrote:

Add to this that fatter tyres actually have LESS rolling resistance and the point is weakened further. See:

http://velonews.competitor.com/2012/03/ ... ren_209268

That link is looking at 25/23and going to 28mm road tyres, is of no relevance at all when looking at MTB tyres! That is well known but on road bikes the aero benefit of slimmer tyres outweighs the rolling resistance penalty at a typical 18-20mph days race speed.

Also you have to factor in pressure and rim width, but generally wider tyres have more rolling resistance on smoother surfaces while as the surface gets rougher the benefit moves back to more conformal fatter softer tyres, there is no single lowest!

benpinnick

Its a weight thing mainly - thin tyres weigh less, and so since you don't need the extra grip on the back (really) there's no need to carry around that weight penalty.

Monty Dog

Give me a fatter tyre every time - better grip, more control - a rounder tyre will give more progressive handling too - too skinny and you get a noticeable transition that leads to washing-out.

timbooth

Thanks for all of the replies - I still don't get it. All the comments are on why a wider front tyre is good, but nothing to explain why you wouldn't want the same benefits on the back (except the comment on weight). Personally, I've always found grip on the back to be at least as important as the front - sure, you don't want a front-wheel washout but you won't get one if the rear (driving) wheel won't grip enough to get you moving.

I understand that the back wheel drives and the front steers and does most of the braking, but this is to do with tread, not size. Apart from fitting into the frame, the only reason I can see for a skinnier rear tyre is weight. And that seems like a bad place to save 50-100g.

stubs

On a XC fame there often isnt the room to go bigger than 2.1 or 2.2, most XC forks will fit up to a 2.35.

You can never have too much grip at the front you really dont want the front sliding unless you are Steve Peat or Gee Atherton. Grip is less important at the rear you could actually run a completely slick tyre at the rear in dry weather the grip doesnt come from the tread in dry weather it comes from the rubber compound and the tyre carcass deforming. You could even run a slick rear tyre in wet weather if you ran the pressure low enough.

So to summarise you dont need a really wide rear tyre in summer because grip is not as essential and most of it comes from the compound not the tread. In winter a narrow tyre is good for cutting through mud to find grip.

peter413

stubs wrote:

You can never have too much grip at the front you really dont want the front sliding unless you are Steve Peat or Gee Atherton. Grip is less important at the rear you could actually run a completely slick tyre at the rear in dry weather the grip doesnt come from the tread in dry weather it comes from the rubber compound and the tyre carcass deforming. You could even run a slick rear tyre in wet weather if you ran the pressure low enough.

Sorry but that is total rubbish. You can have too much grip on the front because you end up with a tyre that rolls really slowly. And you really couldn't run a slick tyre and ride just as well as you could with a normal tyre. A lot of the grip does come from the tread, how else does the tyre dig into the ground?

stubs

peter413 wrote:

stubs wrote:

You can never have too much grip at the front you really dont want the front sliding unless you are Steve Peat or Gee Atherton. Grip is less important at the rear you could actually run a completely slick tyre at the rear in dry weather the grip doesnt come from the tread in dry weather it comes from the rubber compound and the tyre carcass deforming. You could even run a slick rear tyre in wet weather if you ran the pressure low enough.

Sorry but that is total rubbish. You can have too much grip on the front because you end up with a tyre that rolls really slowly. And you really couldn't run a slick tyre and ride just as well as you could with a normal tyre. A lot of the grip does come from the tread, how else does the tyre dig into the ground?

In dry weather how does the tread dig into the ground. Go and look at a rally car set up for dry surfaces and tell me how much tread they are running.

peter413

stubs wrote:

peter413 wrote:

stubs wrote:

You can never have too much grip at the front you really dont want the front sliding unless you are Steve Peat or Gee Atherton. Grip is less important at the rear you could actually run a completely slick tyre at the rear in dry weather the grip doesnt come from the tread in dry weather it comes from the rubber compound and the tyre carcass deforming. You could even run a slick rear tyre in wet weather if you ran the pressure low enough.

Sorry but that is total rubbish. You can have too much grip on the front because you end up with a tyre that rolls really slowly. And you really couldn't run a slick tyre and ride just as well as you could with a normal tyre. A lot of the grip does come from the tread, how else does the tyre dig into the ground?

In dry weather how does the tread dig into the ground. Go and look at a rally car set up for dry surfaces and tell me how much tread they are running.

I don't need to go look at anything. I know from personal experience that a tyre with not much or no tread has less grip than a more aggressive tyre. And why should the ground being dry stop the tyre digging in? By that logic why not run slick tyres both front and rear?

timbooth

I suppose the last couple of comments explain my confusion! The rules should be the same for front and rear - either the grip comes from tread/size or it doesnt. And either bigger tyres are better (for both ends) or they aren't? Surely? The arguments used for a bigger front apply to a bigger rear?

cooldad

It's fairly easy to control the rear if it lets go, but if the front goes you fall off. So more grip at the front, less weight at the back is apparently the ideal compromise.
But as most of use are mediocre riders, in real life I doubt it's that serious.

I've tried all sizes in various permutations and remain mediocre.

BigAl

cooldad wrote:

But as most of us are mediocre riders, in real life I doubt it's that serious.

Nice of you to say so but, really, I'm not as good as that

ride_whenever

The only reason to run skinnier tyres on the rear is that of clearance. Most frames simply wont accommodate large tyres and mud! Whilst forks don't have huge clearance there isn't really many places for it to clog unless things get really rough in which case the same applies.

Case in point would be me running 2.35 hans dampfs in sids, or 2.4 rocket rons in the same.

Corners R Us

I've recently tried a few different combos on my winter hardtail.

2.35 Maxxis High Roller IIs front and rear. Front good, rear very draggy.

2.35 Maxxis HR II front Tioga 1.8 XC rear. Spot on. Front gives confidence, rear cuts through gloop and slides predictably.

2.35 Maxxis HR up front Panaracer Trailspiker 1.95 (more like a 1.8); as above.

2.25 Intense System 3 up front, 2.1 Intense System 5 out rear: treads not aggressive enough for mud, lots of edgy, unpredictable slides!

Tyre pressures are also critical this time of year. Ran the Maxxis HR at 30psi and they were very wooden. 25psy, spot on. Running the rear Tioga and Trialspiker at 30 psi.

Hope this helps.

Corners

jimothy78

No-one yet has mentioned weight distribution.

In a neutral attack postion, with your weight on your pedals, not on your bar, then depending on the geometry of your bike, 60-70% of your weight is borne by the rear tyre, and the remaining 30-40% on your front. This would mean that if you ran the same tread pattern, compound, tyre size and pressure on both wheels you'd have much less (in some cases as little as half as much) grip on the front than the rear, which would be very hard to control. You compensate for this by using some or all of the following on the front tyre:

a) a more aggressive tread pattern
b) a softer compound
c) a wider tyre (more contact area)
d) less pressure (more contact area)

Of course, most riders would prefer to have more grip on the front, so we over-compensate, and tend to use more of these than necessary to simply equalise the grip, in order to set the balance of grip towards the front, rather than the rear.

timbooth

jimothy78 wrote:

No-one yet has mentioned weight distribution.

In a neutral attack postion, with your weight on your pedals, not on your bar, then depending on the geometry of your bike, 60-70% of your weight is borne by the rear tyre, and the remaining 30-40% on your front. This would mean that if you ran the same tread pattern, compound, tyre size and pressure on both wheels you'd have much less (in some cases as little as half as much) grip on the front than the rear, which would be very hard to control. You compensate for this by using some or all of the following on the front tyre:

a) a more aggressive tread pattern
b) a softer compound
c) a wider tyre (more contact area)
d) less pressure (more contact area)

Of course, most riders would prefer to have more grip on the front, so we over-compensate, and tend to use more of these than necessary to simply equalise the grip, in order to set the balance of grip towards the front, rather than the rear.

Thank you - this is the first argument that I have heard, that actually explains why you might run a bigger front tyre. Based on this argument, I'd put a larger front tyre on, but might actually also put a larger rear on as well, as the only thing to lose is 50-100g of weight.
Thanks

rockmonkeysc

There's some proper b0ll0cks in this thread. Even by usual internet forum standards there's some proper horse sh1t.
To the OP, abandon all hope of a sensible answer here. If you want better advice on mountain bike tyre choice you would do better on the knitting world forum.

timbooth

RockmonkeySC wrote:

There's some proper b0ll0cks in this thread. Even by usual internet forum standards there's some proper horse sh1t.
To the OP, abandon all hope of a sensible answer here. If you want better advice on mountain bike tyre choice you would do better on the knitting world forum.

Thanks - I'll head over and ask them what they think!! In all seriousness, I have to agree - most of the arguments (bar the last) explain why one should have bigger tyres, not why one should run a bigger tyre only on the front. I suspect that it is a lot to do with herd-mentality - people copy others, without know/asking why.

Knitting World it is, then!

The Rookie

No one has said you should run a bigger tyre at the front, what has bene said is the reason why if one tyre is bigger it is put on the front, there have also been put forward some suggestions why people run one tyre narrower (such as fitment with frame clearance at the rear or to reduce weight and rolling resistance (where it will work).

As an example, the trails near me can get VERY muddy over winter, so I have a 1.8" mud tyre I run on the back when applicable, it gets me up the hills easily when a lot of my mates grind to a halt with no traction, better still I get through muddy sections when they stop and have to put a foot down in that 2" deep mud, I don't run it year round and I only run it when there is a benefit to running it, I still run a more conventional 2 point 'something' at the front for control and stability on the non muddy sections.

timbooth

Rookie, "if one tyre is bigger " implies that people aren't choosing to buy one tyre bigger than the other. But they are and they do - they buy a front tyre bigger than the back, but without being able to explain why (other than mud clearance). "More grip" (same for the rear), "More traction" (same for the rear), "More control", etc, etc. Do you get my point?

Perhaps I should have asked - "Why would you run a skinnier/less knobbly rear tyre than you are able to?"

And to get it out of the way in the beginning - 'rolling resistance' is a weak answer - BOTH wheels have rolling resistance, so to imply that your bike is fast with a tiny rear tyre, whilst still pushing a massive tractor tyre at the front doesn't make sense (physically-speaking). I was hoping to learn 'the science' behind this trend.

The Rookie

TimBooth wrote:

"Why would you run a skinnier/less knobbly rear tyre than you are able to?"

And to get it out of the way in the beginning - 'rolling resistance' is a weak answer

Weight and rolling resistance.....

RR is not a weak answer - you saying it doesn't make it so, any reduction is a reduction, and why (ignoring width) many people run a harder compound on the rear than the front. A lot of XC racers run a Ralph front with a Ron rear as the Ron has less rolling resistance, even if they tend to run the same width. (happens to be my summer option as it happens)

The rear accounts for about 70% of the RR when pedalling in the saddle (as the rear takes about 70% of the weight when seated), hence why the rear magnifies the benefit of a faster rolling tyre at the rear, I know 2 people who have 26" fat bike tyres out front coupled to a (similar outside diameter) normal width (2.1-2.25") 29er rear for just that same reason.

njee20

A lot of XC racers run a Ralph front with a Ron rear as the Ron has less rolling resistance, even if they tend to run the same width.

Even more so a Furious Fred on the back, I remember Ian Bibby (decent Elite rider) running a FF at Dalby in the wet. Most people don't need as much tread on the back as they think, even in the wet if you can power through stuff you're often ok. It provides a safety blanket though.

timbooth

"The rear accounts for about 70% of the RR". Yes, it also accounts for about 100% of the propulsion, so wouldn't a bigger rear tyre give better traction?

dusk

If the rear isn't slipping then it doesn't need more traction and doesn't need to be bigger/wider

The Rookie

Traction is only usually an issue when it's muddy when a narrower tyre tends to cut through mud better anyway, which is why dedicated mud tyres like the Panaracer Mud XC and the Black shark are 1.8 and 1.5" respectively.

kajjal

Few riders spend most of the ride on the limits of their bikes traction to really notice. As long as the handling of the bike is consistent and feels right to you then that is all you need.

njee20

Few riders spend most of the ride on the limits of their bikes traction to really notice.

Which was my point, at least on the back you can generally get away with a far slicker/lighter tyre with little discernible difference in traction.