Bike Geometry and Cornering Grip?

Jul 23, 17 16:06

Post #1 of 19 (2227 views)

Is there any relationship between bike geometry and cornering stability/grip? E.g. like a higher Cg significantly affecting cornering forces?

I'm just wondering because on my old bike, a 1st-gen Felt AR, I felt like a cornering pro in Cat 1-2 criteriums, etc. Even in the rain - I'd be the guy who'd attack when the first drops fell, taking a the wet chicane at 35MPH, etc.

Then this year I switched to a late-model Cervelo S5. In crits I went down twice in the rain, fortunately not taking anyone else with me in either instance. Both times completely shocking me - "WTF am I doing on the ground?" I chalked them up to maybe crossing a slick, painted line in one instance or possibly an oil slick in the 2nd. Both time were the rear wheel slipping out from underneath me.

But then today while descending good, dry pavement, I got loose for an instant on a technical descent (but saved it). And I wasn't even being what I'd consider all that aggressive. So now my former confidence is completely shot.

- I don't think it's the tires/inflation. I run a variety of well-respected tires, and they're the same tires I ran on the Felt. And I don't run stupid pressure. I'm 170lb. and I'm running clinchers at about 90PSI on wider rims (HED +). Schwalbe Pro One and Vittoria Speed, generally. 23mm

-I'm not a pro, but I like to think I corner pretty well. I know how to set up a turn. I don't brake while turning. I've followed former pro crit riders, e.g. Rahsaan Bahati on the last lap of crits for years now without having anyone yell at me or any sort of drama. I know about body and foot placement/pressure in turns, etc. I'm certainly not pedal-striking. I understand "counter-steering." (not to bring up that whole thread again).

So I'm sort of eyeballing my S5, perhaps unfairly. Is there something geometry-related that could explain it, at least partially.

Or is it just dumb bad luck maybe combined with overestimated skill? The latter is possible, but I've been road racing for for 10 years, and am Cat 2. I made it hundreds of mass starts in Cat 4 and Cat 3 without ever going down. My two embarrassing crashes at Cat 2 were my first since Cat 5 stupidity around a decade ago. So my record is really good with crashes in general.

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 23, 17 20:39

Post #2 of 19 (2170 views)

I believe a higher CoM would only affect agility ona bicycle since your rolling moment is neutralized by your cornering angle. You would have to lean slightly more for higher CoM, but I dont think it's game changing.

What I would think would be a primary factor is fore/aft weight distribution. Say for instance that the ratio of front or rear contact patches to the bottom bracket is different, in this case maybe the BB is closer to the rear contact patch, causing you to load the rear tire more than on your previous bike. Only adding or removing load on the handlebars would help, moving the CoM from BB or saddle contacts towards the front.

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 23, 17 21:25

Post #3 of 19 (2157 views)

Comparing the geo charts the Felt is a longer wheel base frame with the same angles [56cm] .. less bb drop of the S5 is due to a more lay in and less steer in cornering approach .. I would put an eye on the tires' rubber edit: grip ..

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___/\___/\___/\___ the s u r f b o a r d of the K u r p f a l z is the r o a d b i k e .. oSo >>

Last edited by: sausskross: Jul 23, 17 21:32

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 23, 17 22:28

Post #4 of 19 (2118 views)

How long have you been running the Jet+ rims? I was told in no uncertain terms by a HED technician to start at 85psi "max" and to "go down" from there with a 22mm Attack. I'm also 170lbs. Bike + rider in race trim everything comes to 194lbs. Keep in mind your 23s are probably closer to 26mm on those rims and when you account for casing tension 90psi is probably closer to 110-115psi on a true 23mm tire. Also, on a fast tire like the Corsa Speed, the penalty for running 10psi lower than optimum is only a couple of watts (check out the curve on BRR's chart).

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 23, 17 22:39

Post #5 of 19 (2114 views)

Longer chainstays, shorter front center, slightly lower BB on AR compared to S5. My experience is that chainstay length has a huge effect on cornering. I'm a fan of longer chainstays.

As previous poster noted, your weight distribution will be quite different between the two bikes. The AR's suiting you more. Moving saddle forward and longer stem may improve S5's handling. Could be worth a try.

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 24, 17 2:03

Post #6 of 19 (2078 views)

I don't recall the size bike you're on, will use 56 as an example, the comparison holds for other sizes pretty well

Almost the same head tube angle and same rake gives the same trail (obviously that would be something important to you). Only real front end difference is the Felt has a 1.125 lower race and the Cervelo is 1.375
Felt has much longer chainstays
Cervelo is 2mm higher off the ground.

Technically the Felt puts more weight on the front wheel with the longer stays and slightly less front centre. I'd be surprised if the 2mm difference in CoG height had any real impact on handling.

Was there anything that could have kicked up your rear wheel descending? Maybe your weight dist was better with the AR.

I went from 2nd gen AR to 2nd gen S5 (both 58s) and found the S5 to be a bit more confident for driving hard into corners with the stiffer front end. But overall behaviour the same. AR gen 1 and 2 quite different though.

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 24, 17 4:42

Post #7 of 19 (2044 views)

Ok to answer your question....sorta.

The maximum available grip available to you is determined by the following factors

Tire compound
Normal force applied through the contact patch
Lean angle
Slip angle (the difference between the direction the tire is pointed and the direction you are actually going (yes they are different, small difference but the difference is important)

None of those things has to do directly w/ the geometry of your bike....but what does have to do with the geometry (and stiffness) of your bike are the following

The fore/aft balance of weight applied to the 2 tire. If the new bike has moved you forward or backwards you may be upsetting the balance between the 2 tires
If the frame is not torsionally stiff enough (or dampens too much) it may be filtering out the little feedback your tires are trying to give you and therefore you are overworking them with out knowing
If the geometry doesn't have enough trail you may not be getting the feedback through the steering moment that you are used to

Most of the geometry parameters effect how willing the bike is able to initiate a turn...not necessarily what the maximum capable lateral force it can generate in a corner. The ability to initiate a turn, and react to small changes while in a turn is hugely important, but isn't the decider in a steady state turning event.

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 24, 17 9:24

Post #8 of 19 (1991 views)

What size / angle stem were you running? are running now?Maybe the stem is too long and forcing more of your weight to the front wheel?

Any changes in saddle / saddle height? A saddle slightly too high could also force your body forward.

Re: Bike Geometry and Cornering Grip? [bloodyshogun] [ In reply to ]

Jul 24, 17 13:05

Post #9 of 19 (1949 views)

Lot of very good answers in this thread. Thank you, all. Haven't had time to fully digest them.

To answer this, no change in saddle height, but I have a high saddle height in general. 84cm - very long legs. So my Cg isn't great for cornering in general.

I have a 100cm -6 stem sitting on top of 15mm of spacer (plus the cone/cap thing). This on the 58cm S5 frame. The Felt was also a "58." My bar and saddle positions are very close to what they were on the Felt.. I tried to match it, and got it to within a few mm - just didn't jigger with spacers to get an exact match. Same bars, same saddle - just swapped out the frame.

I have a forward position - Adamo "Attack" saddle slammed forward. I came to road from triathlon, so my road position is very TT-like.

I think you may be on to something though. At the advice of an experienced racer, I've been spending almost 100% of my time in the drops. That might shift my weight forward a bit. I also need to look at my effective seat tube angle with the slammed saddle. I thought i matched that, but I need to verify.

Also good information above on tire pressure that I'll look into.

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 24, 17 13:18

Post #10 of 19 (1940 views)

I think what matters more is how stretched you are (Shoulder angle, not seat tube angle). I ride very low and stay on the drops during descends (better than staying on the hoods). However, if I am too stretched out, I can't scoot my body back far enough to put weight on the rear wheel.

And sometimes, it's just a matter of luck. There was a year I crashed 6 times in a row. It took me half a year to build my confidence back up (I was riding with elbow pads to ease my mind), but no problems since.

Last edited by: bloodyshogun: Jul 24, 17 13:31

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 24, 17 13:28

Post #11 of 19 (1928 views)

This is going to sound weird....and I haven't done it with a bike....but it might be informative.

Set up 2 decent bathroom scales next to a wall, put one wheel on each scale and get on. What do they read when you are in your drops(you are looking at the front/(front + rear) = nose percent)? What about in when you are on the hoods? Move the seat back to a more normal place..and adjust the stem accordingly....what now?

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 24, 17 13:44

Post #12 of 19 (1918 views)

My Felt AR is by far the best handling bike I've ever had. Way better than my old Trek, Specialized, GT, Litespeed, and Colnago.

Love the way it drills turns.

Last edited by: rubik: Jul 24, 17 13:44

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 24, 17 16:25

Post #13 of 19 (1876 views)

Cervelo is quite a stiff ride compared to the AR.
The extra flex of the AR may very well help keep the wheels in contact with the ground.

If you have an Adamo seat slammed forward you really are at the extreme end of forward weight bias for a road bike.
This would suggest that either the front breaks first in genuine overload conditions or the rear breaks first in choppy road conditions.

Motorbike racers have found in recent years that too stiff of a frame hurts handling and that is on machines with suspension.
So it is reasonable to believe that the stiff Cervelo is having trouble keeping the rear wheel in contact with the road especially as you have set your seat up far out of any reasonably normal setup for weight distribution.

I fell off of my nineties road bike set up with a forward post far too many times.
I have not fell off of a road going bike since loosing that bad setup.
An Adamo slammed forward is easily the equal screw up to a Profile forward seat post and standard saddle.

I suggest you put on a regular saddle and learn to pedal in a normal road position.
Don't just fudge around and go back a little, there is no middle ground, either you are forward or you are back, there is a no mans land in the middle where you will struggle to make power.

Re: Bike Geometry and Cornering Grip? [lyrrad] [ In reply to ]

Jul 25, 17 6:52

Post #14 of 19 (1814 views)

lyrrad wrote:

I suggest you put on a regular saddle and learn to pedal in a normal road position.

Ick. My slammed forward position is about 3cm off the center of the rail on the saddle, which 84cm away from the BB doesn't come out to much angle at all. And I don't see off the top of the saddle, generally speaking. The Attack gives quite a bit of fore-aft range, which I use. And I've got years and years of pure comfort on the saddle (other than very recently sliding across the ground at 30MPH, which isn't that comfortable).

But I'll keep an open mind and factor that in as an option. I could make the road bike consistent with my track bike which has slacker geometry.

Re: Bike Geometry and Cornering Grip? [GreenPlease] [ In reply to ]

Jul 25, 17 6:54

Post #15 of 19 (1811 views)

GreenPlease wrote:

How long have you been running the Jet+ rims? I was told in no uncertain terms by a HED technician to start at 85psi "max" and to "go down" from there with a 22mm Attack. I'm also 170lbs. Bike + rider in race trim everything comes to 194lbs. Keep in mind your 23s are probably closer to 26mm on those rims and when you account for casing tension 90psi is probably closer to 110-115psi on a true 23mm tire. Also, on a fast tire like the Corsa Speed, the penalty for running 10psi lower than optimum is only a couple of watts (check out the curve on BRR's chart).

For about 2 years now. But I'll consider running lower than my 90. Particularly in the rain.

Re: Bike Geometry and Cornering Grip? [pyrahna] [ In reply to ]

Jul 25, 17 6:58

Post #16 of 19 (1807 views)

pyrahna wrote:

This is going to sound weird....and I haven't done it with a bike....but it might be informative.

Set up 2 decent bathroom scales next to a wall, put one wheel on each scale and get on. What do they read when you are in your drops(you are looking at the front/(front + rear) = nose percent)? What about in when you are on the hoods? Move the seat back to a more normal place..and adjust the stem accordingly....what now?

That's interesting. I think the mechanical engineer guys at my work have a big platform thing for measuring Cg. I might ask if they could sneak me on there. Though I'd have no baseline - no way to rebuild the old bike.

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 25, 17 7:04

Post #17 of 19 (1802 views)

That is way overkill.....the fact of the matter is that your CG height doesn't actually matter for a two wheel vehicle (bike, motorcycle) for cornering speed. It matters for braking but we aren't talking about that. I know that sounds counter intuitive but the truth is that you will lean the bike over to get around the corner and that takes care of the cg height.

The bathroom scale trick takes care of the fore/aft balance question and that is what matters for cornering.

Re: Bike Geometry and Cornering Grip? [pyrahna] [ In reply to ]

Jul 25, 17 7:24

Post #18 of 19 (1790 views)

pyrahna wrote:

I know that sounds counter intuitive but the truth is that you will lean the bike over to get around the corner and that takes care of the cg height.

Yes, but my lay understanding of cornering physics is that the more you lean, the greater the lateral force vector, and the more you depend on your tires to maintain adhesion. And that tall/high riders have an inherent disadvantage in cornering in that respect.

All 3 of my incidents were a pure lateral slide of the rear tire.

So factoring in all of the above -

1) My rear tire isn't weighted as much as it used to be, reducing friction. Possibly position related.
2) Inflation isn't correct/optimal
3) Damping characteristics of the frame sometimes doesn't keep tire in contact with the ground.
4) Skill
5) Luck (bad)
6) The one guy who said my rear tire could be more weighted (still trying to think about that).

2) is possible, but isn't something I've changed. 3) is possible, but seems unlikely given the reasonably good road conditions. 4) is always there, and I'm taking a "beginner's mind" in looking at my cornering.

Re: Bike Geometry and Cornering Grip? [trail] [ In reply to ]

Jul 25, 17 7:37

Post #19 of 19 (1782 views)

The amount of lateral force you need depends on your mass and the speed at which you are trying to change directions. Lean angle (inclination angle w.r.t. the tire) increases the amount of capability for the tire to generate lateral force (to a point). You are correct that a higher cg will need more lean angle but for the most part the difference is pretty moot.

Ideally, from a lateral force perspective, you want 1/2 your weight on each tire while cornering...this will allow you to take the most advantage of the tires you have. This is because the tires generate more grip w/ more load applied to them....but that curve falls off...i.e. 50% more load does not mean 50% more lateral force...it might mean 40% more lateral force. This assumes you are not applying tractive force to the rear wheel by pedaling hard in the corner....if that is the case you want to have a rear bias to add more capability to the rear tire to handle the combined lateral and longitudinal load.

If you have an extreme forward position and you changed to a bike that has even more of a forward bias....then you are making a less than ideal situation worse.

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