Hubbry Logo
Flat spotFlat spotMain
Open search
Flat spot
Community hub
Flat spot
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Flat spot
Flat spot
Flat spot
View on Wikipedia
from Wikipedia
A severe flat spot on a rail wheel.

A flat spot, or wheel flat, also called spalling or shelling, is a fault in railroad wheel shape. A flat spot occurs when a rolling stock's wheelset stops rotating while the train is still in motion. Friction causes the part of the wheel against the hardened steel of the rails to ablate. Flat spots are usually caused on the rail transport when emergency brake are applied, or when conditions (such as during autumn (leaves) and winter (ice)) create slippery rails causing wheels to lock up while the train is still moving.

Railways

[edit]

NORAC guidelines state that the train may continue at normal speed if the flat spot is less than 2.5 inches long, or, in the case of multiple adjacent flat spots, each is shorter than 2 inches.[1] The fault is removed later in the wheelset turning process, using a wheel lathe. However, because of the heat suffered while being dragged along the rail and the impacts suffered afterward, these wheels are more likely to break due to changes in the alloy structure.

If the flat spot is sufficiently large, strands of molten metal may have stuck on one side of the flat spot, making it impossible for the wheel to turn due to insufficient clearance between the rolling surface and the brake block. In this case, the wheelset must be replaced immediately.[2]

In extreme cases such as the 1971 Salem, Illinois derailment a wheel with an untreated flat spot can damage the track and cause a derailment.

Vehicles

[edit]

Engine response

[edit]

In automobile parlance, a flat spot occurs when the driver presses the gas pedal and there is a delay in the engine's response. This fault was more common before cars had electronic fuel injection.[3] Many engines, especially modified ones, still have flat spots in the torque curve due to resonances in the intake system, although manufacturers try to eliminate these by use of a plenum chamber, careful design and testing.

Car tires

[edit]
A Formula One car with smoke coming from the front right tire after a brake lockup
Lewis Hamilton locks his brakes during heavy braking

A literal flat spot can occur on car tires if the vehicle is parked without moving for some time (generally longer than a week), and the tire deformation at the bottom of the wheel becomes semi-permanent. The flat spot gradually relieves itself when the car is driven but can temporarily give similar symptoms to an unbalanced wheel. Cars being laid up for extended periods, or intermittently-used caravans and trailers, should be kept on axle stands (tyres not in contact with the ground) or have the tires over-inflated to eliminate or reduce this problem. "Tire savers", curved wheel stands, are also available for use during storage. These reduce or avoid the problem by cradling the lower part of the tire tread and preventing the usual deformation where it rests on the ground.

Another cause seen frequently in racing is locking the wheels during heavy braking.[4]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Flat spot

View on Grokipedia
from Grokipedia
A flat spot is a term used in automotive engineering, particularly referring to either a deformation in vehicle tires or a hesitation in engine power delivery. In the context of automotive tires, it describes a localized deformation or wear on the tread surface where the tire becomes temporarily or permanently flattened due to prolonged stationary load or excessive braking force. This occurs primarily in radial tires reinforced with steel belts or nylon plies, as these materials can "set" in a deformed position when the tire cools after being under the vehicle's weight. In internal combustion engines, a flat spot refers to a temporary loss of power or hesitation during acceleration, often due to fuel delivery or ignition issues. Flat spots from , often called "cold spots," typically develop after a sits idle for weeks or months, especially in cooler temperatures where the rubber hardens and compresses unevenly against the ground. These are usually temporary and can self-correct after driving at highway speeds for 10–20 miles, allowing the to warm and reshape through flexing. However, if the deformation persists, it leads to noticeable vibrations through the or , uneven tread wear, reduced traction, and increased road noise, potentially compromising safety and requiring professional inspection or replacement. In contrast, flat spots caused by hard —common in motorsports or stops—result from lockup and skidding, which abrades the tread and creates a permanent, irregular patch that does not round out with use. Such damage is more severe in high-performance scenarios, where locked during cornering can rapidly wear the , affecting handling and increasing the risk of blowouts. Prevention strategies include avoiding long-term parking on the same spot by rotating s periodically, using tire cradles or to relieve load, maintaining proper , and employing anti-lock braking systems (ABS) to minimize skidding. Regular vehicle movement, even short drives, helps mitigate the issue, particularly for seasonal or stored vehicles.

Tire Flat Spots

Causes and Formation

A flat spot on a refers to a localized deformation or flattening of the tread surface, resulting from prolonged static loading under the vehicle's weight. This condition typically arises when the remains stationary for extended periods, allowing the with the ground to compress and harden. While often temporary, severe cases can lead to permanent damage requiring replacement. The primary cause of flat spotting is extended parking or storage, particularly in cold temperatures, where the rubber compound loses flexibility and the underlying cap plies conform to the flattened shape. Cold weather exacerbates this by reducing pressure and causing the rubber to "set" in the compressed position, as the viscoelastic properties of the material allow it to retain deformation when cooled after prior flexing during use. The physical process involves the tire's sidewall and tread compressing under load over time; as the tire cools, the hardens unevenly due to these viscoelastic characteristics, creating a bulge elsewhere on the tire that manifests as a when driving resumes. Temporary flat spots, common after days or weeks of inactivity, typically resolve after driving several miles, as the heat generated from rolling—through repeated deflections—restores the rubber's pliability and evens out the tread. In contrast, permanent flat spots develop after prolonged storage of a month or more, where the deformation becomes embedded and does not fully recover, often necessitating professional inspection or replacement. Secondary causes include underinflation or overloading, which distribute unevenly and amplify compression, as well as extreme hard braking or skidding that abrades a specific tread area, such as in high-performance motorsports applications. Specific examples include flat spots forming on vehicles stored for winter, where subfreezing temperatures accelerate the hardening process, or on high-performance cars left unused for weeks at airports during travel, leading to noticeable ride disturbances upon reactivation.

Detection and Symptoms

Flat spots on tires manifest through distinct visual and driving symptoms that drivers can observe during routine checks or operation. Visually, deformation flat spots (from ) appear as a flattened area on the tread without significant loss of tread depth, typically 1-2 inches wide, which may present as a subtle bulge or indentation when the tire is removed from the vehicle for ; braking-induced flat spots may show as worn or abraded patches. This localized irregularity disrupts the 's uniform profile, making it evident upon close examination of the tread pattern. During driving, flat spots often produce noticeable vibrations or thumping noises, particularly at speeds above 40 mph, accompanied by uneven ride quality or shimmy that is more pronounced on smooth roads. These sensations arise from the tire's irregular contact with the road surface, creating rhythmic disturbances that can feel like a mild pulsation through the vehicle's . Similar vibrations may occasionally mimic those from flat spots during , though tire-related issues are typically speed-dependent rather than throttle-responsive. To detect flat spots, owners can perform basic inspection methods by jacking up the , rotating the by hand, and visually checking the tread for flat areas or irregularities in depth. Complementing this, using a helps rule out underinflation as a factor, while professional alignment checks can differentiate flat spots from other forms of uneven wear such as cupping or feathering. Flat spots commonly affect all four tires symmetrically if resulting from prolonged , but may appear asymmetrically, often on the front tires, if linked to hard braking events. They are particularly prevalent in radial tires due to their flexible sidewall construction, which allows greater deformation under load. For more precise diagnosis, specialized tools like force balancing machines are employed, which detect flat spots by measuring radial variation across the tire's ; values exceeding 25 lbs (110 N) typically indicate a significant issue requiring attention. These machines simulate road conditions to quantify imbalances, providing that confirms the presence of flat spotting without invasive disassembly.

Prevention and Correction

Preventing tire flat spots involves routine maintenance and mindful storage practices to minimize the stress on tire treads and sidewalls. Regular tire rotations every 5,000 to 8,000 miles, combined with monthly inflation checks to maintain the manufacturer's recommended pressure—typically 32-35 for most passenger vehicles—helps distribute wear evenly and reduces deformation risk under load. For vehicles in storage, using tire cradles or jack stands offloads weight from the tires, preserving their round shape, while parking in a heated garage during cold weather prevents rubber hardening from low temperatures that exacerbate flat spotting. Adopting careful driving habits further aids prevention, particularly for high-performance with softer compounds that are more prone to deformation. Avoiding extreme braking, which can lock the wheels and imprint flat areas on the tread, and allowing gradual warm-up periods before aggressive maneuvers help maintain tire integrity. In contexts, tire warmers applied before sessions prevent cold-induced setting of the rubber. and Firestone both emphasize operating the vehicle regularly, even short distances every few weeks, to keep tires flexible. Once flat spots form, correction depends on their severity, with temporary cases often resolving through controlled . For minor impressions from short-term parking, at speeds of 50 mph or more for 20-30 miles generates sufficient heat to reshape the , typically eliminating vibrations within 20 minutes of sustained travel. Persistent or permanent flat spots, common after prolonged storage, require professional intervention such as buffing, shaving, or full replacement, as the deformation may compromise safety and handling. Replacement costs average $150–$250 per (as of 2025), depending on size and brand, though warranties from manufacturers like may offer pro-rated credits if the issue stems from material defects. For long-term mitigation, upgrading to run-flat tires or those with reinforced sidewalls reduces susceptibility to flat spotting due to their stiffer construction, which better resists deformation under static load. These options, recommended by experts for frequent parkers, provide added durability without eliminating the need for basic maintenance.

Engine Flat Spots

Characteristics in Power Delivery

In the context of internal engines, a flat spot refers to a temporary dip or plateau in the or horsepower curve, typically occurring between 2,000 and 4,000 RPM, resulting in momentary or reduced acceleration response during application. This manifestation is characterized by a sudden loss of power, often perceived as the engine bogging down or stumbling under load, followed by a surge once the RPM passes through the affected range. Such flat spots are particularly prevalent in carbureted engines and older fuel-injected systems, where the transition between idle and main fuel circuits can lead to inconsistent air-fuel mixtures. In contemporary engines with advanced technologies like direct injection and , flat spots are less common in stock configurations but can still occur due to tuning issues or component failures. On a versus RPM graph, the flat spot appears as a or flattened region in the , where output fails to rise proportionally with speed, often due to a temporary drop in in specific RPM bands caused by intake manifold resonance or airflow restrictions. This inefficiency reduces the 's ability to ingest and utilize air effectively, leading to suboptimal and a noticeable lag in power delivery. Unlike a smooth, rising , this dip disrupts the expected linear progression of power, making the feel unresponsive in mid-range acceleration. Specific examples include classic vehicles equipped with Weber carburetors, such as certain European sports cars from the mid-20th century, where improper accelerator pump settings or emulsion tube tuning can exacerbate the flat spot around 2,200-3,500 RPM. In modern turbocharged engines, a similar effect occurs during turbo spool-up lag, where exhaust energy is insufficient to build boost quickly, creating a hesitation below 2,500 RPM until the turbine reaches operational speed. It is important to distinguish this from the intentional flat torque curves in diesel engines, which are engineered for broad, consistent output across a wide RPM range to enhance drivability and load-hauling capability, rather than representing a performance fault. Historically, vehicles from the and experienced reductions due to stringent emissions regulations, which mandated leaner mixtures and restricted to reduce and output. These controls, including early catalytic converters and systems, prioritized environmental compliance over unrestricted , leading to detuned engines.

Common Causes

Fuel system malfunctions often lead to flat spots in delivery, particularly through the creation of lean air-fuel mixtures in mid-RPM ranges. In carbureted engines, improper jetting can result in insufficient fuel delivery during transitional positions, causing as the becomes too lean for optimal . leaks around the manifold or body introduce unmetered air, further leaning the and exacerbating the issue. In electronic (EFI) systems, a faulty airflow (MAF) can misread incoming air volume, leading to reduced and a power valley under load. The ideal air-fuel ratio for engines is 14.7:1 by , and deviations toward a leaner (higher than 14.7:1) in these scenarios directly contribute to by limiting . Ignition system deficiencies, including timing inaccuracies, frequently induce flat spots by disrupting spark delivery at critical RPMs. Incorrect ignition advance curves, often due to malfunctioning s or electronic control modules, fail to optimize spark timing for mid-range , resulting in incomplete and power loss. Worn spark plugs or components can cause intermittent misfires, particularly under , creating a noticeable dip in responsiveness. In forced-induction engines, turbo lag manifests as a flat spot due to the delay in exhaust-driven spool-up, forming a temporary power valley until boost pressure builds sufficiently. Mechanical restrictions within the engine's path contribute to flat spots by impeding efficient . A clogged restricts exhaust flow, increasing backpressure and reducing engine breathing efficiency, which is particularly evident in mid-RPM . flaws in the intake manifold, such as uneven runner lengths, can cause uneven air distribution across cylinders, leading to torque inconsistencies. Camshaft profiles with inadequate mid-range lift limit velocity and volume during that operating regime, resulting in a power deficit common in performance-modified engines like hot rods. Environmental conditions can amplify flat spots by altering combustion dynamics. During cold starts, insufficient fuel enrichment fails to compensate for denser, cooler air and higher fuel volatility needs, prolonging lean conditions until the engine warms. At higher altitudes, reduced air density lowers oxygen availability, leaning the mixture and diminishing power output across the RPM band, with flat spots becoming more pronounced in non-adjusted engines. These issues are notably prevalent in older BMW vehicles equipped with Motronic engine management systems, where sensor calibration for varying conditions can lag, and in modified hot rods tuned primarily for low- or high-RPM performance.

Diagnosis and Mitigation

Diagnostic tools play a crucial role in identifying engine flat spots, which manifest as dips in the curve at specific RPM ranges. Dynamometers are widely used to plot and horsepower curves, allowing technicians to pinpoint points by observing irregularities such as valleys in output between 2500-3000 RPM under part-throttle conditions. OBD-II scanners detect related issues by retrieving error codes for fuel trim deviations or misfires, which can contribute to uneven power delivery; for instance, monitoring live for misfire counts and short-term fuel trims helps isolate intermittent problems without triggering a . gauges assist in by measuring manifold ; a steady low reading around 8-14 inHg often indicates leaks that cause flat spots, as they disrupt air-fuel mixture consistency. A systematic step-by-step begins with road testing to identify points, such as sluggish acceleration under load, followed by connecting an OBD-II scanner to check for codes related to ignition or fuel delivery. Next, inspect spark plugs for fouling or wear, which can lead to misfires causing power dips, and perform fuel pressure testing at the rail, targeting 40-60 PSI for most port-injected engines to ensure adequate supply without excess that might flood cylinders. If pressures fall outside this range, it signals pump or regulator issues contributing to flat spots. Mitigation strategies focus on optimizing and ignition delivery for smoother curves. For carbureted engines, rejetting the with appropriately sized jets and adjusting the accelerator pump can eliminate off-idle by improving atomization during transitions. In fuel-injected systems, ECU remapping refines air- ratios and to fill valleys, often achieving broader power bands. Upgrading ignition components, such as replacing points with electronic distributors like Pertronix systems, provides consistent spark energy to reduce misfire-induced flat spots. Exhaust modifications, including headers, reduce backpressure to enhance scavenging and mid-range , minimizing dips without sacrificing low-end response. Specific examples include addressing turbocharged engine flat spots with boost controllers, such as manual or electronic units that adjust actuation to build pressure more progressively, eliminating lag-related hesitations between 2500-4000 RPM. Aftermarket solutions like performance chips or standalone ECUs offer plug-and-play remapping, while tuning costs typically range from $200 for basic adjustments to $1,000 for comprehensive sessions including hardware (as of 2025). Advanced tips emphasize dyno tuning sessions, where real-time adjustments to fuel maps and timing under load help achieve peak without valleys, often requiring 3-5 hours to refine for specific modifications.

Consequences

Performance Impacts

Tire flat spots compromise traction and grip by creating uneven contact with the road surface, resulting in longer stopping distances and diminished control during braking maneuvers. This unevenness arises from localized tread deformation, which disrupts the tire's ability to maintain consistent , particularly under lock-up conditions where ABS systems help mitigate but cannot fully eliminate the effects. The irregular tread profile also elevates , forcing the engine to expend additional energy to maintain speed and thereby reducing fuel economy. Temporary vibrations from these flat spots further contribute to this inefficiency until the tire warms and the deformation dissipates, typically after sustained driving. Additionally, the accelerated uneven shortens overall lifespan, as the affected areas degrade faster than the rest of the tread, leading to premature replacement needs. Engine flat spots manifest as inconsistent , particularly noticeable in with manual transmissions where precise modulation is required for smooth power delivery. This disrupts drivability, creating momentary lapses in responsiveness that affect merging, , and overall vehicle control. During these periods of suboptimal , engines may produce higher emissions due to irregular fuel-air mixtures, exacerbating environmental impact in the affected RPM range. When and flat spots occur simultaneously, their effects compound, with tire-induced vibrations potentially masking subtle power inconsistencies from the engine, complicating feedback on . This leads to broader inefficiencies, such as a noticeable decline in fuel economy due to heightened resistance and erratic power output. In high-performance contexts like , tire flat spots introduce vibrations that degrade handling precision, reducing through inconsistent lateral grip and potentially costing several seconds per lap in competitive scenarios.

Safety and Handling Risks

Flat spots on tires pose significant safety risks by inducing vibrations that compromise precision, particularly at speeds, where drivers may struggle to maintain directional control. These vibrations arise from the uneven tread contact with the road surface, potentially leading to overcorrection or loss of vehicle stability during sudden maneuvers. In wet conditions, the irregular contact patch from flat spots reduces overall traction, heightening the likelihood of hydroplaning, where the vehicle can lose braking and steering responsiveness entirely. Permanent flat spots further endanger drivers by accelerating uneven wear and weakening the tire's structural integrity, which may result in blowouts under load—similar to risks observed in severely worn tires. As of 2023, the National Highway Traffic Safety Administration (NHTSA) reports nearly 11,000 tire-related crashes annually in the United States, resulting in over 600 fatalities, with tire failures like blowouts contributing significantly. Engine flat spots introduce hesitation in power delivery, creating sudden lapses in that can prove hazardous during or merging into , where consistent response is essential to avoid collisions. This unpredictability is particularly acute in situations, such as evasive actions, amplifying the of loss of control. Handling is further impaired by reduced cornering stability from tire flat spots' uneven contact, which diminishes grip and can cause the vehicle to understeer or oversteer unexpectedly. Notable incidents underscore these dangers; for instance, during the at the , Kimi Räikkönen's severely flat-spotted front tire generated intense vibrations that fractured his suspension, resulting in a high-speed spin and retirement from the race. Drivers experiencing symptoms like persistent vibrations or thumping noises should immediately pull over to a safe location, activate hazard lights, and inspect the tires to mitigate escalation to a full .

References

  1. https://www.continental-tires.com/products/b2c/tire-knowledge/flat-spotting/
  2. https://www.michelinman.com/auto/auto-tips-and-advice/tire-damage/tire-flat-spotting
  3. https://privatefleet.com.au/glossary/what-is-a-flat-spot/
  4. https://www.firestonecompleteautocare.com/blog/tires/what-causes-tire-flat-spots/
  5. https://www.tires-easy.com/blog/flat-spotting-when-your-tire-sits-too-long/
  6. https://www.raceramps.com/blog/how-to/prevent-flat-spots/b/rrht4/
  7. https://www.startrescue.co.uk/breakdown-cover/motoring-advice/safety-and-security/flat-spots-on-tyres-causes-solutions-and-prevention
  8. https://www.tirerack.com/upgrade-garage/what-is-temporary-tire-flatspotting
  9. https://www.tirereview.com/rfv-tire-industry-part-1/
  10. https://www.consumerreports.org/cars/tire-buying-maintenance/how-to-save-money-when-buying-replacement-tires-a6799675738/
  11. https://www.kbb.com/car-advice/run-flat-tires-pros-and-cons/
  12. https://cartechnical.co.uk/jargonbuster/35-f/272-flat-spot
  13. https://www.jiminglese.com/weber-tech
  14. https://www.sae.org/publications/technical-papers/content/2023-01-0444/
  15. http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm
  16. https://www.garrettmotion.com/knowledge-center-category/turbo-replacement/turbo-lag-causes-effects-and-solutions/
  17. https://www.ncbi.nlm.nih.gov/books/NBK218144/
  18. https://grassrootsmotorsports.com/forum/grm/learn-me-carb-nonsense-specifically-flat-spots/163198/page1/
  19. https://www.ccautomotive.com/acceleration-lag-can-be-caused-by-a-faulty-mass-airflow-sensor/
  20. https://www.hptuners.com/articles/air-fuel-ratios-lambda-and-stoichiometry-explained/
  21. https://www.justanswer.com/dodge/9w605-flat-spot-when-accelerating.html
  22. https://www.donsservicecenter.com/blog/what-happens-when-your-car-s-catalytic-converter-starts-clogging
  23. https://www.hotrod.com/features/cam-talk-september-1989-982-1434-32-1
  24. https://knowhow.napaonline.com/engine-performance-depend-climate-altitude/
  25. https://forums.mye28.com/viewtopic.php?t=66119
  26. https://torqlogic.com/the-science-behind-dyno-testing-what-it-means-for-your-tuned-vehicle/
  27. https://asmtuning.co/how-to-read-dyno-graphs/
  28. https://www.foxwelldiag.com/blogs/car-diagnostic/obd2-scanner-misfires
  29. https://www.motor.com/magazine-summary/fuel-trim-data-powerful-diagnostic-tool/
  30. https://www.tuneruniversity.com/blog/2011/05/check-your-engines-health-with-a-vacuum-gauge/
  31. https://www.hotrod.com/features/leak-tweak-april-1991-982-1472-106-1
  32. https://www.onallcylinders.com/2017/03/10/ask-away-jeff-smith-step-step-guide-curing-engine-hesitation/
  33. https://www.xtoolonline.com/articles/how-to-check-fuel-pump-pressure
  34. https://www.hagerty.com/media/opinion/the-hack-mechanic/upgrading-the-distributor-comes-with-plenty-of-issues-and-choices/
  35. https://www.diygokarts.com/community/threads/the-exhaustive-guide-to-back-pressure.22264/
  36. https://www.hpacademy.com/previous-webinars/345-boost-control-trouble-shooting/
  37. https://www.enhancedstreetperformance.com/dyno/dyno-tuning-pricing
  38. https://www.eccustoms.com/info/why-dyno/
  39. https://www.tirerack.com/upgrade-garage/what-is-permanent-tire-flatspotting
  40. https://www.tires-easy.com/blog/how-to-spot-irregular-tire-wear/
  41. https://www.sl113.org/forums/index.php?topic=30299.0
  42. https://www.sae.org/publications/technical-papers/content/2015-01-2196/
  43. https://rnrtires.com/tips-guides/signs-that-your-tires-may-be-beyond-repair/
  44. https://golden-horse.ae/blog/flat-spots-on-tires-symptoms-and-fixes/
  45. https://www.handylawutah.com/blog/2022/10/how-bad-tires-increase-the-risk-of-car-accidents/
  46. https://www.nhtsa.gov/vehicle-safety/tires
  47. https://www.youtube.com/watch?v=ob69GFZR1fs
  48. https://jeffwylerspringfieldtoyota.net/blog/2024/december/23/5-warning-signs-you-need-new-tires.htm
  49. https://www.fleschnerlaw.com/blog/flat-tire-safety/
Add your contribution
Related Hubs
User Avatar
No comments yet.